Mapping of absolute stresses around two California earthquakes reveals a very weak crust

Mapping of absolute stresses around two California earthquakes reveals a very weak crust | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Mapping of absolute stresses around two California earthquakes reveals a very weak crust Siyuan Zhang, Heidi Houston, Binhao Wang, Hao Zhang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4555753/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Absolute amplitudes of shear stresses that drive crustal earthquakes are not well known. There is a long-standing divergence between the values inferred from lab experiments and stress changes during faulting. Two large earthquakes near Ridgecrest, California with M6.4 and 7.1 provide a natural laboratory to determine the in-situ average shear stress in the crust off the main faults. Here we use the change in faulting geometries of abundant small earthquakes together with stress changes imposed by doublet slip to determine full deviatoric stress tensors both before and after it. We first invert suites of focal mechanisms for stress orientations and ratios between eigenvalues. We then invert for the 3-D full deviatoric tensors constrained by the stress orientations, stress ratios, and the coseismic stress change due to the doublet. We applied this method using two doublet slip models and two endmember approaches: first dividing the region into 12 blocks surrounding the mainshock faults, and second performing 9,200 separate inversions offset by ~ 1 km. To obtain reliable results, we use the 3-D relationship rather than a common 2-D strike-slip simplification, define inversion regions that do not cross the main faults, and include only high-quality events a few km away from the main faults to avoid large heterogeneities in the co-seismic stress change. Deviatoric stresses are only a few percent of levels expected at seismogenic depths from Byerlee friction, except for regions near the doublet hypocenters where they are up to only ~ 7.5%. Our approach yields strong evidence for a very weak continental crust, which bears on earthquake and geodynamic modeling, as well as earthquake recurrence behavior and hazard, suggesting near-complete stress drops in the mainshock doublet and a low chance of imminent large slips there. Earth and environmental sciences/Solid Earth sciences/Tectonics Earth and environmental sciences/Solid Earth sciences/Seismology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction The strength of the crust has been a long-standing question in seismology, tectonics, and geodynamics. The seismogenic crust is almost entirely inaccessible to direct measurements of stress and strength. Analysis of earthquake focal mechanisms provides the primary method of gauging in-situ stress conditions and suites of them can be inverted to yield spatially-averaged normalized stress orientations (e.g., refs. 1–2). The misalignment of those orientations with a major fault has been used to constrain friction on the fault (e.g., refs. 3–6). Changes in those orientations due to large earthquakes have been used in a two-dimensional analytic framework to constrain the fraction of pre-existing shear stress relieved by the mainshock stress drop (e.g., refs. 7–9). Ref. 10 proposed a three-dimensional framework, which can further yield the absolute deviatoric stress tensors before and after a mainshock given adequate focal mechanisms and mainshock slip distribution. If the amplitude of stress before an earthquake has a similar order of magnitude to the coseismic stress drop, the stress orientations rotate by an amount depending on the geometrical relationships. Studies have observed stress rotations indicating low background stress amplitudes and weak faults (e.g., refs. 8–11). A Mw6.4 strike-slip earthquake ruptured an unmapped fault system in Ridgecrest on 4 July 2019, followed by a Mw7.1 strike-slip earthquake 34 hours later. These two earthquakes and their aftershocks have been well-documented and widely studied for multiple purposes. Resolvable rotations of maximum horizontal stress have been observed by comparing stress orientations before and after the M7.1 (refs. 12–15), suggesting that coseismic stress drop relieved much of the background stress. Ref. 13 conducted stress inversions on five blocks that cross the two major faults. They adopted the 2-D simplification proposed by ref. 8 and inferred that the maximum shear stress was ~ 10 MPa near the junction of the two major faults before the M7.1. Ref. 15 inferred a maximum shear stress of 5 MPa following the approach of ref. 13 but using more aftershocks and their own slip model. Ref. 16 interpreted a stress rotation obtained by comparing orientations derived from surface mainshock slip measurements (surmised to represent the pre-mainshock stress state) with orientations from aftershocks at depth, and inferred similar magnitudes of stress along the main faults. These studies rely on the two-dimensional analytic simplification (ref. 8), which does not account for rotations around a non-vertical axis that are observed. They also focus on the aftershock zone close to the mainshock, which is highly complex, subject to mainshock slip heterogeneity, and likely anisotropic. Here, we utilize a three-dimensional framework (as in refs. 10 & 17) to analyze stress rotations and infer full deviatoric stress tensors, using two slip models and multiple approaches to define subregions and allow for different length scales of stress variations. Our analysis inverts for stresses both near and well away from the main faults. The blocks we selected were not allowed to cross the major faults, as we observed differences across the faults. We are able to capture robust features, reinforced by the consistent results from our different inversion approaches and use of only higher-quality focal mechanisms. In addition to resolving spatial stress concentrations before the mainshocks and a pattern of shape ratio change consistent with the mainshock slip, we find exceedingly low deviatoric stresses even off the main faults indicating a very weak crust. 2 Stress orientations before and after the 2019 Ridgecrest doublet We first inverted focal mechanisms for the stress orientations before the M6.4 foreshock (referred to as pre-event) and those after the M7.1 mainshock (referred to as post-event) using the package developed by ref. 2 following the method of ref. 1 (see Methods). We divided the study area into 12 blocks based on the distributions of slip and of seismicity (Fig. 1 ). Each block was further divided into the periods before and after the 2019 Ridgecrest doublet, excluding the M6.4 and M7.1. The 50 best focal mechanisms in each of the 24 spatiotemporal blocks were selected based on their uncertainties (Supplementary Table S1 ), as suggested by ref 11. There is a tradeoff between focal mechanism quality and the number utilized for inversion (see Methods). Events in block 12 were required to occur at least two months after the M7.1 to exclude numerous normal mechanisms related to a more local process of extensional topographic collapse. To stabilize the inversion result, we treated the 12 pre-event blocks as an ensemble with the same stress conditions. This is a reasonable assumption given the absence of a large earthquake during the period, stabilizing those blocks that contain only a few pre-event focal mechanisms. The uniform pre-event stress orientation was determined by inverting 60 events that comprise the 5 best focal mechanisms from each block (Supplementary Table S1 ). Changes in the stress orientations before and after the mainshocks are apparent in Fig. 2 , which shows P- and T-axes and inverted stress orientations for the 12 blocks. The uniform pre-event stress orientation is consistent with the focal mechanisms of the mainshocks, with \({\sigma }_{1}\) striking ~ 45°from the high-slip portion of the M7.1 fault. Large rotations after the earthquake doublet occur in blocks 2, 7, 11, and 12, with Kagan angles (ref. 18) exceeding 30º (Supplementary Table S2). The coseismic stress induced by the mainshock is likely to have released a substantial portion of the pre-event stresses in blocks 2 and 7, altering an almost pure strike-slip regime to one with more normal faulting. In block 11, the stress orientation primarily rotates around \({\sigma }_{2}\) and may be affected by a shift in epicentral locations, Block 12 may have experienced a large decrease in its compressional stress. In block 12, as well as block 7, \({\sigma }_{1}\) and \({\sigma }_{2}\) have a clear trade-off (Supplementary Fig. S3), resulting from the comparable numbers of strike-slip and normal faulting events. The areas surrounding the northwest and southeast rupture termini (blocks 1, 3, 5, 8, and 9) underwent moderate stress rotations, with Kagan angles ranging from 10º to 25º (Supplementary Table S2). Blocks 4, 6 and 10 exhibit nearly identical stress orientations before and after the mainshocks, with Kagan angles less than 10º (Supplementary Table S2). Overall, we observed significant stress rotations in most blocks, indicative of comparable levels of pre-event and coseismic stresses (ref. 11). In addition, shape ratios (see Methods) are generated as part of the current step as well, subsequent to obtaining the stress orientation tensors, and will be discussed along with the stress amplitudes in later sections. 3 Coseismic stress change induced by the 2019 Ridgecrest doublet We calculated the coseismic stress change from published slip distributions (refs. 19–20) based on the analytical static stress solution derived by ref. 21 (see Methods). The two slip models we utilized are consistent in terms of the slip pattern of the M7.1 mainshock (Figs. 3 b & 3 d). Ref. 19 associated the slip of the M6.4 foreshock mostly with the southwest-trending fault, whereas ref. 20 preferred the northwest-trending main fault. The latter result also has a slip patch isolated from the mainshock, and located to the southeast of the junction between the two largest orthogonal faults (Fig. 3 d). The lobes of the Coulomb stress changes on faults parallel to the M7.1 are similar except immediately adjacent to the fault (Figs. 3 a & 3 c). The right-lateral component was released by the slip around the M7.1 mainshock epicenter and accumulated at the two rupture termini. The distribution of the focal mechanisms used in various blocks is also shown in Fig. 3 . We employed different strategies to capture representative coseismic stress change tensors for each block (“mean” and “centroid”; see Methods). Although the “mean” mode results in slightly smaller coseismic stress changes, this reduction does not substantially change the relative magnitudes among the different blocks (Supplementary Fig. S5). 