Multiscale off-fault brecciation records coseismic energy budget of principal fault zone

Multiscale off-fault brecciation records coseismic energy budget of principal fault zone | 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 Multiscale off-fault brecciation records coseismic energy budget of principal fault zone Geri Agroli, Atsushi Okamoto, Masaoki Uno, Noriyoshi Tsuchiya This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3952437/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 8 You are reading this latest preprint version Abstract Breccia and pulverized rock are typical textures in off-fault damage adjacent to a main seismogenic zone. Previously, by estimating the energy required to advance the rupture in this zone using particle size distribution at sub-millimeter/micrometer scales, we could constrain the energy budget during coseismic events. However, whether microscopic estimation is sufficient to capture surface energy fragmentation during an earthquake and the effect of measurement scale variation on calculation of co-seismic energy partitioning remained unclear. Here, we investigated the mechanism of coseismic off-fault damage based on field and microstructural observations of a well-exposed breccia body in Ichinokawa, Japan. We used in situ clast measurements coupled with thin-section analysis of breccia clasts to estimate the energy budget of the damage zone adjacent to the principal slip zone of the median tectonic line. The total surface energy density and corresponding surface energy per unit fault for a width of ~ 500 m of the dynamical damage zone were estimated. The moment magnitude estimated based on surface energy was 5.8–8.3 Mw. In Ichinokawa, off-fault fragmentation is initiated by coseismic activity and is followed by fluid activity. Under dynamic fragmentation conditions, the scale is important to calculate the surface energy. Earth and environmental sciences/Solid earth sciences/Geology/Structural geology Earth and environmental sciences/Solid earth sciences/Petrology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction The damage zone in the vicinity of a principal fault indicates the abrupt change in the stress regime. A transient change in natural permeability leads to fluid infiltration into the damage zone over a period 1–3 . Pseudotachylite is considered a marker of earthquake slip phenomena and is caused by the frictional melting between two rock bodies in a fault zone 4,5 . The formation of pseudotachylite is exclusively driven by high slip rates of 1–0.1 m/s for coseismic and interseismic events, respectively 6,7 . The number of studies on the relationship between seismicity and faults is increasing, and currently, fast rupture is considered generated under extreme stress conditions 4 . The signatures of fast rupture include rock pulverization 8 and periodic injection of fluids, which can be observed in relatively soft rocks such as carbonates 9–11 . When an earthquake occurs, the released elastic energy is translated into ground shaking ( U rad ), frictional heat ( U fh ), and increase in the area of surface energy ( U sa ) 12–14 . The surface energy is a wave that propagates away from the earthquake source and is responsible for initiation and advancement of rupture in the off-fault region 15–17 . Recent research suggests that surface energy is not suitable for calculation of the overall energy balance of the dynamics of earthquake rupture 12,18,19 . Surrounding active and mature continental strike-slip faults, the surface energy is manifested as well-preserved pervasive fragmentation of rocks that occur within the off-fault region (tens to hundreds of meters in scale) as pulverized rocks 20–27 . Pulverized rocks require high to modest strain rates that are achieved through single or successive loading of the intact rock 8,28–32 . This loading represents the pulse of the surface energy during the seismic event. Fragmented rocks at the field and thin-section scales can be analyzed to reconstruct the dynamics of the energy budget experienced by them 33,34 . We performed a multidisciplinary study of pulverized pelitic schists exposed in the off-fault region of the median tectonic line (MTL) in Ichinokawa, Japan. Based on the texture and structure of breccia combined with particle size distribution (PSD) at various observation scales, we redefine the dynamics of the brecciation mechanism and its relationship with the MTL 35 . We performed calculations of surface energy intensities based on the size of breccia fragments at the outcrop and thin-section scales. Our findings reveal a new constraint on the earthquake energy budget and rupture dynamics in the off-fault region of the MTL 12,36 . Our analyses reveal unique fragmentation behavior (pulverization), earthquake energy estimation, fluid circulation, and mineralization in the off-fault region. Therefore, Ichinokawa can serve as a reference site for the study of such phenomena. Results and Discussion Off fault pulverized rock associated with principal slip zone The Sanbagawa belt is a Cretaceous high-pressure metamorphosed accretionary complex consisting mainly of pelitic, psammitic, and mafic schists with minor quantities of metachert, expressing the subduction of marine sediment deposited upon the basaltic oceanic crust 37 . Across Shikoku Island, the Sanbagawa belt and the young accretionary complex in the north are separated by the largest arc-parallel tectonic fault of the MTL. Right-lateral slip displacement, recorded in the latest history, contributes to the deformation of adjacent rock in terms of brittle failure including brecciation 38,39 . The Ichinokawa breccia body is located in central Shikoku approximately 3 km south of Saijo City within the off-fault region of the MTL. The lenticularly restricted breccia body is perpendicular to the MTL with a dimension of 200 × 400 m consisting of low-grade Sanbagawa pelitic schist. It is surrounded by intact pelitic schist with lesser occurrences of psammitic schist. In the north, the rock body is juxtaposed with the Upper Cretaceous sediment formation of Izumi Group bounded by the ENE–WSW trending MTL (Fig. 1 a and Supplementary Fig. 1a). The distance between the MTL and breccia is 50–650 m, representing the width of the dynamics of the damage zone (Supplementary Fig. 1b). The strike of the breccia and clast elongation are subparallel to the MTL (Supplementary Fig. 1c); however, in another outcrop, in situ measurement of the strike of the fault-filling breccia shows that it is perpendicular to the MTL (Supplementary Fig. 1d), suggesting that these developed under the dynamics of rupturing, and the MTL contributes significantly to the brecciation processes (Supplementary Fig. 1e). The pervasive brecciation is well exposed near the Senga-ko adit (Fig. 1 b) and is classified into two types. The breccia-1 (bx-1) is clast supported breccia and is composed solely of pelitic schist clast with a size of up to 5 m (monomict; Fig. 1 c, d). The pelitic schist has typical mineral assemblages of muscovite and quartz with EW sub-horizontal lineation of the Sanbagawa belt (Fig. 1 e, f). Clasts also show little or no rotational block as well as dilatational shear deformation with well-preserved metamorphic lineation. Clasts are mostly angular and exhibit a crackle texture with non-systematically oriented micro- to macrofractures separating the clast. The fine microfracture cuts the lineation and mineral vein filled with dolomite with a lower quantity of quartz grains, which indicates the initial matrix component soon after the beginning of fragmentation (Fig. 1 g, h). The alteration around the clast is absent, indicating that the comminution is mainly governed by mechanical processes. We suggest that these features of bx-1 correspond with the coseismic fragmentation, as it is in the proximity of the principal slip zone in the order of hundreds of meters. The intricate deformation history of the MTL 38,39 , combined with a significant seismic event of magnitude 7–8 40–42 that occurred prior to the formation of Ichinokawa Formation, may have triggered the coseismic fragmentation of the Sanbagawa pelitic schist. Moreover, the asymmetrical damage encompassed bimaterial is prone to generating rock pulverization 22,43 . Ichinokawa Formation juxtaposes with Izumi Formation on the northern side. The wide perturbation zone in this formation is characterized by the development of boudinage along the MTL, and it is opposed to Ichinokawa Formation, where the disturbed zone is very narrow and local, resembling the pulverized outcrop along San Andreas Fault 33,34 . Because Ichinokawa Formation comprises a stiffer rock (schist) with fragmentation characteristics, the Ichinokawa breccia is more likely to occur because of rock pulverization. Fluidization and signature of decarbonation during seismic slip The cohesive pulverized rock observed in Ichinokawa, unlike the typical incohesive pulverized rocks studied so far 24 , can be explained by the presence of breccia-2 (bx-2), which occupies the same outcrop as bx-1 and occurs as an injection-like breccia with various breccia pipe widths ranging from 5 cm to 1 m (Fig. 2 a, b). The bx-2 is matrix-supported with polymict clast consisting of pelitic schist and a metamorphic mineral-bearing vein that is derived from bx-1 (Fig. 1 e). The clast is sub-rounded to sub-angular with an average size of 10–20 cm at outcrop and thin-section scales, with a lower quantity of sub-millimeter particle size observed in backscatter detector (BSE) images (Fig. 2 c). The clast is intensely rotated and shows a fluidized texture, indicating that it experienced in situ transport phenomena and was most likely subjected to subsequent fluid-driven fragmentation. The matrix consists of muscovite, quartz, dolomite, and several sulfide minerals such as pyrite, arsenopyrite, and stibnite/sulfosalt, which make up 59, 19, 18, and 9%, respectively (Fig. 2 d, e). Dolomite constitutes up to 40% (Supplementary Fig. 1f) of the matrix, with two observed precipitation modes: 1) the dolomite encloses another component similar to that of a cockade with distinct oscillatory