Shear wave splitting and seismic velocity structure in the onshore focal area of the 2024 Noto Peninsula earthquake, central Japan, as a fluid-promoted multifault rupture in a compressional inversion area

Shear wave splitting and seismic velocity structure in the onshore focal area of the 2024 Noto Peninsula earthquake, central Japan, as a fluid-promoted multifault rupture in a compressional inversion area | 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 Research Article Shear wave splitting and seismic velocity structure in the onshore focal area of the 2024 Noto Peninsula earthquake, central Japan, as a fluid-promoted multifault rupture in a compressional inversion area Tomomi Okada, Martha Kane Savage, Ayaka Tagami, Ryotaro Fujimura, and 20 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6576798/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The M7.6 Noto Peninsula earthquake occurred on 1 January 2024 in the Noto region of Ishikawa Prefecture. Several faults successively ruptured during the 2024 Noto Peninsula earthquake. Seismic activity in the area has increased since approximately August 2020. Possible causes of this increased seismic activity include stress changes due to crustal deformation and increased fluid pressure. In this study, following an analysis of the seismic swarm area northeast of the Noto Peninsula, S-wave splitting in the epicentral area was investigated using a dense seismic network of temporary and permanent stations. Seismic wave velocity tomography analysis was also conducted to investigate the isotropic seismic wave velocity structure in the onshore source region. The predominant fast direction of anisotropy throughout the peninsula was NE‒SW. This direction generally coincided with the strike direction of faults and folds and may be due to structural anisotropy.. In addition, NW‒SE anisotropy, whose direction generally coincided with the axis of maximum horizontal compression, and north‒southanisotropy were also observed in some areas. In the shallow crust, the hanging wall of the fault has low velocities. This shallow low-velocity hanging wall may be associated with structures that developed during past normal faulting that formed in this region. The low velocity in the shallow part of the hanging wall side of the fault and the anisotropy due to structural properties indicate a complex structure in which the structure of normal faults that developed in this region coexists with a compressional structure due to the present stress field. Low-Vs and high-Vp/Vs regions were identified beneath or at deeper extensions of hypocenters and faults at a depth of 18 km. Slightly larger P-wave velocities were obtained in this region than in the surrounding area. Tertiary igneous rocks are distributed in the target area, suggesting that the low-Vs and high-Vp/Vs regions may represent an old magma reservoir and that fluids released from/through this reservoir were involved in the 2024 Noto Peninsula earthquake and preceded seismic swarm activity that occurredbefore the 2024 earthquake. Shear wave splitting seismic velocity structure earthquake swarm Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction The M7.6 Noto Peninsula earthquake occurred on 1 January 2024 in the Noto region of Ishikawa Prefecture (e.g., Fujii and Satake, 2024 ; Okuwaki et al., 2024 ) (Fig. 1 ). Seismic activity in the area has increased since approximately August 2020. Possible causes of this increased seismic activity include stress changes due to crustal deformation and increased fluid pressure. These changes have been inferred from regional-scale seismic tomography (e.g., Nakajima et al., 2022) and electrical resistivity structures (e.g., Yoshimura et al., 2023 ), and spatio-temporal changes in crustal deformation (Nishimura et al., 2023 ),: hypocenters (e.g., Amezawa et al., 2023 ; Kato, 2024 ; Yoshida et al., 2023a , 2023b, 2024 ), static stress drops (Fukuoka et al., 2024 ), focal mechanisms, and local stress fields (Takano et al., 2024 ; Matsumoto et al., 2024 ). In our previous studies at the eastern end of the Noto Peninsula (Okada et al., 2024a ; Okada and Joint inland aftershock observation group for the 2024 Noto Peninsula