4 Change in maximum shear stresses before and after the 2019 Ridgecrest doublet We combined the stress orientations before and after the doublet with the coseismic stress change to invert for the absolute deviatoric stresses and reconstruct the full pre-event and post-event deviatoric stress tensors (see Methods). Figure 4 illustrates the resulting inverted maximum shear stresses on any plane, obtained from the pre- and post-event deviatoric stress tensors. We observe a consistent pattern of stress increase or decrease across the blocks (Figs. 4 d–e). The changes in blocks 2–3, 5–8, and 11–12 are statistically significant (Supplementary Fig. S6). Remarkably, the two slip models yield similar patterns of stress changes (Figs. 4 d & 4 f). Inverting for different pre-event stress orientations in the 12 blocks gives the same pattern but more variable stress levels (Supplementary Fig. S7d). Such consistency indicates that the different methods of calculating the coseismic stress in each block do not affect the pattern of increase or decrease in maximum shear stress. Employing a stress mode with higher coseismic stress magnitudes amplifies maximum shear stresses (Figs. 4 d–e & Supplementary Fig. S5). Opting for the slip model of ref. 20 yields somewhat reduced stress levels (Fig. 4 f), because the orientations as well as magnitudes of the coseismic stress tensors differ significantly in the near-fault blocks ( 1 – 7 ) for the two slip models. As an alternate approach and a check on the 12-block regionalization and joint inversion, we partitioned the study area into small “pixels” at 0.01º intervals in latitude and longitude (see Methods). The pattern of stress changes resulting from the pixel inversion (Fig. 5 f) remains stable, even when using the slip model of ref. 20, as depicted in Supplementary Fig. S8f. This inversion approach, which leverages more focal mechanisms and allows for pre-event variation, does introduce greater uncertainties; however, it significantly concurs with the results of inverting the 12 blocks (Fig. 4 g & Supplementary Fig. S9). A pronounced stress reduction is evident on both sides of the southwest-trending fault (blocks 5 & 6), correlating with high slip from the M6.4 foreshock according to ref. 19. Adjacent to the M7.1 mainshock epicenter, blocks 2 and 7 display similar patterns of stress decrease to that of blocks 5 and 6. The zero post-event stresses in blocks 2 and 6 reach the bound of the non-negative inversion. Different responses are observed in blocks 3 and 4, at the southeast section of the main fault. The slip model from ref. 19 suggests less slip during the mainshock around the junction of two orthogonal faults (Fig. 3 b). This slip pattern induces an increase in right-lateral shear stress in the northern half of block 4, which is further enhanced by the foreshock (Fig. 3 a). Therefore, the inversion for 12 blocks (Fig. 4 d) indicates an extensive stress decrease in block 3, whereas little change in maximum shear stress occurred in block 4 perhaps due to the smoothing based on the focal mechanism distribution (Fig. 3 ); the contrast between blocks 3 and 4 is consistent with the details from pixel inversion (Fig. 5 f). These different impacts on the two sides argue against defining blocks that cross the fault when investigating earthquakes with complex slip patterns or fault geometry. At the northwest terminus of the mainshock, there is an apparent stress increase over almost all of block 8 and the portion of block 1 proximal to the fault (Figs. 4 d & 5 f). The southeast terminus of the main fault behaves similarly to load the Garlock fault (Fig. 5 f), although not revealed by the inversion for 12 blocks. Since the addressed pre-event and post-event stress levels are contingent upon the amplitude of the coseismic stress through inversion (Eq. 2 ; see Methods), and the rotation of stress orientations is difficult to resolve in areas less affected by the earthquake, blocks 9–12 are either roughly unchanged or moderately reduced in stress. 5 Discussion The change in shape ratio revealed by the pixel inversion (Fig. 5 c) is consistent with the mechanics of strike-slip faulting. The overall pattern of shape ratios in Fig. 5 delineates a four-quadrant pattern, which is bisected by the two major orthogonal faults. Only small differences appear under the assumption of a uniform pre-event stress orientation in each grid (Supplementary Fig. S10c & Fig. 5 c), demonstrating that this pattern is a manifestation of the coseismic stress change. Typically, a double-couple earthquake produces compressions and dilatations across four quadrants as its focal mechanism indicates, which is expected to be reflected in the shape ratio. The western part of our study region, subject to the compression of both the foreshock and mainshock, exhibits the most significant increase in shape ratio (Fig. 5 c) due to its proximity to the substantial slip. Conversely, the decrease in shape ratio in the north quadrant indicates enhanced tensional deviatoric stress. The two quadrants to the southeast generally conform to the expected pattern but exhibit more heterogeneity, less affected by the mainshock slip. This observation further supports that the order of magnitude of the pre-event stress is comparable to that of the coseismic stress change, as otherwise the pre-event compressional or tensional influence would dominate the post-event shape ratio. Although rotations of stress orientation have been observed in various earthquakes, both continental and in subduction zones, negative results and skepticism regarding uncertainties have been noted by some researchers, as reviewed by ref. 11. To address these concerns, we have thoroughly compared different strategies at each stage as previously discussed and summarize those results here. For inverting the stress orientations, we selected only high-quality focal mechanisms (A or B) from the original catalog (refs. 22–23). The 12-block inversion further picked the best 50 focal mechanisms with the least uncertainty in each spatiotemporal bin (Supplementary Table S1 ). Allowing pre-event stress orientations to differ for the 12 blocks prior to the Ridgecrest doublet (Supplementary Fig. S4) yielded remarkably similar results to our preferred method of assuming the same pre-event stress orientation for all 12 blocks (Fig. 2 ), an approach that improves stability and minimizes uncertainty. We considered two different modes of extracting the static coseismic stress and two different slip models to calculate the representative coseismic stress change in each of the 12 blocks, revealing a consistent pattern of maximum shear stress change (Figs. 4 d–f). Solutions that incorporate the uncertainty of focal mechanisms demonstrated the same pattern in a statistical way (Supplementary Fig. S6). The 12-block inversion can be assessed as the most stable approach in this study. The other endmember, the pixel inversion, fully utilizes information from more focal mechanisms and allows higher spatial variation, which introduces more uncertainty. Furthermore, the different slip models show similar stress change patterns (Fig. 5 f & Supplementary Fig. S8f). Practical issues prevent the construction of a combined matrix to guarantee a unique uniform pre-event stress tensor, but we were able to test a uniform pre-event stress orientation at every pixel, using the one from the 12-block inversion (Supplementary Fig. S10), which produces results very similar to Fig. 5 . Along with another test in which the 12-block inversion allows pre-event regional variations (Supplementary Fig. S7), these two approaches balance both stability and variability. All four inversions produce largely consistent spatial patterns of stress increase or decrease, lending credibility to our results. The variability of post-event stress orientations was calculated to be 20.37º for 12-block inversion (see Methods), which contrasts with the pre-event variability of 11.45º if each block is inverted individually (Supplementary Fig. S4). For the pixel inversion, the variability increases from 19.65º to 31.82º. This difference highlights the importance of the Ridgecrest doublet in increasing stress heterogeneity regionally, providing some support to the assumption of a more uniform stress state pre-event. The pre-event maximum shear stress displays a pattern in which the amplitude decreases with increasing distance from the fault (Fig. 5 d & Supplementary Fig. S7), deviating from our assumption of a uniform pre-event stress. It could be a concern that the stress amplitude generally correlates with the magnitude of coseismic stress changes, which could be an artifact. However, the stress rotations in blocks 9–12 (Supplementary Table S2) are significant when compared to their small coseismic stress changes (Supplementary Fig. S5), resulting in their low-stress levels. Additionally, a large portion of block 7 has low pre-event stress amplitudes, despite its substantial coseismic stress change. The stress rotations in areas away from the fault may partially arise from the stress heterogeneity, rather than predominantly influenced by the coseismic stress close to the fault. Other factors like undetected creep that could alter the stress, would rarely play a role on the same time scale but remains possible. The spectrum of our inversion approaches covers various extents of pre-event variations, accounting for those unknown effects. Some studies have also observed back rotation to the pre-event stress orientation within a few months or a few years (refs. 11 & 17). Due to the Omori-type decay of aftershocks, the focal mechanisms we utilized to constrain the post-event stress orientation were primarily from the immediate aftershocks. Nevertheless, we find no apparent trends of back rotations in any of the 12 blocks (Supplementary Fig. S11). In this study, the maximum shear stresses are all below 10 MPa, as low as ~ 2 MPa in the results from the 12-block inversion, which is consistent with previous results (refs. 13, & 15–16). Some previous studies on stress rotations indicated low stress amplitudes on the order of the earthquake stress drops (refs. 7–10), suggesting a weak crust (ref. 11). Considering that the gravity of crustal rocks imparts a compressional normal stress of about 27 MPa per kilometer of depth, such low shear stress values at depths of ~ 5–10 km imply an extraordinarily low friction coefficient and/or extremely high fluid pressure. The peak value of the pixel inversion is ~ 9.5 MPa, 7.5% of the frictional strength from Byerlee’s law at a depth of 8 km (assuming a friction coefficient of 0.6, a rock density of 2700 kg/m 3 , and no cohesion; ref. 24), and the common value of ~ 2 MPa is only 1.6% of it. Ref. 25 documented numerous instances of orthogonal faulting within the Ridgecrest fault zone, which is evidence of a near-zero friction coefficient based on the Coulomb failure criterion. While extremely high fluid pressure may account for low shear stresses in subduction zones, as suggested by ref. 17, a combination of a low friction coefficient and elevated fluid pressure could be responsible for the low maximum shear stresses in the continental crust. Ref. 26 inferred a near-lithostatic fluid pressure below 5 km at the two termini of the coseismic rupture of the M7.1 by inverting fault frictional properties using an afterslip model. It is also worth noting that the maximum shear stresses revealed by this method are only sampling those structures that hold earthquakes, rather than indicating the stress conditions in the middle of intact rock. Therefore, fluid pressure close to lithostatic could be present in fault gouges depending on the permeability, even if the tectonic environment is not as fluid-saturated as in subducting slabs. Prior to the Ridgecrest doublet, the pixel inversion using the slip model from ref. 19 reveals distinct stress concentrations (> 7 MPa) proximate to the high-slip patches of the M6.4 foreshock (Fig. 5 d). The density of nearby focal mechanisms is sufficient to resolve this in-situ feature. Near the epicenter of the M7.1 mainshock, the maximum shear stress is elevated as well, but to 3–4 MPa which is not as high as that on the southwest-trending fault. This finding may explain why the southwest-trending rupture preceded and presumably loaded the M7.1 segment. A similar relationship was inferred for the 2016 Kumamoto earthquake and its large foreshock by investigating the stress rotations caused by both (ref. 9). We have demonstrated a comprehensive framework from focal mechanisms to full deviatoric stress tensors, which can be applied to regions with well-resolved coseismic slip models. Using the best but enough focal mechanisms helps optimize the uncertainty from each focal mechanism, which can be quantitatively leveraged with our new noisy solutions. Calculating the static coseismic stress change and employing the 3-D relationship remove the necessity to estimate the average stress drop needed to employ the less accurate analytic 2-D relationship. A spectrum of approaches with various pre-event