zoning (Fig. 2 f–i), and 2) it forms uniformly along with another matrix component (Fig. 2 j). The dolomite exhibits compositional zoning, with the core having higher magnesium and calcium but lower iron, whereas the rim has a higher iron content, giving rise to the reddish matrix color (Fig. 2 k and Supplementary Fig. 1g, h). The significant quantity of dolomite and carbonic fluid inclusions indicates the existence of a CO 2 -rich fluid, even though the source of the fluid remains unknown (Supplementary Text). The abundance of this fluid along the MTL is manifested as a calcite/dolomite vein within Sanbagawa 44,45 , dolomite-bearing stibnite deposits, and a modern CO 2 hot spring, which indicates CO 2 activity derived from hydrothermal sources 46,47 (Supplementary Fig. 1i). Fluid inclusions in the quartz–stibnite vein contain a low concentration of CO 2 , suggesting that the initial fluid has an elevated CO 2 concentration. The flow texture coupled with the subparallel orientation of the matrix component was observed in the BSE image (Fig. 2 c). In addition, the quartz matrix is distinct from the quartz clast based on the cathodoluminescence (CL) intensity. The quart matrix shows high (CL-bright) and low (CL-dark) CL intensities in the quartz clast, strongly suggesting that fluidization processes dictate the formation of bx-2 through mineral precipitation from the fluid or some degree of reaction occurred between the rising fluid and pelitic schist (Fig. 2 e). The microtexture of sulfide minerals, such as zoning, also supports the notion that the fluidization was caused by pressure fluctuation and change in the fluid-flow regime during coseismic and interseismic events (Supplementary Fig. 2) 48,49 . Particle size distribution and dynamics of off-fault brecciation PSD analysis is a powerful tool to study fragmented rock or other materials 50,51 . The dimensionless D value or fractal dimension represents various mechanisms of clast fragmentation and size reduction (comminution) as mentioned by various authors 52–54 . Particularly in the brecciated system, the D value is a function of energy input applied to the rock during breccia formation 55 , and it has high and low values for high and low energy input processes, respectively. However, the fundamental process of both characterizations are that one emphasizes fault-related fragmentation, whereas the other indicates processes related to hydrothermal systems. Therefore, high and low D values depend on the process. The integrated PSD in Ichinokawa (Fig. 3 ) follows a power law distribution with a slope of 1.65 for the clast spanning 0.3–3.5 mm for the thin section and 20–100 mm for the outcrop (indicated as the gray area). The clast size varies between 5–500 mm, and this wide range of clast sizes represents bx-1 and bx-2 at different observation scales. Under thin section, the particle density has two major trends due to the difference in sampling locations, wherein the high particle density corresponds to bx-1 (Fig. 1 c). The discrepancies in particle density at the outcrop scale is a function of the width of the breccia pipe. According to the density plot, the clast sizes at the outcrop and thin-section scales have mean clast diameters of 20.4 and 0.483 mm, respectively, with modal values of the distribution curves of 0.25 and 12.6 mm for the thin-section and outcrop, respectively (Fig. 3 a). In addition, the distribution of individual samples shows a relatively higher fractal dimension (D value) with more than one size distribution gradient. In the outcrop scale, bx-1 has the highest D value of 3.5, probably due to less variation in the clast size, as the rock was subjected to initial fragmentation (Fig. 3 b and Supplementary Fig. 3a). The bx-2 has a fractal dimension of 1.2–2.9, with a positive correlation with the size of the breccia pipe. The distribution of clasts at the thin-section scale is relatively less heterogeneous, with the fractal dimension of 1.8–2.3 (Fig. 3 c and Supplementary Fig. 3b). In Ichinokawa, the D value is 1.2–3.5 at both the outcrop and thin-section scales (Fig. 3 d). Most of the D values coincide with the fractal dimension of natural and experimental fault gouges 23,56,57 . The distribution of fragments of bx-1 and bx-2 with the widest apertures correspond with high D values, surpassing the theoretical boundary of shear comminution processes 58 , which can be attained by extensive fragmentation and impact loading such as pervasive fracturing and rock pulverization 23,30,53,54 . The aperture of the conduit also contributes to the high-energy input during the dynamics of brecciation 55,59 . A fractal dimension of < 1.68 implies that fragmentation proceeds to the next step with a lower energy input associated with propagation, mechanical/chemical wearing, dilatation, and fragmentation caused by fluids, as observed in Ichinokawa 35,52,55 . The brecciation observed in Ichinokawa is intriguing because it is caused by in situ hydro-fracturing without the rotation of large clasts and fault-filling material consisting of finely crushed rock 35 . We propose a model for the development of breccia based on the dynamics of fracturing in relation to the main tectonic fault and hydrothermal activity (Fig. 4 ) based on our novel investigation on the internal structure of breccia coupled with multiscale PSD analysis. Elastic energy and surface energy (U sa ) that are radiated by the earthquake nucleation (red star) along the MTL are responsible for the advancement of the rupture 16,19 . The bimaterial that is juxtaposed between Sanbagawa Formation and Izumi Group causes off-fault fracturing primarily in the stiffer rock, that is, pelitic schist 60,61 . Subshear rupture and pulverization propagate forward as the recurrence of the earthquake releases elastic energy, and these processes occur recursively 62 (Fig. 4 a, b). These processes caused the formation of bx-1, as rock pulverization occurred close to the MTL and dolomite filled microfractures during the initial brecciation (Fig. 4 b). The formation of bx-1 induces the fracture permeability of pelitic schist leading to an episodic hydrothermal fluid flow to form bx-2 as a breccia dike 9,42 (Fig. 4 c). Fluid pressure may be building up to some extent underneath Ichinokawa, even though the intrusion-driven hydrothermal activity remains uncertain 63 . Energy budget on off-fault damage and an estimation of earthquake magnitude through the multi-scale of breccia Breccia represent phenomena associated with paleoearthquakes indicating deformation and reflect the mechanical processes that occur during coseismic events 11,64,65 . The elastic energy that accumulates and propagates during and after the slip event is translated into surface energy, which advances the rupture of the intact rock 12,19 . We next estimated the energy budget based on off-fault damage using a D value of > 1.68 to perform calculations as the low fractal dimension might represent the low energy fragmentation, as explained in the previous section. Therefore, we assumed that fragmentation occurs in a single system (scale invariance). In addition to using individual breccia data, we also used integrated PSD data (Fig. 3 a). Using those values and the approach described by Johnson et al. 12 , we obtained the area-averaged particle size (L) for the outcrop at 24.30–8.73 mm and for the thin section at 0.65–2.55 mm. Assuming that the specific surface area of pelitic schist is equal to that of granite at approximately 56 J/m 2 66–68 and using surface correction for the most natural gouge of 6.6 27 , we calculated the total surface fracture energy density ( U s ) at 2.25–9.12 × 10 4 J/m 3 and 8.69 × 10 5 to 3.43 × 10 6 J/m 3 for outcrop and thin sections, respectively. The scale-integrated data showed that the surface area and surface energy density are in the range of the energy indicated by individual breccia. It is evident that the fragmentation continues from the macro to micro scales, and the particle size dictates the energy consumption during the fragmentation (Fig. 5 a and Supplementary Table 1). The estimated values of dissipated energy in Ichinokawa are comparable to those of several natural and experimental occurrences of off-fault fragmentation or pulverization. The energy level is apparent at a thin-section scale, as the smallest aggregates are subjected to intense fragmentation processes at high strain rates 12,28,29,69 . The total energy density is distributed across the ~ 500 m (according to outcrop distance with respect to the MTL) wide dynamic damage zone to propagate the rupture through the off-fault region. Integrating the energy density ( U s ) over the damage-zone width in Ichinokawa resulted in the total fracture energy per unit fault area ( U sa ) of 1.12–4.56 × 10 7 J/m 2 for the outcrop and 4.34 × 10 8 to 1.72 × 10 9 J/m 2 for the thin section. The pulverization accumulates the stress of every individual coseismic slip 69 . Thus, the estimation of surface energy for a single earthquake reflects the dynamic loading that contributes to the fragmentation 70 . The total displacement of the principal fault can be used to estimate the number of earthquake events 18,20,27 . However, the estimation of earthquake recurrence based on the slip displacement cannot be deployed in Ichinokawa. Therefore, we used the dolomite matrix as a proxy to obtain the number of earthquake events. Assuming dolomite cementation with apparent oscillatory zoning occurs during the coseismic event, we found five to eight zonings that reflect the minimum earthquake recurrences (Fig. 2 f–i and Supplementary Fig. 1g). The estimation of surface energy released by a single earthquake in Ichinokawa is 1.12–172 MJ/m 2 (Fig. 5 b and Supplementary Table 1). An earlier study suggests that the pulverized rock is a distinct type of rock fragmentation produced by intense loading. Therefore, we estimated the number of loadings subjected onto the intact rock to produce the finest breccia clast in Ichinokawa from the biggest fragment based on the dynamics loading experiment 71 . We obtained approximately 100 loadings required to generate the clast size; in