Earthquake, 2024), we found a high-Vp/Vs body with high Vp and low Vs beneath a low gravity (Bouguer) anomaly (Honda et al., 2008 ; Sawada et al., 2012 ; Sawada and Hiramatsu, 2022 ) at the bottom of the swarm seismicity and interpreted it as an ancient magma body. Seismic anisotropy could provide clues for understanding the stress or fault structure of a focal area. Under differential stress, the preferred alignment of an opened planar crack, in which the plane is aligned with the maximum horizontal stress orientation (SHmax), could cause stress-induced anisotropy (e.g., Nur and Simmons, 1969 ; Nur, 1971 ; Crampin, 1987 ; Aster et al., 1990 ; Savage et al., 1990 ; Gledhill, 1991 ; Aster and Shearer, 1992; Okada et al., 1994 ; Hiramatsu et al., 2010 ; Savage et al., 2015 ; Hiramatsu and Iidaka, 2015 ). Fault parallel alignment of fault fabrics and minerals could cause structure-controlled anisotropy (e.g., Savage et al., 1990 ; Sayers, 1994 ; Boness and Zoback, 2006 ). Furthermore, an anticline folding structure could be manifested as macroscopic anisotropy for seismic waves whose wavelengths are longer than the characteristic length of the structure (Okaya et al., 2019 ). We observed seismic anisotropy using shear wave splitting (e.g., Savage, 1999 ). In shear wave splitting, two split shear waves are observed, and the fast shear wave oscillation direction (FSOD) tends to be parallel to the SHmax orientation or the strike of the fault and anticline axis. In the eastern part of the Noto Peninsula (Okada et al., 2024a ), where SHmax is oriented NW–SE (e.g., Terakawa and Matsu’ura, 2010 ; Matsumoto et al., 2024 ; Tagami and Okada, 2024 ; Takano et al., 2024 ; Tagami et al., 2025), we found stress-induced anisotropy with an FSOD of NW–SE in the southern part of the swarm seismicity and structurally controlled anisotropy with an FSOD of E‒W in the north, where the swarm was more active. In the focal area, Tertiary sedimentary and volcanic rocks are widely distributed (Fig. 1 , Sawada et al., 2013). This finding suggests that the focal area was ina rift zone in the Miocene when the Japan Sea opened (e.g., Okamura et al., 1995 ; Okamura, 2019 ). At present, faulting and deformation are thought to have occurred in the tectonic inversion stage, and old originally formed unfavorably oriented normal faults and optimally oriented reverse faults coexist (e.g., Sibson and Ghisetti, 2018 ). In the case of the 2003 northern Miyagi earthquake (M6.4), NE Japan, Okada et al. ( 2007a ) reported that the hanging wall of the fault has a lower seismic velocity than the footwall. In particular, this situation is considered to reflect the existence of a thick sedimentary layer formed on the hanging wall. Major faults exist along the northern coastline of the peninsula. There are four segments from east to west, Suzu-oki, Wajima-oki, Saruyama-oki, and Monzen-oki, inferred from the continuity of each fault (Inoue and Okamura, 2010). Some studies have suggested that the 2007 Noto-Hanto earthquake (M6.9) corresponded to the Monzen-oki segment (e.g., Hiramatsu et al., 2008 ) and that the Wajima-oki segment ruptured during a historical earthquake in 1729 (Hamada et al., 2016 ). Several faults successively ruptured during the 2024 Noto Peninsula earthquake (e.g., Fujii and Satake, 2024 ; Okuwaki et al., 2024 ). Tagami and Okada ( 2024 ) and Tagami et al. (2025) reported that most of the 2024 Noto Peninsula earthquake faults had a high slip tendency toward the stress field before the 2024 earthquake and thus interpreted the faults as being under conditions that could cause multiple fault ruptures (e.g., Matsuno et al., 2020 for the 2016 M7.8 Kaikoura earthquake). In this study, following an analysis of the seismic swarm area northeast of the Noto Peninsula (Okada et al., 2024a ), S-wave splitting was investigated using temporary and permanent stations in the epicentral area. Seismic wave velocity tomography analysis was also conducted to investigate the average isotropic velocity structure in the onshore source region. The key questions are as follows: 1) Can we observe a preexisting structure related to inversion tectonics in the focal area? 2) How did fluid affect the Noto Peninsula earthquake at the hypocenter and along the focal area of the multifault rupture? 