assumptions allows for exploring details progressively. The four-quadrant pattern of the change in shape ratio and the stress concentrations before the doublet illustrate the potential of this technique to reveal detailed features. The very low deviatoric stresses before and after the mainshocks indicate both weak faults and a weak crust even away from the primary fault zone. This supports the view that the seismogenic portions of the crust do not provide substantial lithospheric strength, which is likely much greater in the mid-crust (ref. 27). Methods Inversion for stress orientations We employed the package developed by ref. 2, which uses the stress inversion of ref. 1. This method minimizes the difference between the direction of shear stress imposed by a normalized stress tensor on a nodal plane and the actual slip on that plane. The deviatoric part of the average background stress orientation can be retrieved given a sufficient number of focal mechanisms that rupture under a consistent stress condition. Under the further assumption that the amplitude of shear stress at failure is the same for all events, the inversion is linear. The shape ratio is defined as $$R=\frac{{\sigma }_{1}-{\sigma }_{2}}{{\sigma }_{1}-{\sigma }_{3}}$$ 1 , where \({\sigma }_{1}\) and \({\sigma }_{3}\) represent the most compressional and tensional axes, respectively. Ref. 2 introduced the notion of fault instability in order to assess which of the two nodal planes is more likely to fail in a given stress state (comprising the stress orientations and shape ratio). This leads to higher accuracy of the shape ratio. Focal mechanisms were taken from a refined catalog using the HASH method (refs. 22–23; extended), including earthquakes from 1981 to 2022 (Fig. 1 ). Only high-quality events (quality A or B) were retained. Events within 2 km of the finite-fault model (ref. 19) after the M7.1 mainshock were excluded (Fig. 1 b) because near the fault the finite-fault models generate highly heterogeneous coseismic stress changes where neighboring sub-faults adjoin. Thus, most of the very-close-in aftershocks of the doublet were excluded (Fig. 8 in ref. 12). The uncertainty of each focal mechanism was utilized to generate noisy solutions for assessing the results (Supplementary Text S1). According to synthetic tests (ref. 2), ~ 50 focal mechaisms provide adequate accuracies in both stress orientation and shape ratio which do not improve much upon including more focal mechanisms. A uniform number of focal mechanisms in each block leads to more comparable uncertainties in inverted stress orientations. In addition to the aforementioned assumptions required to establish the inverse problem, it is desirable that the focal mechanisms be adequately diverse but also consistent with rupture in one stress field (refs. 1, 8, & 13). The diversity of focal mechanisms can be quantified by the RMS angular difference between the focal mechanisms and a “mean focal mechanism” (refs. 8 & 13). We adopt the inverted stress orientation as the “mean focal mechanism”, and measure the angular difference by the Kagan angle (ref. 18). The calculated RMS angular differences are labeled with each stress orientation (upper right corners in Fig. 2 ), mostly satisfying the diversity requirement (greater than ~ 35–40º if the focal mechanism errors are ~ 10–20º, according to ref. 8). The assumption of a consistent stress field can be assessed by the misfit angle, which is the median difference between the observed slip vectors and the ones predicted by the inverted stress orientation. Ideally, diverse and noise-free focal mechanisms would look like two “butterfly wings” of P- or T-axes around \({\sigma }_{1}\) or \({\sigma }_{3}\) orientations, respectively (ref. 2). The deviation from that pattern reflects the misfit angle (Fig. 2 ). Our 12 blocks all satisfy the consistent-stress-field assumption (less than ~ 35–40º misfit angle if the focal mechanism errors are ~ 10–20º, according to ref. 30). Computing representative coseismic stress change for each block We assumed 30GPa for both Lamé parameters to compute the coseismic stress changes, based on the analytical static stress solution in a homogeneous elastic half space derived by ref. 21. The trace of the calculated coseismic stress change tensor was then removed. The following inversion requires one representative coseismic stress change tensor for each block. We employed three different modes to capture it: 1) Following ref. 10, we averaged coseismic stress changes at all post-event hypocenters within a block, referred to as the “mean” coseismic stress mode; 2) We adopted the coseismic stress change at the centroid of post-event hypocenters as the “centroid” coseismic stress mode; 3) In the “pixel inversion”, we adopted the in-situ coseismic stress change at each pixel. The “mean” mode results in slightly smaller coseismic stress changes than the “centroid” mode (Supplementary Fig. S5) when averaging tensors. The “centroid” and the in-situ modes would sample the near-fault heterogeneous coseismic stresses, whereas the “mean” mode would not, due to the removal of the near-fault seismicity. Thus, we opted to use only the “mean” mode in our systematic pixel inversion. Inversion for full deviatoric stress tensors before and after the doublet The stress orientations before and after an earthquake, coupled with the coseismic stress change, can be used to estimate the absolute deviatoric stresses, enabling the reconstruction of the full pre-event and post-event deviatoric stress tensors at seismogenic depths (refs. 10–11). Analytic methods, such as those by refs. 7–8, and inversion approaches (refs 10 & 32) have been introduced in the literature, as reviewed by ref. 11. The analytic methods simplify the problem into a 2-D model, relating the rotation of the stress orientation to the ratio of stress drop to the maximum shear stress (ref. 8) or the shear stress on the fault (ref. 7). Their assumptions about the coseismic stress change have been substantiated for simple strike-slip faults (ref. 32); however, they are not valid in off-fault areas. The inversion approach used here (following refs. 10 & 17) accounts for the full 3D coseismic stress tensors and is capable of addressing complexities in off-fault areas. While the initial application (ref. 10) still divided the fault into several cross-fault segments, our blocks avoid extending over both sides of the fault to eliminate the singularities and uncertainties of coseismic stress changes derived from unsmoothed slip models. Since we treated the 12 blocks as an ensemble prior to the Ridgecrest doublet to enhance stability, the inversion approach from ref. 10 was adapted as follows: $$\left[\begin{array}{c}\varDelta {\varvec{S}}_{1}\\ \begin{array}{c}\varDelta {\varvec{S}}_{2}\\ \dots \end{array}\\ \varDelta {\varvec{S}}_{12}\end{array}\right]=\left[\begin{array}{ccc}{\varvec{S}}_{1}^{after}& \cdots & 0\\ ⋮& \ddots & ⋮\\ 0& \cdots & {\varvec{S}}_{12}^{after}\end{array}\begin{array}{c}{-\varvec{S}}^{before}\\ ⋮\\ {-\varvec{S}}^{before}\end{array}\right]\left[\begin{array}{c}{C}_{1}^{after}\\ \begin{array}{c}{C}_{2}^{after}\\ \dots \end{array}\\ \begin{array}{c}{C}_{12}^{after}\\ {C}^{before}\end{array}\end{array}\right]$$ 2 , where \(\varDelta {\varvec{S}}_{i}\) and \({\varvec{S}}_{1}^{after}\) are the representative deviatoric coseismic stress change and the normalized post-event stress tensor in block \(i\) , respectively. \({\varvec{S}}^{before}\) is the normalized pre-event uniform stress tensor for all 12 blocks. Each of these stress tensors comprises six components arranged vertically. Although only five components of a deviatoric stress tensor are independent, we have constructed the matrix with six components to equally weight the misfits among them as in ref. 17. After re-normalizing the stress orientation tensors by their eigenvalue ranges, \({C}_{i}^{after}\) and \({C}^{before}\) represent the differential stresses (twice the maximum shear stresses) in post-event block \(i\) and for the pre-event unified 12 blocks, respectively. To solve Eq. 2 , we utilized the non-negative least-square function LSQNONNEG, implemented in MATLAB. Pixel inversion We applied a systematic inversion by partitioning the study area into small “pixels” at 0.01º intervals in latitude and longitude. For each pixel before and after the Ridgecrest doublet, we utilized the 45 focal mechanisms before and after the doublet (picked from the quality A or B mechanisms in Fig. 1 ) closest to the pixel to constrain its stress orientations. Near the fault, the coseismic stress changes appear highly heterogeneous, primarily due to the unsmoothed slip as well as the bends between fault segments. As such, we opted to use only the “mean” mode in our systematic inversion. Given the substantial size of the combined matrix (Eq. 2 ) of 91 \(\times\) 101 pixels, amounting to 55146 \(\times\) 9192 elements, it is practically challenging to perform the non-negative inversion of Eq. ( 2 ) on such a scale. Therefore, we inverted for each pixel individually. Thus, here the pre-event shape ratio (Fig. 5 a) and stress (Fig. 5 d) can vary across the study region. Quantifying the variability of stress orientations To quantify the variability of post-event stress orientations, we first determined their centroid tensor using the MATLAB function KMEANS. We represented each stress orientation as a vector comprising 9 components and employed the ‘cosine’ distance metric within KMEANS. We computed the mean Kagan angle between the centroid and each post-event stress orientation to quantify the variability of stress orientations. This procedure reveals increased stress heterogeneity following the doublet for both regionalization approaches. Declarations Data Availability The focal mechanisms used in this study can be found at https://scedc.caltech.edu/data/alt-2011-yang-hauksson-shearer.html . The inverted full deviatoric stress tensors from both block and pixel inversions have been uploaded to https://doi.org/10.5281/zenodo.11475511 . Acknowledgements We are grateful to John Vidale from University of Southern California for invaluable suggestions on constructing this manuscript. Figures were produced using Generic Mapping Tools 6 (ref. 33). References Michael, A. J. Determination of stress from slip data: Faults and folds. J. Geophys. Res.: Solid Earth 89, 11517–11526 (1984). Vavryčuk, V. Iterative joint inversion for stress and fault orientations from focal mechanisms. Geophys. J. Int. 199, 69–77 (2014). Sibson, R. H. A note on fault reactivation. J. Struct. Geol. 7, 751–754 (1985). Lachenbruch, A. H. & McGarr, A. Stress and heat flow. in The San Andreas Fault System, California 261–277 (U.S. Geological Survey, California, United States, 1990). doi: 10.3133/pp1515 . Provost, A. & Houston, H. Orientation of the stress field surrounding the creeping section of the San Andreas Fault: Evidence for a narrow mechanically weak fault zone. J. Geophys. Res.: Solid Earth 106, 11373–11386 (2001). Provost, A. & Houston, H. Stress orientations in northern and central California: Evidence for the evolution of frictional strength along the San Andreas plate boundary system. J. Geophys. Res.: Solid Earth 108, (2003). Yin, Z.