this case, the calculated U sa is 0.11–17.2 MJ/m 2 (Fig. 5 c and Supplementary Table 1). The scale integration estimated the surface energy span at 2.08–73.13 and 0.21–7.31 MJ/m 2 for 10 and 100 earthquake recurrences, respectively. This comparison shows that the energy budget is lower by four-fold compared with the average surface energy obtained from individual breccia data. This also influences the earthquake magnitude estimation. These results are similar to those of other estimates of energy release for single earthquakes compiled by Johnson et al., 2021 12 (Supplementary Fig. 4b). Our energy estimation at the outcrop scale corresponds to those of large displacement faults such as Chelungpu, Punchbowl, and San Andreas Faults 18,20,72 , suggesting that the surface energy in Ichinokawa is stored across the wide damage zone of the MTL, and its significant quantity suggests that it is a non-negligible component for the advancement of the rupture near the principal fault zone. At the thin-section scale, it could be overestimated (by two orders of difference) or it probably records the highest energy dissipation from the MTL. Additionally, a width of 500 m of the dynamical damage zone corresponds to a depth of ~ 5 km. The corresponding depth was confirmed by estimating the pressure based on the isochore analysis of fluid inclusions at 30–190 MPa (Supplementary Fig. 5h). At this depth, approximately 40% of the fracture energy is dissipated in the off-fault medium, and the rest of the energy is directed into the fault 19 . We estimated the ranges of moment magnitude at 5.8–6.9 and 7.1–8.3 based on the records at the outcrop and thin-section scales, respectively (Fig. 5 d). We attempted to integrate the fragmentation of breccia with the moment magnitude of an earthquake, which has a far reaching scope compared with that of usual studies. However, it is necessary to consider many mechanical and physical parameters including the type of converted energy and data derived from numerical and kinematic models, which over or underestimate results. In the future, we can accumulate a significant amount of data to generate a robust energy–seismic moment model, using the breccia records of paleoseismic activity present adjacent to the principal fault. Methods Electron microprobe analysis The mineral compositions of the samples were determined using an electron microprobe analyzer (EPMA; JEOL JXA-8200) at Tohoku University, Japan. X-ray mapping using EPMA was performed with acceleration voltage, beam current, and dwell time of 15 kV, 120 nA, and 60 ms, respectively. The beam diameter was 1 µm. Natural and synthetic standards (wollastonite for Ca and Si, rutile for Ti, eskolaite for Cr, hematite for Fe, manganosite for Mn, periclase for Mg, albite for Na, feldspar for K, halite for Cl and F) were used for calibration. The counting times for the peaks and backgrounds of all major elements were 10 and 5 s, respectively, whereas for the minor elements Cl and F they were 30 and 15 s, respectively. Scanning electron microscopy coupled with cathodoluminescence The internal quartz structure was characterized using a scanning electron microscope equipped with an Oxford cathodoluminescence detector and photomultiplier (SEM-CL) at the Graduate School of Science, Tohoku University, Japan with an accelerating voltage of 25 kV and a beam current of 90 µA. The CL image was analyzed as described by Frelinger et al. and Rusk and Reed 73,74 . Fluid Inclusion microthermometry Fluid inclusion microthermometry was performed on double-polished thick sections (~ 100-µm thickness) of the Sb-bearing vein. Microthermometry was performed using a Linkam THS600 heating/freezing with an operating temperature of − 180–600°C and a measurement error of 0.1°C at the Tohoku University, Japan. Salinities of fluid inclusion in NaCl wt.% equivalent were estimated using the final melting temperatures of ice 75,76 . Raman Raman spectroscopic analysis was applied to identify the phases of fluid inclusion from the Sb-bearing vein using a HORIBA XploRA PLUS Confocal Raman Microscope at the Graduate School of Environmental Studies, Tohoku University. The laser beam was positioned using a built-in Olympus BX optical polarization microscope. Excitation was done at a solid-state wavelength of 532 nm. Spectra were registered using a charge-coupled device detector cooled to − 70°C using a Peltier element. The analysis was carried out in a backscattered geometry. The scattered light was focused using a 100× magnification lens. Spectra were registered at 100–4,200 cm − 1 . The spectrometer was calibrated using a pure silicon line (520.7 cm − 1 ) and an intrinsic laser line (0 cm − 1 ). PSD at macro- and microscales PSD was used to understand the characteristics and formation mechanism of breccia in Ichinokawa. Scanline sampling was deployed both for direct field and rock slab/thin-section measurements to characterize the matrix-to-clast ratio (ε) and the distribution of clast within the breccia. The one-dimensional scanline was conducted to obtain the geometry of the breccia or clast parameter primarily in the long (L) and short (S) axes of the clast/particle for breccia-2 (bx-2; Supplementary Fig. 3c, d). The PSD measurement is a function of the width of breccia pipe. For the rock slab/thin-section, we initially traced the image that fits the sampling line, and then, obtained the clast parameters using ImageJ (Supplementary Fig. 3e, f) 77 . The clast size is provided by the square root of L and S of each particle and it is expressed as \(d={\left(LS\right)}^{1/2}\) . The cumulative probability was calculated from the PSD 56 and then plotted in the log(N) − log(d) diagram. The distributions were fitted using the power law equation as follows: $$N\propto {d}^{-Ds}$$ 1 where d and N denote the particle size (µm) and the cumulative probability of particles > d, respectively, and Ds is the fractal dimension 78,79 . Furthermore, the multiscale particle size integration was performed by normalizing the clast distribution with the measurement area (mm 2 ) to obtain the particle density. Calculation of surface energy and Earthquake magnitude For the estimation of the surface energy using natural fragment size data, we transformed the mean diameter into the area-averaged fragment size. The versatile derivation of this problem was performed according to Johnson et al. 12 by considering the surface area per unit volume of a fragment consisting of n i spheres of diameter s i for bin i as follows: $$\left(\frac{A}{V}\right)=\frac{\varSigma {n}_{i}\left[4\pi {\left(\frac{{s}_{i}}{2}\right)}^{2}\right]}{\varSigma {n}_{i}\left[\frac{4}{3}\pi {\left(\frac{{s}_{i}}{2}\right)}^{3}\right]}=\frac{6\varSigma {n}_{i}{s}_{i}^{2}}{\varSigma {n}_{i}{s}_{i}^{3}}=\frac{6}{{\stackrel{\prime }{s}}_{avg}}=\frac{6}{L}$$ 2 For surface area (L) the Error! Reference source not found. can be rewritten as follows: $$L=\frac{\varSigma {n}_{i}{s}_{i}^{3}}{\varSigma {n}_{i}{s}_{i}^{2}}$$ 3 The count of the fraction for the power-law cumulative distribution of a population of spheres ( n ) can be expressed as follows: $$n\left(s\right)={kDs}^{-D-1}$$ 4 where s is the diameter of the fragment, D is the fractal dimension, and k is the constant. By integrating Error! Reference source not found. s 2 and 4, we can obtain: $$L=\frac{\varSigma {n}_{i}{s}_{i}^{3}}{\varSigma {n}_{i}{s}_{i}^{2}}=\frac{{\int }_{{s}_{min}}^{{s}_{max}}n\left(s\right){s}^{3}ds}{{\int }_{{s}_{min}}^{{s}_{max}}n\left(s\right){s}^{2}ds}=\frac{{\int }_{{s}_{min}}^{{s}_{max}}{kDs}^{-D+2}ds}{{\int }_{{s}_{min}}^{{s}_{max}}{kDs}^{-D+1}ds}$$ 5 Thus, the solution is: $$L=\left(\frac{2-D}{3-D}\right)\left[\frac{{s}_{max}^{\left(3-D\right)}-{s}_{min}^{\left(3-D\right)}}{{s}_{max}^{\left(2-D\right)}-{s}_{min}^{\left(2-D\right)}}\right]$$ 6 The area-averaged fragment size ( L ) as a function of diameter ( s ) and fractal dimension ( D ) allows us to estimate the surface energy density ( U s ; J/m 3 ) using the following equation: $$Us=\frac{6\gamma \lambda }{L}$$ 7 where γ is specific surface energy of the material (J/m 2 ) and λ represents the surface-area correction factor because the breccia fragment is not a perfect sphere or cube. Using the converted energy value from the numerical and kinematic slip model coupled with the seismic moment ( M o ) allows us to generate a simple model of the relationship between energy and seismic moment (Supplementary Fig. 5c, d). Assuming that the energy generated during the earthquake is converted into surface-area energy ( U sa ), we fitted the average energy value of two earthquake recurrences in Ichinokawa (10 and 100 EQ) to get the seismic moment. Subsequently the moment magnitude ( M w ) was estimated using the equation provided by Kanamori and Brodsky 36 as follows: $$Mw=\frac{log\left(Mo\right)}{1.5}-6.07$$ 8 Declarations Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author Contribution G.A, A.O., M.U., and N.T. design the research and carried out fieldwork. G.A. carried out petrological, fluid inclusion analyses, and numerical modelling. All authors contributed to the data interpretation. G.A. prepared the draft of the manuscript. All authors were involved in revising the manuscript and approved the submitted version. Acknowledgements We thank Jun Muto and Mitsuhiro Toriumi for fruitful discussion. Alexey Kotov and Bayarbold Manzhir for assisting during the fieldwork. Diana Mindaleva for drone image and fieldwork assistance. Data Availability The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request. Additional information Supplementary information the online version contains supplementary material available at References Sibson, R. H. Rupture Interaction with Fault Jogs. in Earthquake Source Mechanics 157–167 (American Geophysical Union (AGU), 1986). doi: 10.1029/GM037p0157 . Caine, J. S., Evans, J. P. & Forster, C. B. 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Fracture toughness of schist, amphibolite, and rhyolite from the Sanford Underground Research Facility (SURF), Lead, South Dakota. Sci. Rep. 12, 15941 (2022). Scholz, C. H. The Mechanics of Earthquakes and Faulting . (Cambridge University Press, 2019). doi: 10.1017/9781316681473 . Zhang, Z.