-M. & Rogers, G. C. Rotation of the principal stress directions due to earthquake faulting and its seismological implications. Bull. Seism. Soc. Am. 85, 1513–1517 (1995). Hardebeck, J. L. & Hauksson, E. Crustal stress field in southern California and its implications for fault mechanics. J. Geophys. Res.: Solid Earth 106, 21859–21882 (2001). Yoshida, K. et al. Stress rotations due to the M6.5 foreshock and M7.3 main shock in the 2016 Kumamoto, SW Japan, earthquake sequence. Geophys. Res. Lett. 43, 10,097 – 10,104 (2016). Wesson, R. L. & Boyd, O. S. Stress before and after the 2002 Denali fault earthquake. Geophys. Res. Lett. 34, (2007). Hardebeck, J. L. & Okada, T. Temporal stress changes caused by earthquakes: A review. J. Geophys. Res.: Solid Earth 123, 1350–1365 (2018). Hauksson, E. & Jones, L. M. Seismicity, Stress State, and Style of Faulting of the Ridgecrest-Coso Region from the 1930s to 2019: Seismotectonics of an Evolving Plate Boundary Segment. Bull. Seism. Soc. Am. (2020) doi: 10.1785/0120200051 . Sheng, S. & Meng, L. Stress field variation during the 2019 Ridgecrest earthquake sequence. Geophys. Res. Lett. 47, (2020). Wang, X. & Zhan, Z. Seismotectonics and fault geometries of the 2019 Ridgecrest sequence: Insight from aftershock moment tensor catalog using 3-D Green’s functions. J. Geophys. Res.: Solid Earth 125, (2020). Duan, H. et al. Analysis of coseismic slip distributions and stress variations of the 2019 Mw 6.4 and 7.1 earthquakes in Ridgecrest, California. Tectonophysics 831, 229343 (2022). Milliner, C. W. D., Aati, S. & Avouac, J. P. Fault friction derived from fault bend influence on coseismic slip during the 2019 Ridgecrest Mw 7.1 mainshock. J. Geophys. Res.: Solid Earth 127, (2022). Delbridge, B. G., Houston, H., Bürgmann, R., Kita, S. & Asano, Y. A weak subducting slab at intermediate depths below northeast Japan. Sci. Adv. 10, eadh2106 (2024). Kagan, Y. Y. Simplified algorithms for calculating double-couple rotation. Geophys. J. Int. 171, 411–418 (2007). Xu, X. et al. Surface deformation associated with fractures near the 2019 Ridgecrest earthquake sequence. Science 370, 605–608 (2020). Yue, H. et al. The 2019 Ridgecrest, California earthquake sequence: Evolution of seismic and aseismic slip on an orthogonal fault system. Earth Planet. Sci. Lett. 570, 117066 (2021). Okada, Y. Internal deformation due to shear and tensile faults in a half-space. Bull. Seism. Soc. Am. 82, 1018–1040 (1992). Yang, W., Hauksson, E. & Shearer, P. M. Computing a Large Refined Catalog of Focal Mechanisms for Southern California (1981–2010): Temporal Stability of the Style of Faulting. Bull. Seism. Soc. Am. 102, 1179–1194 (2012). Hauksson, E., Yang, W. & Shearer, P. M. Waveform Relocated Earthquake Catalog for Southern California (1981–2011). Bull. Seism. Soc. Am. 102, 2239–2244 (2012). Byerlee, J. Friction of rocks. pure Appl. Geophys. 116, 615–626 (1978). Ross, Z. E. et al. Hierarchical interlocked orthogonal faulting in the 2019 Ridgecrest earthquake sequence. Science 366, 346–351 (2019). Zhao, Z. & Yue, H. A two-step inversion for fault frictional properties using a temporally varying afterslip model and its application to the 2019 Ridgecrest earthquake. Earth Planet. Sci. Lett. 602, 117932 (2023). Behr, W. M. & Platt, J. P. Brittle faults are weak, yet the ductile middle crust is strong: Implications for lithospheric mechanics. Geophys. Res. Lett. 41, 8067–8075 (2014). Zoback, M. L. First- and second-order patterns of stress in the lithosphere: The World Stress Map Project. J. Geophys. Res.: Solid Earth 97, 11703–11728 (1992). Evans, W. S. et al. A Statistical Method for Associating Earthquakes with Their Source Faults in Southern California. Bull. Seism. Soc. Am. 110, 213–225 (2020). Michael, A. J. Spatial variations in stress within the 1987 Whittier Narrows, California, aftershock sequence: New techniques and results. J. Geophys. Res.: Solid Earth 96, 6303–6319 (1991). Yang, Y., Johnson, K. M. & Chuang, R. Y. Inversion for absolute deviatoric crustal stress using focal mechanisms and coseismic stress changes: The 2011 M9 Tohoku-oki, Japan, earthquake. J. Geophys. Res.: Solid Earth 118, 5516–5529 (2013). Hardebeck, J. L. Earth’s free surface complicates inference of absolute stress from earthquake-induced stress rotations. Geophys. Res. Lett. 51, (2024). Wessel, P. et al. The Generic Mapping Tools Version 6. Geochem., Geophys., Geosystems 20, 5556–5564 (2019). Additional Declarations There is NO Competing Interest. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4555753","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":334928733,"identity":"0fc9c8bf-1134-46be-aca7-8a064ad01f78","order_by":0,"name":"Siyuan Zhang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxklEQVRIiWNgGAWjYFACHhDBVt8P4TETrYWPcWYDiVrkGDccIFaLfP/Zg58LfpkxG99If/iBocI6sYGQFoMD55KlZ/alsZndyDGWYDiTToQWxh4Dad6eYzxALWwMjG2HCWuRb+Yx/s3b81/CeEb6MwbGf0RoYQCaL83zg83AQCLBjIGxgQgtBmd4zKx5G9gSJM68MZZIOJZuTNhh/WeMb/P8YUvgbweG2Icaa1nCDgMBxjYoI4Eo5WDwh3ilo2AUjIJRMAIBAGQ4OsF19+w5AAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-4607-3649","institution":"University of Southern California","correspondingAuthor":true,"prefix":"","firstName":"Siyuan","middleName":"","lastName":"Zhang","suffix":""},{"id":334928734,"identity":"7961132d-df7c-4490-a8a7-004582b9b696","order_by":1,"name":"Heidi Houston","email":"","orcid":"https://orcid.org/0000-0001-8533-4760","institution":"University of Washingto","correspondingAuthor":false,"prefix":"","firstName":"Heidi","middleName":"","lastName":"Houston","suffix":""},{"id":334928735,"identity":"700a0820-ae88-4c30-a2a1-dd0b57325637","order_by":2,"name":"Binhao Wang","email":"","orcid":"","institution":"University of Southern California","correspondingAuthor":false,"prefix":"","firstName":"Binhao","middleName":"","lastName":"Wang","suffix":""},{"id":334928736,"identity":"73bdd031-3f4a-4dfc-834b-444c2aa31a57","order_by":3,"name":"Hao Zhang","email":"","orcid":"","institution":"University of Southern California","correspondingAuthor":false,"prefix":"","firstName":"Hao","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2024-06-10 04:45:06","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4555753/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4555753/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":61736780,"identity":"505344f5-581e-468a-9491-b76333d5ff2c","added_by":"auto","created_at":"2024-08-05 03:33:58","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":781678,"visible":true,"origin":"","legend":"\u003cp\u003eFocal mechanisms in the Ridgecrest region with quality A or B. The left panel shows earthquakes before the M6.4 foreshock, and the right panel is after the M7.1 mainshock, excluding earthquakes closer than 2 km to the fault. Focal mechanisms are color-coded by their stress regimes (ref. 28). SS: strike-slip, NF: normal faulting, NS: primarily normal with a strike-slip component, TF: thrust faulting, TS: primarily thrust with a strike-slip component. Brown lines show fault traces from the latest community fault model (CFM6.1; ref. 29). Black dashed lines are the boundaries of our manually-picked blocks, with their indices inside. The epicenters of the doublet are shown by red stars.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4555753/v1/7f973c69b5d08cc54505e969.jpeg"},{"id":61737145,"identity":"722d919c-c99e-4ae7-ab63-a1f29013c289","added_by":"auto","created_at":"2024-08-05 03:41:58","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":281948,"visible":true,"origin":"","legend":"\u003cp\u003eChange of stress orientations in different blocks. The red and blue symbols show P- and T-axes, respectively from the focal mechanisms used for each stress inversion. All axes are plotted in a lower-hemisphere projection (zenithal equal-area projection). Numbers at the upper right corner of each hemisphere indicate the RMS angular difference on top and misfit angle below (both in degrees).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4555753/v1/de4aca54415a02b3cafe4c12.png"},{"id":61736430,"identity":"ebf1097c-520b-4129-9642-cd5d9695cdba","added_by":"auto","created_at":"2024-08-05 03:25:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1628057,"visible":true,"origin":"","legend":"\u003cp\u003eCoulomb stress change at a depth of 6 km using two slip models (friction coefficient 0.4). Orientation of the receiver fault is shown in the upper-right corner. (a) Coulomb stress change calculated using the slip model from ref. 19, shown in (b). Cyan crosses represent the sub-fault centers. Green diamonds signify the earthquake locations used for inverting the uniform stress orientation prior to slip, whereas post-event focal mechanisms are color-coded according to their depths. (c) Same as (a) but calculated using the slip model from ref. 20, shown in (d).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4555753/v1/2e6dcd214f1a3d3e32a59444.png"},{"id":61736426,"identity":"bab69ff0-a504-476d-8f5a-373d2cc8f8e1","added_by":"auto","created_at":"2024-08-05 03:25:58","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":170708,"visible":true,"origin":"","legend":"\u003cp\u003eInverted maximum shear stresses in 12 manually-picked blocks and the corresponding shape ratios. The upper three panels exhibit results directly from the stress orientation inversion. (a) Shape ratios before and after the 2019 Ridgecrest doublet. (b) RMS angular differences between the focal mechanisms and the inverted stress orientation for each block, which quantifies focal mechanism diversity. (c) Misfit angles between the observed and predicted slip directions of the focal mechanisms. (d–f) Inverted maximum shear stresses using different coseismic stress modes and slip models. (g) (inset) Increase in maximum shear stress plotted in a map view, using the same colorbar as in Fig. 5f.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4555753/v1/cc06885cdbe27f53689cfaae.png"},{"id":61736427,"identity":"546c3ccc-f589-4d24-8cf2-5a016ff3d18f","added_by":"auto","created_at":"2024-08-05 03:25:58","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":681514,"visible":true,"origin":"","legend":"\u003cp\u003eShape ratios and maximum shear stresses inverted by gridding the study region into small “pixels”. The coseismic stress change is calculated using the slip model from ref. 19 and the “mean” stress mode. The centers of the sub-faults are depicted by black crosses. The color bars are truncated for better visibility.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4555753/v1/20f2045688a908f38ce1362b.png"},{"id":77141352,"identity":"4768d08a-a653-4f5f-a454-31c0f77c5903","added_by":"auto","created_at":"2025-02-25 13:43:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4133294,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4555753/v1/d8759a2b-6d93-45e8-b0e7-6995f17604df.pdf"},{"id":61736431,"identity":"25773d37-f49f-42a1-b843-b221a28ae0f6","added_by":"auto","created_at":"2024-08-05 03:25:58","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3200145,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Supplementfirstsubmission.docx","url":"https://assets-eu.researchsquare.com/files/rs-4555753/v1/44f2fe6354c4e6d670fc1629.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Mapping of absolute stresses around two California earthquakes reveals a very weak crust","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eThe strength of the crust has been a long-standing question in seismology, tectonics, and geodynamics. The seismogenic crust is almost entirely inaccessible to direct measurements of stress and strength. Analysis of earthquake focal mechanisms provides the primary method of gauging in-situ stress conditions and suites of them can be inverted to yield spatially-averaged normalized stress orientations (e.g., refs. 1\u0026ndash;2). The misalignment of those orientations with a major fault has been used to constrain friction on the fault (e.g., refs. 3\u0026ndash;6). Changes in those orientations due to large earthquakes have been used in a two-dimensional analytic framework to constrain the fraction of pre-existing shear stress relieved by the mainshock stress drop (e.g., refs. 7\u0026ndash;9). Ref. 10 proposed a three-dimensional framework, which can further yield the absolute deviatoric stress tensors before and after a mainshock given adequate focal mechanisms and mainshock slip distribution. If the amplitude of stress before an earthquake has a similar order of magnitude to the coseismic stress drop, the stress orientations rotate by an amount depending on the geometrical relationships. Studies have observed stress rotations indicating low background stress amplitudes and weak faults (e.g., refs. 8\u0026ndash;11).