-X. & Ouchterlony, F. Energy Requirement for Rock Breakage in Laboratory Experiments and Engineering Operations: A Review. Rock Mech. Rock Eng. 55, 629–667 (2022). Yao, W., Xu, Y. & Xia, K. Damage Evolution During Rock Pulverization Induced by Dynamic Compressive Loading. J. Geophys. Res. Solid Earth 125, e2020JB019388 (2020). Doan, M. L. & D’Hour, V. Effect of initial damage on rock pulverization along faults. J. Struct. Geol. 45, 113–124 (2012). Smith, Z. D. & Griffith, W. A. Evolution of Pulverized Fault Zone Rocks by Dynamic Tensile Loading During Successive Earthquakes. Geophys. Res. Lett. 49, e2022GL099971 (2022). Rockwell, T. et al. Chemical and Physical Characteristics of Pulverized Tejon Lookout Granite Adjacent to the San Andreas and Garlock Faults: Implications for Earthquake Physics. Pure Appl. Geophys. 2009 16610 166, 1725–1746 (2009). Frelinger, S. N., Ledvina, M. D., Kyle, J. R. & Zhao, D. Scanning electron microscopy cathodoluminescence of quartz: Principles, techniques and applications in ore geology. Ore Geol. Rev. 65, 840–852 (2015). Rusk, B. & Reed, M. Scanning electron microscope-cathodoluminescence analysis of quartz reveals complex growth histories in veins from the Butte porphyry copper deposit, Montana. Geology 30, 727–730 (2002). Bodnar, R. J. Introduction to Fluid Inclusions. Fluid Incl.-Anal. Interpret. 32, 81–99 (2003). Roedder, E. Fluid inclusion studies of ore deposits in the Viburnum Trend, Southeast Missouri. Econ. Geol. 72, 474–479 (1977). Schneider, C. A., Rasband, W. S. & Eliceiri, K. W. NIH Image to ImageJ: 25 years of image analysis. Nat. Methods 9, 671–675 (2012). Turcotte, D. L. Fractals in geology: what are they and what are they good for? GSA Today 1, 1–4 (1991). Turcotte, L. Fractals and Fragmentation. N. Y. 91, 1921–1926 (1986). Lanari, P. et al. XMapTools: A MATLAB©-based program for electron microprobe X-ray image processing and geothermobarometry. Comput. Geosci. 62, 227–240 (2014). Additional Declarations No competing interests reported. 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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-3952437","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":275126590,"identity":"d724a1fd-a4fc-4736-9cdb-b0df3af3d05a","order_by":0,"name":"Geri Agroli","email":"","orcid":"","institution":"Tohoku University","correspondingAuthor":false,"prefix":"","firstName":"Geri","middleName":"","lastName":"Agroli","suffix":""},{"id":275126591,"identity":"f5a93164-bcac-4cf7-acab-37f6a9fdfc7d","order_by":1,"name":"Atsushi Okamoto","email":"","orcid":"","institution":"Tohoku University","correspondingAuthor":false,"prefix":"","firstName":"Atsushi","middleName":"","lastName":"Okamoto","suffix":""},{"id":275126592,"identity":"0ad71532-f108-42e3-9b12-b0ada9a0c274","order_by":2,"name":"Masaoki Uno","email":"","orcid":"","institution":"Tohoku University","correspondingAuthor":false,"prefix":"","firstName":"Masaoki","middleName":"","lastName":"Uno","suffix":""},{"id":275126593,"identity":"ec99c6e9-42c2-4518-bf21-d0c132cbcc86","order_by":3,"name":"Noriyoshi Tsuchiya","email":"data:image/png;base64,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","orcid":"","institution":"Tohoku University","correspondingAuthor":true,"prefix":"","firstName":"Noriyoshi","middleName":"","lastName":"Tsuchiya","suffix":""}],"badges":[],"createdAt":"2024-02-13 02:24:45","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3952437/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3952437/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":51802736,"identity":"f7b83d4d-54db-43d8-88b2-9f6e86fb54c0","added_by":"auto","created_at":"2024-02-29 09:44:20","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2558720,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRepresentative field occurrences of breccia-1 in Ichinokawa.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Geological maps of the Shikoku and Ichinokawa areas where the apparent relationship between the MTL and Ichinokawa breccia in terms of distances are observed. \u003cstrong\u003eb\u003c/strong\u003e The well-known Senga-ko adit; the entrance to the main stibnite body, including the famous giant stibnite preserve in the Saijo City museum. \u003cstrong\u003ec\u003c/strong\u003e Crackle breccia with minimum clast-rotation; breccia-1 (bx-1). \u003cstrong\u003ed \u003c/strong\u003eAerial photograph and schematic illustration showing the relationship between bx-1 and breccia-2 (bx-2), where the pervasive bx-2 intrudes the mega clast of bx-1 with the respective mineral vein (qtz) parallel to schistosity and microfracture filled with dolomite. \u003cstrong\u003ee\u003c/strong\u003e Hand specimen of low-grade Sanbagawa pelitic schist and \u003cstrong\u003ef\u003c/strong\u003e its typical mineral assemblages that become a clast component of Ichinokawa breccia. \u003cstrong\u003eg\u003c/strong\u003e Mode I microfracture in pelitic schist. \u003cstrong\u003eh\u003c/strong\u003e The fracture filled by dolomite and a lower quantity of quartz observed under backscatter detector image, which indicates the earliest generation of the matrix after the formation of bx-1.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-3952437/v1/26ae0ff3513211e596880cc3.png"},{"id":51802737,"identity":"1c16b6d1-b2ab-4310-89d5-055bdc42fdcf","added_by":"auto","created_at":"2024-02-29 09:44:20","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3260429,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTextural and microscopic observations of breccia-2. a\u003c/strong\u003e Breccia-2 with chaotic polymict clasts. \u003cstrong\u003eb\u003c/strong\u003e Various breccia widths and sharp boundaries are the unique features of bx-2. \u003cstrong\u003ec\u003c/strong\u003e The relationship between clast and matrix in bx-2. \u003cstrong\u003ed\u003c/strong\u003e Matrix–mineral phase maps and their relative abundances estimated using XmapTools\u003csup\u003e80\u003c/sup\u003e. \u003cstrong\u003ee\u003c/strong\u003e SEM-CL of the quartz matrix that depicts the crystallographic orientation of the quartz. \u003cstrong\u003ef\u003c/strong\u003e Dolomite coating of the quartz clast and \u003cstrong\u003eg–i\u003c/strong\u003e show the oscillatory zoning with different gray bands in the backscatter detector image. \u003cstrong\u003ej\u003c/strong\u003e Perfectly double-terminated dolomite crystal. \u003cstrong\u003ek\u003c/strong\u003e Elemental mapping of dolomite shows distinct characteristics between the core and rim.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-3952437/v1/001e96c6b63cb61ccd1cb7a7.png"},{"id":51802735,"identity":"3224cc99-b078-4e7a-b48d-82133f2a23a2","added_by":"auto","created_at":"2024-02-29 09:44:20","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":196906,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eParticle size distribution (PSD).\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Scale-integrated distribution of clast size as a function of clast density. The N(D) is the number of grains normalized to the measurement area at the outcrop and thin-section scales. Distributions of clasts at the (\u003cstrong\u003eb\u003c/strong\u003e) outcrop and (\u003cstrong\u003ec\u003c/strong\u003e) thin-section scales. \u003cstrong\u003ed\u003c/strong\u003e The relationship between fractal dimensions (D) and various rock deformation mechanisms from natural observations to experimental cases modified after Muto et al.\u003csup\u003e23\u003c/sup\u003e\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-3952437/v1/eba6f47eec0dd39a7f430788.png"},{"id":51802738,"identity":"cfd215ce-6ca8-460b-a145-92a05539886e","added_by":"auto","created_at":"2024-02-29 09:44:20","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":462509,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eBrecciation model in Ichinokawa\u003c/strong\u003e. \u003cstrong\u003ea\u003c/strong\u003e Schematic view of subshear rupture and energy propagation correspond to the activity along the MTL \u003cstrong\u003eb\u003c/strong\u003e Initial pulverization driven by rupture propagation during the coseismic event. \u003cstrong\u003ec\u003c/strong\u003eFluid-induced brecciation as the consequence of elevated fluid pressure together with perfect macroscopic images of bipyramidal quartz and stibnite.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-3952437/v1/5ddf6a925ae792f6985aff65.png"},{"id":51802945,"identity":"f7670380-7304-4695-8440-b8da9d033c30","added_by":"auto","created_at":"2024-02-29 09:52:20","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":334577,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSurface energy calculation in Ichinokawa. a\u003c/strong\u003e Surface area versus energy density in Ichinokawa based on outcrop and thin section \u003cstrong\u003eb\u003c/strong\u003e and \u003cstrong\u003ec\u003c/strong\u003e Estimation of surface energy per unit fault/damage zone for a single earthquake (U\u003csub\u003esa\u003c/sub\u003e) assuming total earthquake recurrences of 10 and 100 to advance the rupture in Ichinokawa. \u003cstrong\u003ed\u003c/strong\u003e Calculated earthquake magnitudes according to the average surface energy for the 10 and 100 earthquake recurrence scenarios at the outcrop and thin-section scales.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-3952437/v1/c991489d0a7d3e15985baa78.png"},{"id":51803336,"identity":"d51e6f66-93d4-44ec-9556-ec586701d7f2","added_by":"auto","created_at":"2024-02-29 10:00:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5054226,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3952437/v1/2354715d-8808-496a-bd8d-96d9c1366366.pdf"},{"id":51802740,"identity":"c86677d6-3ea9-4e78-89f5-ab33b87f9130","added_by":"auto","created_at":"2024-02-29 09:44:21","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":13064655,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-3952437/v1/c77d0cb50d765836d076226c.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Multiscale off-fault brecciation records coseismic energy budget of principal fault zone","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe damage zone in the vicinity of a principal fault indicates the abrupt change in the stress regime. A transient change in natural permeability leads to fluid infiltration into the damage zone over a period\u003csup\u003e1\u0026ndash;3\u003c/sup\u003e. Pseudotachylite is considered a marker of earthquake slip phenomena and is caused by the frictional melting between two rock bodies in a fault zone\u003csup\u003e4,5\u003c/sup\u003e. The formation of pseudotachylite is exclusively driven by high slip rates of 1\u0026ndash;0.1 m/s for coseismic and interseismic events, respectively\u003csup\u003e6,7\u003c/sup\u003e. The number of studies on the relationship between seismicity and faults is increasing, and currently, fast rupture is considered generated under extreme stress conditions\u003csup\u003e4\u003c/sup\u003e. The signatures of fast rupture include rock pulverization\u003csup\u003e8\u003c/sup\u003e and periodic injection of fluids, which can be observed in relatively soft rocks such as carbonates\u003csup\u003e9\u0026ndash;11\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eWhen an earthquake occurs, the released elastic energy is translated into ground shaking (\u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003erad\u003c/em\u003e\u003c/sub\u003e), frictional heat (\u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003efh\u003c/em\u003e\u003c/sub\u003e), and increase in the area of surface energy (\u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003esa\u003c/em\u003e\u003c/sub\u003e)\u003csup\u003e12\u0026ndash;14\u003c/sup\u003e. The surface energy is a wave that propagates away from the earthquake source and is responsible for initiation and advancement of rupture in the off-fault region\u003csup\u003e15\u0026ndash;17\u003c/sup\u003e. Recent research suggests that surface energy is not suitable for calculation of the overall energy balance of the dynamics of earthquake rupture\u003csup\u003e12,18,19\u003c/sup\u003e. Surrounding active and mature continental strike-slip faults, the surface energy is manifested as well-preserved pervasive fragmentation of rocks that occur within the off-fault region (tens to hundreds of meters in scale) as pulverized rocks\u003csup\u003e20\u0026ndash;27\u003c/sup\u003e. Pulverized rocks require high to modest strain rates that are achieved through single or successive loading of the intact rock\u003csup\u003e8,28\u0026ndash;32\u003c/sup\u003e. This loading represents the pulse of the surface energy during the seismic event. Fragmented rocks at the field and thin-section scales can be analyzed to reconstruct the dynamics of the energy budget experienced by them\u003csup\u003e33,34\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eWe performed a multidisciplinary study of pulverized pelitic schists exposed in the off-fault region of the median tectonic line (MTL) in Ichinokawa, Japan. Based on the texture and structure of breccia combined with particle size distribution (PSD) at various observation scales, we redefine the dynamics of the brecciation mechanism and its relationship with the MTL\u003csup\u003e35\u003c/sup\u003e. We performed calculations of surface energy intensities based on the size of breccia fragments at the outcrop and thin-section scales. Our findings reveal a new constraint on the earthquake energy budget and rupture dynamics in the off-fault region of the MTL\u003csup\u003e12,36\u003c/sup\u003e. Our analyses reveal unique fragmentation behavior (pulverization), earthquake energy estimation, fluid circulation, and mineralization in the off-fault region. Therefore, Ichinokawa can serve as a reference site for the study of such phenomena.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eOff fault pulverized rock associated with principal slip zone\u003c/h2\u003e \u003cp\u003eThe Sanbagawa belt is a Cretaceous high-pressure metamorphosed accretionary complex consisting mainly of pelitic, psammitic, and mafic schists with minor quantities of metachert, expressing the subduction of marine sediment deposited upon the basaltic oceanic crust\u003csup\u003e37\u003c/sup\u003e. Across Shikoku Island, the Sanbagawa belt and the young accretionary complex in the north are separated by the largest arc-parallel tectonic fault of the MTL. Right-lateral slip displacement, recorded in the latest history, contributes to the deformation of adjacent rock in terms of brittle failure including brecciation\u003csup\u003e38,39\u003c/sup\u003e. The Ichinokawa breccia body is located in central Shikoku approximately 3 km south of Saijo City within the off-fault region of the MTL. The lenticularly restricted breccia body is perpendicular to the MTL with a dimension of 200 \u0026times; 400 m consisting of low-grade Sanbagawa pelitic schist. It is surrounded by intact pelitic schist with lesser occurrences of psammitic schist. In the north, the rock body is juxtaposed with the Upper Cretaceous sediment formation of Izumi Group bounded by the ENE\u0026ndash;WSW trending MTL (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea and Supplementary Fig.\u0026nbsp;1a). The distance between the MTL and breccia is 50\u0026ndash;650 m, representing the width of the dynamics of the damage zone (Supplementary Fig.\u0026nbsp;1b). The strike of the breccia and clast elongation are subparallel to the MTL (Supplementary Fig.\u0026nbsp;1c); however, in another outcrop, \u003cem\u003ein situ\u003c/em\u003e measurement of the strike of the fault-filling breccia shows that it is perpendicular to the MTL (Supplementary Fig.\u0026nbsp;1d), suggesting that these developed under the dynamics of rupturing, and the MTL contributes significantly to the brecciation processes (Supplementary Fig.\u0026nbsp;1e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe pervasive brecciation is well exposed near the Senga-ko adit (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) and is classified into two types. The breccia-1 (bx-1) is clast supported breccia and is composed solely of pelitic schist clast with a size of up to 5 m (monomict; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, d). The pelitic schist has typical mineral assemblages of muscovite and quartz with EW sub-horizontal lineation of the Sanbagawa belt (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee, f). Clasts also show little or no rotational block as well as dilatational shear deformation with well-preserved metamorphic lineation. Clasts are mostly angular and exhibit a crackle texture with non-systematically oriented micro- to macrofractures separating the clast. The fine microfracture cuts the lineation and mineral vein filled with dolomite with a lower quantity of quartz grains, which indicates the initial matrix component soon after the beginning of fragmentation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg, h). The alteration around the clast is absent, indicating that the comminution is mainly governed by mechanical processes. We suggest that these features of bx-1 correspond with the coseismic fragmentation, as it is in the proximity of the principal slip zone in the order of hundreds of meters. The intricate deformation history of the MTL\u003csup\u003e38,39\u003c/sup\u003e, combined with a significant seismic event of magnitude 7\u0026ndash;8\u003csup\u003e40\u0026ndash;42\u003c/sup\u003e that occurred prior to the formation of Ichinokawa Formation, may have triggered the coseismic fragmentation of the Sanbagawa pelitic schist. Moreover, the asymmetrical damage encompassed bimaterial is prone to generating rock pulverization\u003csup\u003e22,43\u003c/sup\u003e. Ichinokawa Formation juxtaposes with Izumi Formation on the northern side. The wide perturbation zone in this formation is characterized by the development of boudinage along the MTL, and it is opposed to Ichinokawa Formation, where the disturbed zone is very narrow and local, resembling the pulverized outcrop along San Andreas Fault\u003csup\u003e33,34\u003c/sup\u003e. Because Ichinokawa Formation comprises a stiffer rock (schist) with fragmentation characteristics, the Ichinokawa breccia is more likely to occur because of rock pulverization.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eFluidization and signature of decarbonation during seismic slip\u003c/h2\u003e \u003cp\u003eThe cohesive pulverized rock observed in Ichinokawa, unlike the typical incohesive pulverized rocks studied so far\u003csup\u003e24\u003c/sup\u003e, can be explained by the presence of breccia-2 (bx-2), which occupies the same outcrop as bx-1 and occurs as an injection-like breccia with various breccia pipe widths ranging from 5 cm to 1 m (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, b). The bx-2 is matrix-supported with polymict clast consisting of pelitic schist and a metamorphic mineral-bearing vein that is derived from bx-1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). The clast is sub-rounded to sub-angular with an average size of 10\u0026ndash;20 cm at outcrop and thin-section scales, with a lower quantity of sub-millimeter particle size observed in backscatter detector (BSE) images (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). The clast is intensely rotated and shows a fluidized texture, indicating that it experienced \u003cem\u003ein situ\u003c/em\u003e transport phenomena and was most likely subjected to subsequent fluid-driven fragmentation. The matrix consists of muscovite, quartz, dolomite, and several sulfide minerals such as pyrite, arsenopyrite, and stibnite/sulfosalt, which make up 59, 19, 18, and 9%, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, e). Dolomite constitutes up to 40% (Supplementary Fig.\u0026nbsp;1f) of the matrix, with two observed precipitation modes: 1) the dolomite encloses another component similar to that of a cockade with distinct oscillatory zoning (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef\u0026ndash;i), and 2) it forms uniformly along with another matrix component (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej). The dolomite exhibits compositional zoning, with the core having higher magnesium and calcium but lower iron, whereas the rim has a higher iron content, giving rise to the reddish matrix color (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ek and Supplementary Fig.