\u003c/p\u003e \u003cp\u003eA Mw6.4 strike-slip earthquake ruptured an unmapped fault system in Ridgecrest on 4 July 2019, followed by a Mw7.1 strike-slip earthquake 34 hours later. These two earthquakes and their aftershocks have been well-documented and widely studied for multiple purposes. Resolvable rotations of maximum horizontal stress have been observed by comparing stress orientations before and after the M7.1 (refs. 12\u0026ndash;15), suggesting that coseismic stress drop relieved much of the background stress. Ref. 13 conducted stress inversions on five blocks that cross the two major faults. They adopted the 2-D simplification proposed by ref. 8 and inferred that the maximum shear stress was ~\u0026thinsp;10 MPa near the junction of the two major faults before the M7.1. Ref. 15 inferred a maximum shear stress of 5 MPa following the approach of ref. 13 but using more aftershocks and their own slip model. Ref. 16 interpreted a stress rotation obtained by comparing orientations derived from surface mainshock slip measurements (surmised to represent the pre-mainshock stress state) with orientations from aftershocks at depth, and inferred similar magnitudes of stress along the main faults. These studies rely on the two-dimensional analytic simplification (ref. 8), which does not account for rotations around a non-vertical axis that are observed. They also focus on the aftershock zone close to the mainshock, which is highly complex, subject to mainshock slip heterogeneity, and likely anisotropic.\u003c/p\u003e \u003cp\u003eHere, we utilize a three-dimensional framework (as in refs. 10 \u0026amp; 17) to analyze stress rotations and infer full deviatoric stress tensors, using two slip models and multiple approaches to define subregions and allow for different length scales of stress variations. Our analysis inverts for stresses both near and well away from the main faults. The blocks we selected were not allowed to cross the major faults, as we observed differences across the faults. We are able to capture robust features, reinforced by the consistent results from our different inversion approaches and use of only higher-quality focal mechanisms. In addition to resolving spatial stress concentrations before the mainshocks and a pattern of shape ratio change consistent with the mainshock slip, we find exceedingly low deviatoric stresses even off the main faults indicating a very weak crust.\u003c/p\u003e"},{"header":"2 Stress orientations before and after the 2019 Ridgecrest doublet","content":"\u003cp\u003eWe first inverted focal mechanisms for the stress orientations before the M6.4 foreshock (referred to as pre-event) and those after the M7.1 mainshock (referred to as post-event) using the package developed by ref. 2 following the method of ref. 1 (see Methods). We divided the study area into 12 blocks based on the distributions of slip and of seismicity (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Each block was further divided into the periods before and after the 2019 Ridgecrest doublet, excluding the M6.4 and M7.1. The 50 best focal mechanisms in each of the 24 spatiotemporal blocks were selected based on their uncertainties (Supplementary Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), as suggested by ref 11. There is a tradeoff between focal mechanism quality and the number utilized for inversion (see Methods). Events in block 12 were required to occur at least two months after the M7.1 to exclude numerous normal mechanisms related to a more local process of extensional topographic collapse. To stabilize the inversion result, we treated the 12 pre-event blocks as an ensemble with the same stress conditions. This is a reasonable assumption given the absence of a large earthquake during the period, stabilizing those blocks that contain only a few pre-event focal mechanisms. The uniform pre-event stress orientation was determined by inverting 60 events that comprise the 5 best focal mechanisms from each block (Supplementary Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eChanges in the stress orientations before and after the mainshocks are apparent in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, which shows P- and T-axes and inverted stress orientations for the 12 blocks. The uniform pre-event stress orientation is consistent with the focal mechanisms of the mainshocks, with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{1}\\)\u003c/span\u003e\u003c/span\u003e striking\u0026thinsp;~\u0026thinsp;45\u0026deg;from the high-slip portion of the M7.1 fault. Large rotations after the earthquake doublet occur in blocks 2, 7, 11, and 12, with Kagan angles (ref. 18) exceeding 30\u0026ordm; (Supplementary Table S2). The coseismic stress induced by the mainshock is likely to have released a substantial portion of the pre-event stresses in blocks 2 and 7, altering an almost pure strike-slip regime to one with more normal faulting. In block 11, the stress orientation primarily rotates around \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{2}\\)\u003c/span\u003e\u003c/span\u003e and may be affected by a shift in epicentral locations, Block 12 may have experienced a large decrease in its compressional stress. In block 12, as well as block 7, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{1}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{2}\\)\u003c/span\u003e\u003c/span\u003e have a clear trade-off (Supplementary Fig. S3), resulting from the comparable numbers of strike-slip and normal faulting events. The areas surrounding the northwest and southeast rupture termini (blocks 1, 3, 5, 8, and 9) underwent moderate stress rotations, with Kagan angles ranging from 10\u0026ordm; to 25\u0026ordm; (Supplementary Table S2). Blocks 4, 6 and 10 exhibit nearly identical stress orientations before and after the mainshocks, with Kagan angles less than 10\u0026ordm; (Supplementary Table S2). Overall, we observed significant stress rotations in most blocks, indicative of comparable levels of pre-event and coseismic stresses (ref. 11). In addition, shape ratios (see Methods) are generated as part of the current step as well, subsequent to obtaining the stress orientation tensors, and will be discussed along with the stress amplitudes in later sections.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"3 Coseismic stress change induced by the 2019 Ridgecrest doublet","content":"\u003cp\u003eWe calculated the coseismic stress change from published slip distributions (refs. 19\u0026ndash;20) based on the analytical static stress solution derived by ref. 21 (see Methods). The two slip models we utilized are consistent in terms of the slip pattern of the M7.1 mainshock (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb \u0026amp; \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). Ref. 19 associated the slip of the M6.4 foreshock mostly with the southwest-trending fault, whereas ref. 20 preferred the northwest-trending main fault. The latter result also has a slip patch isolated from the mainshock, and located to the southeast of the junction between the two largest orthogonal faults (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). The lobes of the Coulomb stress changes on faults parallel to the M7.1 are similar except immediately adjacent to the fault (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea \u0026amp; \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). The right-lateral component was released by the slip around the M7.1 mainshock epicenter and accumulated at the two rupture termini.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe distribution of the focal mechanisms used in various blocks is also shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. We employed different strategies to capture representative coseismic stress change tensors for each block (\u0026ldquo;mean\u0026rdquo; and \u0026ldquo;centroid\u0026rdquo;; see Methods). Although the \u0026ldquo;mean\u0026rdquo; mode results in slightly smaller coseismic stress changes, this reduction does not substantially change the relative magnitudes among the different blocks (Supplementary Fig. S5).\u003c/p\u003e"},{"header":"4 Change in maximum shear stresses before and after the 2019 Ridgecrest doublet","content":"\u003cp\u003eWe combined the stress orientations before and after the doublet with the coseismic stress change to invert for the absolute deviatoric stresses and reconstruct the full pre-event and post-event deviatoric stress tensors (see Methods). Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the resulting inverted maximum shear stresses on any plane, obtained from the pre- and post-event deviatoric stress tensors. We observe a consistent pattern of stress increase or decrease across the blocks (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed\u0026ndash;e). The changes in blocks 2\u0026ndash;3, 5\u0026ndash;8, and 11\u0026ndash;12 are statistically significant (Supplementary Fig. S6). Remarkably, the two slip models yield similar patterns of stress changes (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed \u0026amp; \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). Inverting for different pre-event stress orientations in the 12 blocks gives the same pattern but more variable stress levels (Supplementary Fig. S7d). Such consistency indicates that the different methods of calculating the coseismic stress in each block do not affect the pattern of increase or decrease in maximum shear stress. Employing a stress mode with higher coseismic stress magnitudes amplifies maximum shear stresses (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed\u0026ndash;e \u0026amp; Supplementary Fig. S5). Opting for the slip model of ref. 20 yields somewhat reduced stress levels (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef), because the orientations as well as magnitudes of the coseismic stress tensors differ significantly in the near-fault blocks (\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e) for the two slip models.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs an alternate approach and a check on the 12-block regionalization and joint inversion, we partitioned the study area into small \u0026ldquo;pixels\u0026rdquo; at 0.01\u0026ordm; intervals in latitude and longitude (see Methods). The pattern of stress changes resulting from the pixel inversion (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef) remains stable, even when using the slip model of ref. 20, as depicted in Supplementary Fig. S8f. This inversion approach, which leverages more focal mechanisms and allows for pre-event variation, does introduce greater uncertainties; however, it significantly concurs with the results of inverting the 12 blocks (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg \u0026amp; Supplementary Fig. S9). A pronounced stress reduction is evident on both sides of the southwest-trending fault (blocks 5 \u0026amp; 6), correlating with high slip from the M6.4 foreshock according to ref. 19. Adjacent to the M7.1 mainshock epicenter, blocks 2 and 7 display similar patterns of stress decrease to that of blocks 5 and 6. The zero post-event stresses in blocks 2 and 6 reach the bound of the non-negative inversion.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDifferent responses are observed in blocks 3 and 4, at the southeast section of the main fault. The slip model from ref. 19 suggests less slip during the mainshock around the junction of two orthogonal faults (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This slip pattern induces an increase in right-lateral shear stress in the northern half of block 4, which is further enhanced by the foreshock (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Therefore, the inversion for 12 blocks (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed) indicates an extensive stress decrease in block 3, whereas little change in maximum shear stress occurred in block 4 perhaps due to the smoothing based on the focal mechanism distribution (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e); the contrast between blocks 3 and 4 is consistent with the details from pixel inversion (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef). These different impacts on the two sides argue against defining blocks that cross the fault when investigating earthquakes with complex slip patterns or fault geometry.