\u0026nbsp;1g, h).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe significant quantity of dolomite and carbonic fluid inclusions indicates the existence of a CO\u003csub\u003e2\u003c/sub\u003e-rich fluid, even though the source of the fluid remains unknown (Supplementary Text). The abundance of this fluid along the MTL is manifested as a calcite/dolomite vein within Sanbagawa\u003csup\u003e44,45\u003c/sup\u003e, dolomite-bearing stibnite deposits, and a modern CO\u003csub\u003e2\u003c/sub\u003e hot spring, which indicates CO\u003csub\u003e2\u003c/sub\u003e activity derived from hydrothermal sources\u003csup\u003e46,47\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;1i). Fluid inclusions in the quartz\u0026ndash;stibnite vein contain a low concentration of CO\u003csub\u003e2\u003c/sub\u003e, suggesting that the initial fluid has an elevated CO\u003csub\u003e2\u003c/sub\u003e concentration.\u003c/p\u003e \u003cp\u003eThe flow texture coupled with the subparallel orientation of the matrix component was observed in the BSE image (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). In addition, the quartz matrix is distinct from the quartz clast based on the cathodoluminescence (CL) intensity. The quart matrix shows high (CL-bright) and low (CL-dark) CL intensities in the quartz clast, strongly suggesting that fluidization processes dictate the formation of bx-2 through mineral precipitation from the fluid or some degree of reaction occurred between the rising fluid and pelitic schist (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). The microtexture of sulfide minerals, such as zoning, also supports the notion that the fluidization was caused by pressure fluctuation and change in the fluid-flow regime during coseismic and interseismic events (Supplementary Fig.\u0026nbsp;2)\u003csup\u003e48,49\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eParticle size distribution and dynamics of off-fault brecciation\u003c/h2\u003e \u003cp\u003ePSD analysis is a powerful tool to study fragmented rock or other materials\u003csup\u003e50,51\u003c/sup\u003e. The dimensionless D value or fractal dimension represents various mechanisms of clast fragmentation and size reduction (comminution) as mentioned by various authors\u003csup\u003e52\u0026ndash;54\u003c/sup\u003e. Particularly in the brecciated system, the D value is a function of energy input applied to the rock during breccia formation\u003csup\u003e55\u003c/sup\u003e, and it has high and low values for high and low energy input processes, respectively. However, the fundamental process of both characterizations are that one emphasizes fault-related fragmentation, whereas the other indicates processes related to hydrothermal systems. Therefore, high and low D values depend on the process.\u003c/p\u003e \u003cp\u003eThe integrated PSD in Ichinokawa (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) follows a power law distribution with a slope of 1.65 for the clast spanning 0.3\u0026ndash;3.5 mm for the thin section and 20\u0026ndash;100 mm for the outcrop (indicated as the gray area). The clast size varies between 5\u0026ndash;500 mm, and this wide range of clast sizes represents bx-1 and bx-2 at different observation scales. Under thin section, the particle density has two major trends due to the difference in sampling locations, wherein the high particle density corresponds to bx-1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). The discrepancies in particle density at the outcrop scale is a function of the width of the breccia pipe. According to the density plot, the clast sizes at the outcrop and thin-section scales have mean clast diameters of 20.4 and 0.483 mm, respectively, with modal values of the distribution curves of 0.25 and 12.6 mm for the thin-section and outcrop, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn addition, the distribution of individual samples shows a relatively higher fractal dimension (D value) with more than one size distribution gradient. In the outcrop scale, bx-1 has the highest D value of 3.5, probably due to less variation in the clast size, as the rock was subjected to initial fragmentation (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and Supplementary Fig.\u0026nbsp;3a). The bx-2 has a fractal dimension of 1.2\u0026ndash;2.9, with a positive correlation with the size of the breccia pipe. The distribution of clasts at the thin-section scale is relatively less heterogeneous, with the fractal dimension of 1.8\u0026ndash;2.3 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec and Supplementary Fig.\u0026nbsp;3b).\u003c/p\u003e \u003cp\u003eIn Ichinokawa, the D value is 1.2\u0026ndash;3.5 at both the outcrop and thin-section scales (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). Most of the D values coincide with the fractal dimension of natural and experimental fault gouges\u003csup\u003e23,56,57\u003c/sup\u003e. The distribution of fragments of bx-1 and bx-2 with the widest apertures correspond with high D values, surpassing the theoretical boundary of shear comminution processes\u003csup\u003e58\u003c/sup\u003e, which can be attained by extensive fragmentation and impact loading such as pervasive fracturing and rock pulverization\u003csup\u003e23,30,53,54\u003c/sup\u003e. The aperture of the conduit also contributes to the high-energy input during the dynamics of brecciation\u003csup\u003e55,59\u003c/sup\u003e. A fractal dimension of \u0026lt;\u0026thinsp;1.68 implies that fragmentation proceeds to the next step with a lower energy input associated with propagation, mechanical/chemical wearing, dilatation, and fragmentation caused by fluids, as observed in Ichinokawa\u003csup\u003e35,52,55\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe brecciation observed in Ichinokawa is intriguing because it is caused by \u003cem\u003ein situ\u003c/em\u003e hydro-fracturing without the rotation of large clasts and fault-filling material consisting of finely crushed rock\u003csup\u003e35\u003c/sup\u003e. We propose a model for the development of breccia based on the dynamics of fracturing in relation to the main tectonic fault and hydrothermal activity (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) based on our novel investigation on the internal structure of breccia coupled with multiscale PSD analysis. Elastic energy and surface energy (U\u003csub\u003esa\u003c/sub\u003e) that are radiated by the earthquake nucleation (red star) along the MTL are responsible for the advancement of the rupture\u003csup\u003e16,19\u003c/sup\u003e. The bimaterial that is juxtaposed between Sanbagawa Formation and Izumi Group causes off-fault fracturing primarily in the stiffer rock, that is, pelitic schist\u003csup\u003e60,61\u003c/sup\u003e. Subshear rupture and pulverization propagate forward as the recurrence of the earthquake releases elastic energy, and these processes occur recursively\u003csup\u003e62\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, b). These processes caused the formation of bx-1, as rock pulverization occurred close to the MTL and dolomite filled microfractures during the initial brecciation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). The formation of bx-1 induces the fracture permeability of pelitic schist leading to an episodic hydrothermal fluid flow to form bx-2 as a breccia dike\u003csup\u003e9,42\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). Fluid pressure may be building up to some extent underneath Ichinokawa, even though the intrusion-driven hydrothermal activity remains uncertain\u003csup\u003e63\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e \u003cb\u003eEnergy budget on off-fault damage and an estimation of earthquake magnitude through the multi-scale of breccia\u003c/b\u003e \u003c/p\u003e \u003cp\u003eBreccia represent phenomena associated with paleoearthquakes indicating deformation and reflect the mechanical processes that occur during coseismic events\u003csup\u003e11,64,65\u003c/sup\u003e. The elastic energy that accumulates and propagates during and after the slip event is translated into surface energy, which advances the rupture of the intact rock\u003csup\u003e12,19\u003c/sup\u003e. We next estimated the energy budget based on off-fault damage using a D value of \u0026gt;\u0026thinsp;1.68 to perform calculations as the low fractal dimension might represent the low energy fragmentation, as explained in the previous section. Therefore, we assumed that fragmentation occurs in a single system (scale invariance). In addition to using individual breccia data, we also used integrated PSD data (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eUsing those values and the approach described by Johnson et al.