\u003c/p\u003e \u003cp\u003eAt the northwest terminus of the mainshock, there is an apparent stress increase over almost all of block 8 and the portion of block 1 proximal to the fault (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed \u0026amp; \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef). The southeast terminus of the main fault behaves similarly to load the Garlock fault (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef), although not revealed by the inversion for 12 blocks. Since the addressed pre-event and post-event stress levels are contingent upon the amplitude of the coseismic stress through inversion (Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; see Methods), and the rotation of stress orientations is difficult to resolve in areas less affected by the earthquake, blocks 9\u0026ndash;12 are either roughly unchanged or moderately reduced in stress.\u003c/p\u003e"},{"header":"5 Discussion","content":"\u003cp\u003eThe change in shape ratio revealed by the pixel inversion (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec) is consistent with the mechanics of strike-slip faulting. The overall pattern of shape ratios in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e delineates a four-quadrant pattern, which is bisected by the two major orthogonal faults. Only small differences appear under the assumption of a uniform pre-event stress orientation in each grid (Supplementary Fig. S10c \u0026amp; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec), demonstrating that this pattern is a manifestation of the coseismic stress change. Typically, a double-couple earthquake produces compressions and dilatations across four quadrants as its focal mechanism indicates, which is expected to be reflected in the shape ratio. The western part of our study region, subject to the compression of both the foreshock and mainshock, exhibits the most significant increase in shape ratio (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec) due to its proximity to the substantial slip. Conversely, the decrease in shape ratio in the north quadrant indicates enhanced tensional deviatoric stress. The two quadrants to the southeast generally conform to the expected pattern but exhibit more heterogeneity, less affected by the mainshock slip. This observation further supports that the order of magnitude of the pre-event stress is comparable to that of the coseismic stress change, as otherwise the pre-event compressional or tensional influence would dominate the post-event shape ratio.\u003c/p\u003e \u003cp\u003eAlthough rotations of stress orientation have been observed in various earthquakes, both continental and in subduction zones, negative results and skepticism regarding uncertainties have been noted by some researchers, as reviewed by ref. 11. To address these concerns, we have thoroughly compared different strategies at each stage as previously discussed and summarize those results here. For inverting the stress orientations, we selected only high-quality focal mechanisms (A or B) from the original catalog (refs. 22\u0026ndash;23). The 12-block inversion further picked the best 50 focal mechanisms with the least uncertainty in each spatiotemporal bin (Supplementary Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Allowing pre-event stress orientations to differ for the 12 blocks prior to the Ridgecrest doublet (Supplementary Fig. S4) yielded remarkably similar results to our preferred method of assuming the same pre-event stress orientation for all 12 blocks (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), an approach that improves stability and minimizes uncertainty. We considered two different modes of extracting the static coseismic stress and two different slip models to calculate the representative coseismic stress change in each of the 12 blocks, revealing a consistent pattern of maximum shear stress change (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed\u0026ndash;f). Solutions that incorporate the uncertainty of focal mechanisms demonstrated the same pattern in a statistical way (Supplementary Fig. S6). The 12-block inversion can be assessed as the most stable approach in this study. The other endmember, the pixel inversion, fully utilizes information from more focal mechanisms and allows higher spatial variation, which introduces more uncertainty. Furthermore, the different slip models show similar stress change patterns (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef \u0026amp; Supplementary Fig. S8f). Practical issues prevent the construction of a combined matrix to guarantee a unique uniform pre-event stress tensor, but we were able to test a uniform pre-event stress orientation at every pixel, using the one from the 12-block inversion (Supplementary Fig. S10), which produces results very similar to Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Along with another test in which the 12-block inversion allows pre-event regional variations (Supplementary Fig. S7), these two approaches balance both stability and variability. All four inversions produce largely consistent spatial patterns of stress increase or decrease, lending credibility to our results.\u003c/p\u003e \u003cp\u003eThe variability of post-event stress orientations was calculated to be 20.37\u0026ordm; for 12-block inversion (see Methods), which contrasts with the pre-event variability of 11.45\u0026ordm; if each block is inverted individually (Supplementary Fig. S4). For the pixel inversion, the variability increases from 19.65\u0026ordm; to 31.82\u0026ordm;. This difference highlights the importance of the Ridgecrest doublet in increasing stress heterogeneity regionally, providing some support to the assumption of a more uniform stress state pre-event.\u003c/p\u003e \u003cp\u003eThe pre-event maximum shear stress displays a pattern in which the amplitude decreases with increasing distance from the fault (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed \u0026amp; Supplementary Fig. S7), deviating from our assumption of a uniform pre-event stress. It could be a concern that the stress amplitude generally correlates with the magnitude of coseismic stress changes, which could be an artifact. However, the stress rotations in blocks 9\u0026ndash;12 (Supplementary Table S2) are significant when compared to their small coseismic stress changes (Supplementary Fig. S5), resulting in their low-stress levels. Additionally, a large portion of block 7 has low pre-event stress amplitudes, despite its substantial coseismic stress change. The stress rotations in areas away from the fault may partially arise from the stress heterogeneity, rather than predominantly influenced by the coseismic stress close to the fault. Other factors like undetected creep that could alter the stress, would rarely play a role on the same time scale but remains possible. The spectrum of our inversion approaches covers various extents of pre-event variations, accounting for those unknown effects. Some studies have also observed back rotation to the pre-event stress orientation within a few months or a few years (refs. 11 \u0026amp; 17). Due to the Omori-type decay of aftershocks, the focal mechanisms we utilized to constrain the post-event stress orientation were primarily from the immediate aftershocks. Nevertheless, we find no apparent trends of back rotations in any of the 12 blocks (Supplementary Fig. S11).\u003c/p\u003e \u003cp\u003eIn this study, the maximum shear stresses are all below 10 MPa, as low as ~\u0026thinsp;2 MPa in the results from the 12-block inversion, which is consistent with previous results (refs. 13, \u0026amp; 15\u0026ndash;16). Some previous studies on stress rotations indicated low stress amplitudes on the order of the earthquake stress drops (refs. 7\u0026ndash;10), suggesting a weak crust (ref. 11). Considering that the gravity of crustal rocks imparts a compressional normal stress of about 27 MPa per kilometer of depth, such low shear stress values at depths of ~\u0026thinsp;5\u0026ndash;10 km imply an extraordinarily low friction coefficient and/or extremely high fluid pressure. The peak value of the pixel inversion is ~\u0026thinsp;9.5 MPa, 7.5% of the frictional strength from Byerlee\u0026rsquo;s law at a depth of 8 km (assuming a friction coefficient of 0.6, a rock density of 2700 kg/m\u003csup\u003e3\u003c/sup\u003e, and no cohesion; ref. 24), and the common value of ~\u0026thinsp;2 MPa is only 1.6% of it. Ref. 25 documented numerous instances of orthogonal faulting within the Ridgecrest fault zone, which is evidence of a near-zero friction coefficient based on the Coulomb failure criterion. While extremely high fluid pressure may account for low shear stresses in subduction zones, as suggested by ref. 17, a combination of a low friction coefficient and elevated fluid pressure could be responsible for the low maximum shear stresses in the continental crust. Ref. 26 inferred a near-lithostatic fluid pressure below 5 km at the two termini of the coseismic rupture of the M7.1 by inverting fault frictional properties using an afterslip model. It is also worth noting that the maximum shear stresses revealed by this method are only sampling those structures that hold earthquakes, rather than indicating the stress conditions in the middle of intact rock. Therefore, fluid pressure close to lithostatic could be present in fault gouges depending on the permeability, even if the tectonic environment is not as fluid-saturated as in subducting slabs.\u003c/p\u003e \u003cp\u003ePrior to the Ridgecrest doublet, the pixel inversion using the slip model from ref. 19 reveals distinct stress concentrations (\u0026gt;\u0026thinsp;7 MPa) proximate to the high-slip patches of the M6.4 foreshock (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). The density of nearby focal mechanisms is sufficient to resolve this in-situ feature. Near the epicenter of the M7.1 mainshock, the maximum shear stress is elevated as well, but to 3\u0026ndash;4 MPa which is not as high as that on the southwest-trending fault. This finding may explain why the southwest-trending rupture preceded and presumably loaded the M7.1 segment. A similar relationship was inferred for the 2016 Kumamoto earthquake and its large foreshock by investigating the stress rotations caused by both (ref. 9).\u003c/p\u003e \u003cp\u003eWe have demonstrated a comprehensive framework from focal mechanisms to full deviatoric stress tensors, which can be applied to regions with well-resolved coseismic slip models. Using the best but enough focal mechanisms helps optimize the uncertainty from each focal mechanism, which can be quantitatively leveraged with our new noisy solutions. Calculating the static coseismic stress change and employing the 3-D relationship remove the necessity to estimate the average stress drop needed to employ the less accurate analytic 2-D relationship. A spectrum of approaches with various pre-event assumptions allows for exploring details progressively. The four-quadrant pattern of the change in shape ratio and the stress concentrations before the doublet illustrate the potential of this technique to reveal detailed features.\u003c/p\u003e \u003cp\u003eThe very low deviatoric stresses before and after the mainshocks indicate both weak faults and a weak crust even away from the primary fault zone. This supports the view that the seismogenic portions of the crust do not provide substantial lithospheric strength, which is likely much greater in the mid-crust (ref. 27).