\u003csup\u003e12\u003c/sup\u003e, we obtained the area-averaged particle size (L) for the outcrop at 24.30\u0026ndash;8.73 mm and for the thin section at 0.65\u0026ndash;2.55 mm. Assuming that the specific surface area of pelitic schist is equal to that of granite at approximately 56 J/m\u003csup\u003e2 66\u0026ndash;68\u003c/sup\u003e and using surface correction for the most natural gouge of 6.6\u003csup\u003e27\u003c/sup\u003e, we calculated the total surface fracture energy density (\u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e) at 2.25\u0026ndash;9.12 \u0026times; 10\u003csup\u003e4\u003c/sup\u003e J/m\u003csup\u003e3\u003c/sup\u003e and 8.69 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e to 3.43 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e J/m\u003csup\u003e3\u003c/sup\u003e for outcrop and thin sections, respectively. The scale-integrated data showed that the surface area and surface energy density are in the range of the energy indicated by individual breccia. It is evident that the fragmentation continues from the macro to micro scales, and the particle size dictates the energy consumption during the fragmentation (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and Supplementary Table\u0026nbsp;1). The estimated values of dissipated energy in Ichinokawa are comparable to those of several natural and experimental occurrences of off-fault fragmentation or pulverization. The energy level is apparent at a thin-section scale, as the smallest aggregates are subjected to intense fragmentation processes at high strain rates\u003csup\u003e12,28,29,69\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe total energy density is distributed across the ~\u0026thinsp;500 m (according to outcrop distance with respect to the MTL) wide dynamic damage zone to propagate the rupture through the off-fault region. Integrating the energy density (\u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e) over the damage-zone width in Ichinokawa resulted in the total fracture energy per unit fault area (\u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003esa\u003c/em\u003e\u003c/sub\u003e) of 1.12\u0026ndash;4.56 \u0026times; 10\u003csup\u003e7\u003c/sup\u003e J/m\u003csup\u003e2\u003c/sup\u003e for the outcrop and 4.34 \u0026times; 10\u003csup\u003e8\u003c/sup\u003e to 1.72 \u0026times; 10\u003csup\u003e9\u003c/sup\u003e J/m\u003csup\u003e2\u003c/sup\u003e for the thin section. The pulverization accumulates the stress of every individual coseismic slip\u003csup\u003e69\u003c/sup\u003e. Thus, the estimation of surface energy for a single earthquake reflects the dynamic loading that contributes to the fragmentation\u003csup\u003e70\u003c/sup\u003e. The total displacement of the principal fault can be used to estimate the number of earthquake events\u003csup\u003e18,20,27\u003c/sup\u003e. However, the estimation of earthquake recurrence based on the slip displacement cannot be deployed in Ichinokawa.\u003c/p\u003e \u003cp\u003eTherefore, we used the dolomite matrix as a proxy to obtain the number of earthquake events. Assuming dolomite cementation with apparent oscillatory zoning occurs during the coseismic event, we found five to eight zonings that reflect the minimum earthquake recurrences (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef\u0026ndash;i and Supplementary Fig.\u0026nbsp;1g). The estimation of surface energy released by a single earthquake in Ichinokawa is 1.12\u0026ndash;172 MJ/m\u003csup\u003e2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and Supplementary Table\u0026nbsp;1). An earlier study suggests that the pulverized rock is a distinct type of rock fragmentation produced by intense loading. Therefore, we estimated the number of loadings subjected onto the intact rock to produce the finest breccia clast in Ichinokawa from the biggest fragment based on the dynamics loading experiment\u003csup\u003e71\u003c/sup\u003e. We obtained approximately 100 loadings required to generate the clast size; in this case, the calculated \u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003esa\u003c/em\u003e\u003c/sub\u003e is 0.11\u0026ndash;17.2 MJ/m\u003csup\u003e2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec and Supplementary Table\u0026nbsp;1). The scale integration estimated the surface energy span at 2.08\u0026ndash;73.13 and 0.21\u0026ndash;7.31 MJ/m\u003csup\u003e2\u003c/sup\u003e for 10 and 100 earthquake recurrences, respectively. This comparison shows that the energy budget is lower by four-fold compared with the average surface energy obtained from individual breccia data. This also influences the earthquake magnitude estimation.\u003c/p\u003e \u003cp\u003eThese results are similar to those of other estimates of energy release for single earthquakes compiled by Johnson et al., 2021\u003csup\u003e12\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;4b). Our energy estimation at the outcrop scale corresponds to those of large displacement faults such as Chelungpu, Punchbowl, and San Andreas Faults\u003csup\u003e18,20,72\u003c/sup\u003e, suggesting that the surface energy in Ichinokawa is stored across the wide damage zone of the MTL, and its significant quantity suggests that it is a non-negligible component for the advancement of the rupture near the principal fault zone. At the thin-section scale, it could be overestimated (by two orders of difference) or it probably records the highest energy dissipation from the MTL. Additionally, a width of 500 m of the dynamical damage zone corresponds to a depth of ~\u0026thinsp;5 km. The corresponding depth was confirmed by estimating the pressure based on the isochore analysis of fluid inclusions at 30\u0026ndash;190 MPa (Supplementary Fig.\u0026nbsp;5h). At this depth, approximately 40% of the fracture energy is dissipated in the off-fault medium, and the rest of the energy is directed into the fault\u003csup\u003e19\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eWe estimated the ranges of moment magnitude at 5.8\u0026ndash;6.9 and 7.1\u0026ndash;8.3 based on the records at the outcrop and thin-section scales, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). We attempted to integrate the fragmentation of breccia with the moment magnitude of an earthquake, which has a far reaching scope compared with that of usual studies. However, it is necessary to consider many mechanical and physical parameters including the type of converted energy and data derived from numerical and kinematic models, which over or underestimate results. In the future, we can accumulate a significant amount of data to generate a robust energy\u0026ndash;seismic moment model, using the breccia records of paleoseismic activity present adjacent to the principal fault.\u003c/p\u003e \u003c/div\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eElectron microprobe analysis\u003c/h2\u003e \u003cp\u003eThe mineral compositions of the samples were determined using an electron microprobe analyzer (EPMA; JEOL JXA-8200) at Tohoku University, Japan. X-ray mapping using EPMA was performed with acceleration voltage, beam current, and dwell time of 15 kV, 120 nA, and 60 ms, respectively. The beam diameter was 1 \u0026micro;m. Natural and synthetic standards (wollastonite for Ca and Si, rutile for Ti, eskolaite for Cr, hematite for Fe, manganosite for Mn, periclase for Mg, albite for Na, feldspar for K, halite for Cl and F) were used for calibration. The counting times for the peaks and backgrounds of all major elements were 10 and 5 s, respectively, whereas for the minor elements Cl and F they were 30 and 15 s, respectively.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eScanning electron microscopy coupled with cathodoluminescence\u003c/h2\u003e \u003cp\u003eThe internal quartz structure was characterized using a scanning electron microscope equipped with an Oxford cathodoluminescence detector and photomultiplier (SEM-CL) at the Graduate School of Science, Tohoku University, Japan with an accelerating voltage of 25 kV and a beam current of 90 \u0026micro;A. The CL image was analyzed as described by Frelinger et al. and Rusk and Reed\u003csup\u003e73,74\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eFluid Inclusion microthermometry\u003c/h2\u003e \u003cp\u003eFluid inclusion microthermometry was performed on double-polished thick sections (~\u0026thinsp;100-\u0026micro;m thickness) of the Sb-bearing vein. Microthermometry was performed using a Linkam THS600 heating/freezing with an operating temperature of \u0026minus;\u0026thinsp;180\u0026ndash;600\u0026deg;C and a measurement error of 0.1\u0026deg;C at the Tohoku University, Japan. Salinities of fluid inclusion in NaCl wt.% equivalent were estimated using the final melting temperatures of ice\u003csup\u003e75,76\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eRaman\u003c/h2\u003e \u003cp\u003eRaman spectroscopic analysis was applied to identify the phases of fluid inclusion from the Sb-bearing vein using a HORIBA XploRA PLUS Confocal Raman Microscope at the Graduate School of Environmental Studies, Tohoku University. The laser beam was positioned using a built-in Olympus BX optical polarization microscope. Excitation was done at a solid-state wavelength of 532 nm. Spectra were registered using a charge-coupled device detector cooled to \u0026minus;\u0026thinsp;70\u0026deg;C using a Peltier element. The analysis was carried out in a backscattered geometry. The scattered light was focused using a 100\u0026times; magnification lens. Spectra were registered at 100\u0026ndash;4,200 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The spectrometer was calibrated using a pure silicon line (520.7 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and an intrinsic laser line (0 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003ePSD at macro- and microscales\u003c/h2\u003e \u003cp\u003ePSD was used to understand the characteristics and formation mechanism of breccia in Ichinokawa. Scanline sampling was deployed both for direct field and rock slab/thin-section measurements to characterize the matrix-to-clast ratio (ε) and the distribution of clast within the breccia. The one-dimensional scanline was conducted to obtain the geometry of the breccia or clast parameter primarily in the long (L) and short (S) axes of the clast/particle for breccia-2 (bx-2; Supplementary Fig.\u0026nbsp;3c, d). The PSD measurement is a function of the width of breccia pipe. For the rock slab/thin-section, we initially traced the image that fits the sampling line, and then, obtained the clast parameters using ImageJ (Supplementary Fig.\u0026nbsp;3e, f)\u003csup\u003e77\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe clast size is provided by the square root of L and S of each particle and it is expressed as \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(d={\\left(LS\\right)}^{1/2}\\)\u003c/span\u003e\u003c/span\u003e. The cumulative probability was calculated from the PSD\u003csup\u003e56\u003c/sup\u003e and then plotted in the log(N)\u0026thinsp;\u0026minus;\u0026thinsp;log(d) diagram. The distributions were fitted using the power law equation as follows:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$N\\propto {d}^{-Ds}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere d and N denote the particle size (\u0026micro;m) and the cumulative probability of particles\u0026thinsp;\u0026gt;\u0026thinsp;d, respectively, and Ds is the fractal dimension\u003csup\u003e78,79\u003c/sup\u003e. Furthermore, the multiscale particle size integration was performed by normalizing the clast distribution with the measurement area (mm\u003csup\u003e2\u003c/sup\u003e) to obtain the particle density.