\u003c/p\u003e "},{"header":"Methods","content":"\u003cp\u003e \u003cem\u003eInversion for stress orientations\u003c/em\u003e \u003c/p\u003e \u003cp\u003eWe employed the package developed by ref. 2, which uses the stress inversion of ref. 1. This method minimizes the difference between the direction of shear stress imposed by a normalized stress tensor on a nodal plane and the actual slip on that plane. The deviatoric part of the average background stress orientation can be retrieved given a sufficient number of focal mechanisms that rupture under a consistent stress condition. Under the further assumption that the amplitude of shear stress at failure is the same for all events, the inversion is linear. The shape ratio is defined as\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$R=\\frac{{\\sigma }_{1}-{\\sigma }_{2}}{{\\sigma }_{1}-{\\sigma }_{3}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{1}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{3}\\)\u003c/span\u003e\u003c/span\u003e represent the most compressional and tensional axes, respectively. Ref. 2 introduced the notion of fault instability in order to assess which of the two nodal planes is more likely to fail in a given stress state (comprising the stress orientations and shape ratio). This leads to higher accuracy of the shape ratio.\u003c/p\u003e \u003cp\u003eFocal mechanisms were taken from a refined catalog using the HASH method (refs. 22\u0026ndash;23; extended), including earthquakes from 1981 to 2022 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Only high-quality events (quality A or B) were retained. Events within 2 km of the finite-fault model (ref. 19) after the M7.1 mainshock were excluded (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) because near the fault the finite-fault models generate highly heterogeneous coseismic stress changes where neighboring sub-faults adjoin. Thus, most of the very-close-in aftershocks of the doublet were excluded (Fig.\u0026nbsp;8 in ref. 12). The uncertainty of each focal mechanism was utilized to generate noisy solutions for assessing the results (Supplementary Text S1).\u003c/p\u003e \u003cp\u003eAccording to synthetic tests (ref. 2), ~\u0026thinsp;50 focal mechaisms provide adequate accuracies in both stress orientation and shape ratio which do not improve much upon including more focal mechanisms. A uniform number of focal mechanisms in each block leads to more comparable uncertainties in inverted stress orientations.\u003c/p\u003e \u003cp\u003eIn addition to the aforementioned assumptions required to establish the inverse problem, it is desirable that the focal mechanisms be adequately diverse but also consistent with rupture in one stress field (refs. 1, 8, \u0026amp; 13). The diversity of focal mechanisms can be quantified by the RMS angular difference between the focal mechanisms and a \u0026ldquo;mean focal mechanism\u0026rdquo; (refs. 8 \u0026amp; 13). We adopt the inverted stress orientation as the \u0026ldquo;mean focal mechanism\u0026rdquo;, and measure the angular difference by the Kagan angle (ref. 18). The calculated RMS angular differences are labeled with each stress orientation (upper right corners in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), mostly satisfying the diversity requirement (greater than ~\u0026thinsp;35\u0026ndash;40\u0026ordm; if the focal mechanism errors are ~\u0026thinsp;10\u0026ndash;20\u0026ordm;, according to ref. 8).\u003c/p\u003e \u003cp\u003eThe assumption of a consistent stress field can be assessed by the misfit angle, which is the median difference between the observed slip vectors and the ones predicted by the inverted stress orientation. Ideally, diverse and noise-free focal mechanisms would look like two \u0026ldquo;butterfly wings\u0026rdquo; of P- or T-axes around \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{1}\\)\u003c/span\u003e\u003c/span\u003e or \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma }_{3}\\)\u003c/span\u003e\u003c/span\u003e orientations, respectively (ref. 2). The deviation from that pattern reflects the misfit angle (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Our 12 blocks all satisfy the consistent-stress-field assumption (less than ~\u0026thinsp;35\u0026ndash;40\u0026ordm; misfit angle if the focal mechanism errors are ~\u0026thinsp;10\u0026ndash;20\u0026ordm;, according to ref. 30).\u003c/p\u003e \u003cp\u003e \u003cem\u003eComputing representative coseismic stress change for each block\u003c/em\u003e \u003c/p\u003e \u003cp\u003eWe assumed 30GPa for both Lam\u0026eacute; parameters to compute the coseismic stress changes, based on the analytical static stress solution in a homogeneous elastic half space derived by ref. 21. The trace of the calculated coseismic stress change tensor was then removed.\u003c/p\u003e \u003cp\u003eThe following inversion requires one representative coseismic stress change tensor for each block. We employed three different modes to capture it: 1) Following ref. 10, we averaged coseismic stress changes at all post-event hypocenters within a block, referred to as the \u0026ldquo;mean\u0026rdquo; coseismic stress mode; 2) We adopted the coseismic stress change at the centroid of post-event hypocenters as the \u0026ldquo;centroid\u0026rdquo; coseismic stress mode; 3) In the \u0026ldquo;pixel inversion\u0026rdquo;, we adopted the in-situ coseismic stress change at each pixel.\u003c/p\u003e \u003cp\u003eThe \u0026ldquo;mean\u0026rdquo; mode results in slightly smaller coseismic stress changes than the \u0026ldquo;centroid\u0026rdquo; mode (Supplementary Fig. S5) when averaging tensors. The \u0026ldquo;centroid\u0026rdquo; and the in-situ modes would sample the near-fault heterogeneous coseismic stresses, whereas the \u0026ldquo;mean\u0026rdquo; mode would not, due to the removal of the near-fault seismicity. Thus, we opted to use only the \u0026ldquo;mean\u0026rdquo; mode in our systematic pixel inversion.\u003c/p\u003e \u003cp\u003e \u003cem\u003eInversion for full deviatoric stress tensors before and after the doublet\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe stress orientations before and after an earthquake, coupled with the coseismic stress change, can be used to estimate the absolute deviatoric stresses, enabling the reconstruction of the full pre-event and post-event deviatoric stress tensors at seismogenic depths (refs. 10\u0026ndash;11). Analytic methods, such as those by refs. 7\u0026ndash;8, and inversion approaches (refs 10 \u0026amp; 32) have been introduced in the literature, as reviewed by ref. 11. The analytic methods simplify the problem into a 2-D model, relating the rotation of the stress orientation to the ratio of stress drop to the maximum shear stress (ref. 8) or the shear stress on the fault (ref. 7). Their assumptions about the coseismic stress change have been substantiated for simple strike-slip faults (ref. 32); however, they are not valid in off-fault areas. The inversion approach used here (following refs. 10 \u0026amp; 17) accounts for the full 3D coseismic stress tensors and is capable of addressing complexities in off-fault areas. While the initial application (ref. 10) still divided the fault into several cross-fault segments, our blocks avoid extending over both sides of the fault to eliminate the singularities and uncertainties of coseismic stress changes derived from unsmoothed slip models.\u003c/p\u003e \u003cp\u003eSince we treated the 12 blocks as an ensemble prior to the Ridgecrest doublet to enhance stability, the inversion approach from ref. 10 was adapted as follows:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\left[\\begin{array}{c}\\varDelta {\\varvec{S}}_{1}\\\\ \\begin{array}{c}\\varDelta {\\varvec{S}}_{2}\\\\ \\dots \\end{array}\\\\ \\varDelta {\\varvec{S}}_{12}\\end{array}\\right]=\\left[\\begin{array}{ccc}{\\varvec{S}}_{1}^{after}\u0026amp; \\cdots \u0026amp; 0\\\\ ⋮\u0026amp; \\ddots \u0026amp; ⋮\\\\ 0\u0026amp; \\cdots \u0026amp; {\\varvec{S}}_{12}^{after}\\end{array}\\begin{array}{c}{-\\varvec{S}}^{before}\\\\ ⋮\\\\ {-\\varvec{S}}^{before}\\end{array}\\right]\\left[\\begin{array}{c}{C}_{1}^{after}\\\\ \\begin{array}{c}{C}_{2}^{after}\\\\ \\dots \\end{array}\\\\ \\begin{array}{c}{C}_{12}^{after}\\\\ {C}^{before}\\end{array}\\end{array}\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\varDelta {\\varvec{S}}_{i}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\varvec{S}}_{1}^{after}\\)\u003c/span\u003e\u003c/span\u003e are the representative deviatoric coseismic stress change and the normalized post-event stress tensor in block \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(i\\)\u003c/span\u003e\u003c/span\u003e, respectively. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\varvec{S}}^{before}\\)\u003c/span\u003e\u003c/span\u003e is the normalized pre-event uniform stress tensor for all 12 blocks. Each of these stress tensors comprises six components arranged vertically. Although only five components of a deviatoric stress tensor are independent, we have constructed the matrix with six components to equally weight the misfits among them as in ref. 17. After re-normalizing the stress orientation tensors by their eigenvalue ranges, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({C}_{i}^{after}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({C}^{before}\\)\u003c/span\u003e\u003c/span\u003e represent the differential stresses (twice the maximum shear stresses) in post-event block \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(i\\)\u003c/span\u003e\u003c/span\u003e and for the pre-event unified 12 blocks, respectively. To solve Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, we utilized the non-negative least-square function LSQNONNEG, implemented in MATLAB.\u003c/p\u003e \u003cp\u003e \u003cem\u003ePixel inversion\u003c/em\u003e \u003c/p\u003e \u003cp\u003eWe applied a systematic inversion by partitioning the study area into small \u0026ldquo;pixels\u0026rdquo; at 0.01\u0026ordm; intervals in latitude and longitude. For each pixel before and after the Ridgecrest doublet, we utilized the 45 focal mechanisms before and after the doublet (picked from the quality A or B mechanisms in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) closest to the pixel to constrain its stress orientations. Near the fault, the coseismic stress changes appear highly heterogeneous, primarily due to the unsmoothed slip as well as the bends between fault segments. As such, we opted to use only the \u0026ldquo;mean\u0026rdquo; mode in our systematic inversion. Given the substantial size of the combined matrix (Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) of 91\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\times\\)\u003c/span\u003e\u003c/span\u003e101 pixels, amounting to 55146\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\times\\)\u003c/span\u003e\u003c/span\u003e9192 elements, it is practically challenging to perform the non-negative inversion of Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) on such a scale. Therefore, we inverted for each pixel individually. Thus, here the pre-event shape ratio (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea) and stress (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed) can vary across the study region.\u003c/p\u003e \u003cp\u003e \u003cem\u003eQuantifying the variability of stress orientations\u003c/em\u003e \u003c/p\u003e \u003cp\u003eTo quantify the variability of post-event stress orientations, we first determined their centroid tensor using the MATLAB function KMEANS. We represented each stress orientation as a vector comprising 9 components and employed the \u0026lsquo;cosine\u0026rsquo; distance metric within KMEANS. We computed the mean Kagan angle between the centroid and each post-event stress orientation to quantify the variability of stress orientations. This procedure reveals increased stress heterogeneity following the doublet for both regionalization approaches.