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eCalculation of surface energy and Earthquake magnitude\u003c/h2\u003e \u003cp\u003eFor the estimation of the surface energy using natural fragment size data, we transformed the mean diameter into the area-averaged fragment size. The versatile derivation of this problem was performed according to Johnson et al.\u003csup\u003e12\u003c/sup\u003e by considering the surface area per unit volume of a fragment consisting of \u003cem\u003en\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e spheres of diameter \u003cem\u003es\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e for bin \u003cem\u003ei\u003c/em\u003e as follows:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\left(\\frac{A}{V}\\right)=\\frac{\\varSigma {n}_{i}\\left[4\\pi {\\left(\\frac{{s}_{i}}{2}\\right)}^{2}\\right]}{\\varSigma {n}_{i}\\left[\\frac{4}{3}\\pi {\\left(\\frac{{s}_{i}}{2}\\right)}^{3}\\right]}=\\frac{6\\varSigma {n}_{i}{s}_{i}^{2}}{\\varSigma {n}_{i}{s}_{i}^{3}}=\\frac{6}{{\\stackrel{\\prime }{s}}_{avg}}=\\frac{6}{L}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eFor surface area (L) the \u003cb\u003eError! Reference source not found.\u003c/b\u003ecan be rewritten as follows:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$L=\\frac{\\varSigma {n}_{i}{s}_{i}^{3}}{\\varSigma {n}_{i}{s}_{i}^{2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe count of the fraction for the power-law cumulative distribution of a population of spheres (\u003cem\u003en\u003c/em\u003e) can be expressed as follows:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$n\\left(s\\right)={kDs}^{-D-1}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003es\u003c/em\u003e is the diameter of the fragment, \u003cem\u003eD\u003c/em\u003e is the fractal dimension, and \u003cem\u003ek\u003c/em\u003e is the constant. By integrating \u003cb\u003eError! Reference source not found.\u003c/b\u003es 2 and 4, we can obtain:\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$L=\\frac{\\varSigma {n}_{i}{s}_{i}^{3}}{\\varSigma {n}_{i}{s}_{i}^{2}}=\\frac{{\\int }_{{s}_{min}}^{{s}_{max}}n\\left(s\\right){s}^{3}ds}{{\\int }_{{s}_{min}}^{{s}_{max}}n\\left(s\\right){s}^{2}ds}=\\frac{{\\int }_{{s}_{min}}^{{s}_{max}}{kDs}^{-D+2}ds}{{\\int }_{{s}_{min}}^{{s}_{max}}{kDs}^{-D+1}ds}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThus, the solution is:\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$$L=\\left(\\frac{2-D}{3-D}\\right)\\left[\\frac{{s}_{max}^{\\left(3-D\\right)}-{s}_{min}^{\\left(3-D\\right)}}{{s}_{max}^{\\left(2-D\\right)}-{s}_{min}^{\\left(2-D\\right)}}\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe area-averaged fragment size (\u003cem\u003eL\u003c/em\u003e) as a function of diameter (\u003cem\u003es\u003c/em\u003e) and fractal dimension (\u003cem\u003eD\u003c/em\u003e) allows us to estimate the surface energy density (\u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e; J/m\u003csup\u003e3\u003c/sup\u003e) using the following equation:\u003cdiv id=\"Equ7\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e\n$$Us=\\frac{6\\gamma \\lambda }{L}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere γ is specific surface energy of the material (J/m\u003csup\u003e2\u003c/sup\u003e) and λ represents the surface-area correction factor because the breccia fragment is not a perfect sphere or cube.\u003c/p\u003e \u003cp\u003eUsing the converted energy value from the numerical and kinematic slip model coupled with the seismic moment (\u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003eo\u003c/em\u003e\u003c/sub\u003e) allows us to generate a simple model of the relationship between energy and seismic moment (Supplementary Fig.\u0026nbsp;5c, d). Assuming that the energy generated during the earthquake is converted into surface-area energy (\u003cem\u003eU\u003c/em\u003e\u003csub\u003e\u003cem\u003esa\u003c/em\u003e\u003c/sub\u003e), we fitted the average energy value of two earthquake recurrences in Ichinokawa (10 and 100 EQ) to get the seismic moment. Subsequently the moment magnitude (\u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003ew\u003c/em\u003e\u003c/sub\u003e) was estimated using the equation provided by Kanamori and Brodsky\u003csup\u003e36\u003c/sup\u003e as follows:\u003cdiv id=\"Equ8\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ8\" name=\"EquationSource\"\u003e\n$$Mw=\\frac{log\\left(Mo\\right)}{1.5}-6.07$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e8\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting Interest\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eG.A, A.O., M.U., and N.T. design the research and carried out fieldwork. G.A. carried out petrological, fluid inclusion analyses, and numerical modelling. All authors contributed to the data interpretation. G.A. prepared the draft of the manuscript. All authors were involved in revising the manuscript and approved the submitted version.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eWe thank Jun Muto and Mitsuhiro Toriumi for fruitful discussion. Alexey Kotov and Bayarbold Manzhir for assisting during the fieldwork. Diana Mindaleva for drone image and fieldwork assistance.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e \u003cp\u003eThe datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003eAdditional information\u003c/p\u003e\n\u003cp\u003eSupplementary information the online version contains supplementary material available at\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSibson, R. H. Rupture Interaction with Fault Jogs. in Earthquake Source Mechanics 157\u0026ndash;167 (American Geophysical Union (AGU), 1986). doi:\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1029/GM037p0157\u003c/span\u003e\u003cspan address=\"10.1029/GM037p0157\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCaine, J. S., Evans, J. P. \u0026amp; Forster, C. B. Fault zone architecture and permeability structure. Geology 24, 1025\u0026ndash;1028 (1996).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFaulkner, D. R. \u003cem\u003eet al.\u003c/em\u003e A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones. J. Struct. Geol. 32, 1557\u0026ndash;1575 (2010).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRowe, C. D. \u0026amp; Griffith, W. A. Do faults preserve a record of seismic slip: A second opinion. J. Struct. Geol. 78, 1\u0026ndash;26 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSibson, R. H. Generation of Pseudotachylyte by Ancient Seismic Faulting. Geophys. J. Int. 43, 775\u0026ndash;794 (1975).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSibson, R. H. Earthquakes and Rock Deformation in Crustal Fault Zones. Annu. Rev. Earth Planet. 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Geosci. 62, 227\u0026ndash;240 (2014).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-3952437/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3952437/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBreccia and pulverized rock are typical textures in off-fault damage adjacent to a main seismogenic zone. Previously, by estimating the energy required to advance the rupture in this zone using particle size distribution at sub-millimeter/micrometer scales, we could constrain the energy budget during coseismic events. However, whether microscopic estimation is sufficient to capture surface energy fragmentation during an earthquake and the effect of measurement scale variation on calculation of co-seismic energy partitioning remained unclear. Here, we investigated the mechanism of coseismic off-fault damage based on field and microstructural observations of a well-exposed breccia body in Ichinokawa, Japan. We used in situ clast measurements coupled with thin-section analysis of breccia clasts to estimate the energy budget of the damage zone adjacent to the principal slip zone of the median tectonic line. The total surface energy density and corresponding surface energy per unit fault for a width of ~\u0026thinsp;500 m of the dynamical damage zone were estimated. The moment magnitude estimated based on surface energy was 5.8\u0026ndash;8.3 Mw. In Ichinokawa, off-fault fragmentation is initiated by coseismic activity and is followed by fluid activity. Under dynamic fragmentation conditions, the scale is important to calculate the surface energy.\u003c/p\u003e","manuscriptTitle":"Multiscale off-fault brecciation records coseismic energy budget of principal fault zone","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-02-29 09:44:15","doi":"10.21203/rs.3.rs-3952437/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-03-15T08:06:26+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-03-11T02:48:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"16f0d55d-b7f0-42b6-bdc0-0737e4a4914d","date":"2024-03-01T17:45:14+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-03-01T16:15:27+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-03-01T16:12:52+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-02-27T06:51:57+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-02-27T06:50:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-02-13T02:13:26+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2b788678-a624-4899-b994-44e37a925fc9","owner":[],"postedDate":"February 29th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":28994685,"name":"Earth and environmental sciences/Solid earth sciences/Geology/Structural geology"},{"id":28994686,"name":"Earth and environmental sciences/Solid earth sciences/Petrology"}],"tags":[],"updatedAt":"2024-05-22T03:38:33+00:00","versionOfRecord":[],"versionCreatedAt":"2024-02-29 09:44:15","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3952437","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3952437","identity":"rs-3952437","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}