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eData Availability\u003c/h2\u003e \u003cp\u003eThe focal mechanisms used in this study can be found at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://scedc.caltech.edu/data/alt-2011-yang-hauksson-shearer.html\u003c/span\u003e\u003cspan address=\"https://scedc.caltech.edu/data/alt-2011-yang-hauksson-shearer.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. The inverted full deviatoric stress tensors from both block and pixel inversions have been uploaded to \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.5281/zenodo.11475511\u003c/span\u003e\u003cspan address=\"10.5281/zenodo.11475511\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe are grateful to John Vidale from University of Southern California for invaluable suggestions on constructing this manuscript. Figures were produced using Generic Mapping Tools 6 (ref. 33).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMichael, A. J. Determination of stress from slip data: Faults and folds. J. Geophys. Res.: Solid Earth 89, 11517\u0026ndash;11526 (1984).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVavryčuk, V. Iterative joint inversion for stress and fault orientations from focal mechanisms. Geophys. J. Int. 199, 69\u0026ndash;77 (2014).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSibson, R. H. A note on fault reactivation. J. Struct. Geol. 7, 751\u0026ndash;754 (1985).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLachenbruch, A. H. \u0026amp; McGarr, A. Stress and heat flow. in \u003cem\u003eThe San Andreas Fault System, California\u003c/em\u003e 261\u0026ndash;277 (U.S. Geological Survey, California, United States, 1990). doi:\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3133/pp1515\u003c/span\u003e\u003cspan address=\"10.3133/pp1515\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eProvost, A. \u0026amp; Houston, H. Orientation of the stress field surrounding the creeping section of the San Andreas Fault: Evidence for a narrow mechanically weak fault zone. J. Geophys. Res.: Solid Earth 106, 11373\u0026ndash;11386 (2001).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eProvost, A. \u0026amp; Houston, H. Stress orientations in northern and central California: Evidence for the evolution of frictional strength along the San Andreas plate boundary system. J. Geophys. Res.: Solid Earth 108, (2003).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYin, Z.-M. \u0026amp; Rogers, G. C. Rotation of the principal stress directions due to earthquake faulting and its seismological implications. Bull. Seism. Soc. Am. 85, 1513\u0026ndash;1517 (1995).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHardebeck, J. L. \u0026amp; Hauksson, E. Crustal stress field in southern California and its implications for fault mechanics. J. Geophys. Res.: Solid Earth 106, 21859\u0026ndash;21882 (2001).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYoshida, K. \u003cem\u003eet al.\u003c/em\u003e Stress rotations due to the M6.5 foreshock and M7.3 main shock in the 2016 Kumamoto, SW Japan, earthquake sequence. Geophys. Res. Lett. 43, 10,097\u0026thinsp;\u0026ndash;\u0026thinsp;10,104 (2016).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWesson, R. L. \u0026amp; Boyd, O. S. Stress before and after the 2002 Denali fault earthquake. Geophys. Res. Lett. 34, (2007).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHardebeck, J. L. \u0026amp; Okada, T. Temporal stress changes caused by earthquakes: A review. J. Geophys. Res.: Solid Earth 123, 1350\u0026ndash;1365 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHauksson, E. \u0026amp; Jones, L. M. Seismicity, Stress State, and Style of Faulting of the Ridgecrest-Coso Region from the 1930s to 2019: Seismotectonics of an Evolving Plate Boundary Segment. Bull. Seism. Soc. Am. (2020) doi:\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1785/0120200051\u003c/span\u003e\u003cspan address=\"10.1785/0120200051\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSheng, S. \u0026amp; Meng, L. Stress field variation during the 2019 Ridgecrest earthquake sequence. Geophys. Res. Lett. 47, (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, X. \u0026amp; Zhan, Z. Seismotectonics and fault geometries of the 2019 Ridgecrest sequence: Insight from aftershock moment tensor catalog using 3-D Green\u0026rsquo;s functions. J. Geophys. Res.: Solid Earth 125, (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDuan, H. \u003cem\u003eet al.\u003c/em\u003e Analysis of coseismic slip distributions and stress variations of the 2019 Mw 6.4 and 7.1 earthquakes in Ridgecrest, California. Tectonophysics 831, 229343 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMilliner, C. W. D., Aati, S. \u0026amp; Avouac, J. P. Fault friction derived from fault bend influence on coseismic slip during the 2019 Ridgecrest Mw 7.1 mainshock. J. Geophys. Res.: Solid Earth 127, (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDelbridge, B. G., Houston, H., B\u0026uuml;rgmann, R., Kita, S. \u0026amp; Asano, Y. A weak subducting slab at intermediate depths below northeast Japan. Sci. Adv. 10, eadh2106 (2024).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKagan, Y. Y. Simplified algorithms for calculating double-couple rotation. Geophys. J. Int. 171, 411\u0026ndash;418 (2007).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXu, X. \u003cem\u003eet al.\u003c/em\u003e Surface deformation associated with fractures near the 2019 Ridgecrest earthquake sequence. Science 370, 605\u0026ndash;608 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYue, H. \u003cem\u003eet al.\u003c/em\u003e The 2019 Ridgecrest, California earthquake sequence: Evolution of seismic and aseismic slip on an orthogonal fault system. Earth Planet. Sci. Lett. 570, 117066 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOkada, Y. Internal deformation due to shear and tensile faults in a half-space. Bull. Seism. Soc. Am. 82, 1018\u0026ndash;1040 (1992).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang, W., Hauksson, E. \u0026amp; Shearer, P. M. Computing a Large Refined Catalog of Focal Mechanisms for Southern California (1981\u0026ndash;2010): Temporal Stability of the Style of Faulting. Bull. Seism. Soc. Am. 102, 1179\u0026ndash;1194 (2012).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHauksson, E., Yang, W. \u0026amp; Shearer, P. M. Waveform Relocated Earthquake Catalog for Southern California (1981\u0026ndash;2011). Bull. Seism. Soc. Am. 102, 2239\u0026ndash;2244 (2012).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eByerlee, J. Friction of rocks. pure Appl. Geophys. 116, 615\u0026ndash;626 (1978).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRoss, Z. E. \u003cem\u003eet al.\u003c/em\u003e Hierarchical interlocked orthogonal faulting in the 2019 Ridgecrest earthquake sequence. Science 366, 346\u0026ndash;351 (2019).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhao, Z. \u0026amp; Yue, H. A two-step inversion for fault frictional properties using a temporally varying afterslip model and its application to the 2019 Ridgecrest earthquake. Earth Planet. Sci. Lett. 602, 117932 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBehr, W. M. \u0026amp; Platt, J. P. Brittle faults are weak, yet the ductile middle crust is strong: Implications for lithospheric mechanics. Geophys. Res. Lett. 41, 8067\u0026ndash;8075 (2014).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZoback, M. L. First- and second-order patterns of stress in the lithosphere: The World Stress Map Project. J. Geophys. Res.: Solid Earth 97, 11703\u0026ndash;11728 (1992).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eEvans, W. S. \u003cem\u003eet al.\u003c/em\u003e A Statistical Method for Associating Earthquakes with Their Source Faults in Southern California. Bull. Seism. Soc. Am. 110, 213\u0026ndash;225 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMichael, A. J. Spatial variations in stress within the 1987 Whittier Narrows, California, aftershock sequence: New techniques and results. J. Geophys. Res.: Solid Earth 96, 6303\u0026ndash;6319 (1991).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang, Y., Johnson, K. M. \u0026amp; Chuang, R. Y. Inversion for absolute deviatoric crustal stress using focal mechanisms and coseismic stress changes: The 2011 M9 Tohoku-oki, Japan, earthquake. J. Geophys. Res.: Solid Earth 118, 5516\u0026ndash;5529 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHardebeck, J. L. Earth\u0026rsquo;s free surface complicates inference of absolute stress from earthquake-induced stress rotations. Geophys. Res. Lett. 51, (2024).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWessel, P. \u003cem\u003eet al.\u003c/em\u003e The Generic Mapping Tools Version 6. Geochem., Geophys., Geosystems 20, 5556\u0026ndash;5564 (2019).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4555753/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4555753/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAbsolute amplitudes of shear stresses that drive crustal earthquakes are not well known. There is a long-standing divergence between the values inferred from lab experiments and stress changes during faulting. Two large earthquakes near Ridgecrest, California with M6.4 and 7.1 provide a natural laboratory to determine the in-situ average shear stress in the crust off the main faults. Here we use the change in faulting geometries of abundant small earthquakes together with stress changes imposed by doublet slip to determine full deviatoric stress tensors both before and after it. We first invert suites of focal mechanisms for stress orientations and ratios between eigenvalues. We then invert for the 3-D full deviatoric tensors constrained by the stress orientations, stress ratios, and the coseismic stress change due to the doublet. We applied this method using two doublet slip models and two endmember approaches: first dividing the region into 12 blocks surrounding the mainshock faults, and second performing 9,200 separate inversions offset by ~\u0026thinsp;1 km. To obtain reliable results, we use the 3-D relationship rather than a common 2-D strike-slip simplification, define inversion regions that do not cross the main faults, and include only high-quality events a few km away from the main faults to avoid large heterogeneities in the co-seismic stress change. Deviatoric stresses are only a few percent of levels expected at seismogenic depths from Byerlee friction, except for regions near the doublet hypocenters where they are up to only\u0026thinsp;~\u0026thinsp;7.5%. Our approach yields strong evidence for a very weak continental crust, which bears on earthquake and geodynamic modeling, as well as earthquake recurrence behavior and hazard, suggesting near-complete stress drops in the mainshock doublet and a low chance of imminent large slips there.\u003c/p\u003e","manuscriptTitle":"Mapping of absolute stresses around two California earthquakes reveals a very weak crust","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-08-05 03:25:53","doi":"10.21203/rs.3.rs-4555753/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"2f3a2586-4294-4719-b374-86138942a3b5","owner":[],"postedDate":"August 5th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":35472141,"name":"Earth and environmental sciences/Solid Earth sciences/Tectonics"},{"id":35472142,"name":"Earth and environmental sciences/Solid Earth sciences/Seismology"}],"tags":[],"updatedAt":"2025-03-11T18:55:12+00:00","versionOfRecord":[],"versionCreatedAt":"2024-08-05 03:25:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4555753","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4555753","identity":"rs-4555753","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}