2. Data and methods Waveform data were collected from 20 temporary stations in the source region from the end of June 2022 to April 2024 (Sakai et al., 2022; Okada et al., 2024b) and from 70 permanent stations of Kyoto University, University of Tokyo, Nagoya University, AIST, NIED and JMA. Seismographs were short-period (natural frequency: 1 Hz or 2 Hz) at most temporary and permanent stations and broadband at one permanent station. The sampling frequency was 100 Hz or 250 Hz. For S-wave splitting, MFAST (Savage et al., 2010), was used. It is based on Silver and Chan (1991), and performs a cluster analysis of the results of analyses with time windows of various locations and lengths. The measurements were graded from A (best) to D (worst) depending on the consistency between the measurements in different time windows (Savage et al., 2010 for details). This approach is expected to provide a stable and fast estimation of the S-wave anisotropy. We used MFAST on waveform data from permanent and temporary stations in the epicenter area from January 1st to March 15th, 2024. A previous study in the same area (Okada et al., 2024a) examined data from June 2022 to May 8th, 2023. Figure S1 shows the measurements from 2019 through 2024. The coverage of data becomes better because the aftershock area becomes wider. Waveforms with an incident angle greater than 35 degrees at the surface were not used in the analysis because of phase shifts of the waveform due to S-to-P conversion at the free surface. The incident angles were calculated with a velocity structure (Ueno et al., 2002) that had an S-wave velocity of 2.84 km/s at the surface. We examined 10162 waveforms (event‒station pairs), of which 2217 were grade A, and used in the analysis. The double-difference tomography method was used for the seismic wave velocity structure (Zhang and Thurber, 2003, 2006); 2282 earthquakes from February 2020 to March 15th, 2024, were used in the analysis (Fig. 2). We excluded all offshore events that had a ray coverage of less than 180 degrees. The initial hypocenter locations, structure, and arrival time data were obtained from the Japan Meteorological Agency (JMA) catalog with the JMA one-dimensional velocity model (Ueno et al., 2002), and the arrival times at temporary stations were obtained via manual picking for M > 1.5 events and the automated arrival time picking system of Horiuchi et al. (2014) for smaller-magnitude events. Fig. 2 shows the hypocenter and station map. We used a grid with horizontal intervals of 5.6 km at N340°E and 4.3 km at N70°E and depth nodes at depths of 0, 3, 6, 12, 18, 24, 30, 36, and 48 km. Compared with the initial model, the root-mean-square of the arrival time residual in the final tomographic model decreased from 0.246 seconds to 0.088 seconds. We estimated that the spatial resolution of the obtained model ranged from one grid (approximately 6 km) in the area of dense seismicity to two grids (approximately 12 km) in the surrounding area based on a checkerboard resolution test (CRT) (Fig. S2). The hypocenter location error, estimated through the CRT, was approximately 100 m horizontally and 280 m vertically. An isotropic velocity was assumed in this seismic velocity tomography, which can be obtained when there is good recovery of the checkerboard pattern (c.f., Zhao et al. 2016). 3. Results 3.1 Spatial distribution of the S-wave polarization anisotropy Figure 3 shows the spatial distribution of the S-wave polarization anisotropy. To characterize the spatial distribution of the anisotropy, we used the TESSA (Johnson et al., 2011 ) method to obtain a two-dimensional spatial average of the anisotropy direction, weighting the FSODs by the inverse of the distance (from the station to the grid cell) squared and assigning them to each grid block that each ray passed through. We plot rose diagram of the FSODs in grids and indicate the mean direction (computed using circular statistics) for each grid block only if the standard deviation of the data is less than 30° and the standard error of the mean is less than 10°. These criteria allow us to exclude blocks that exhibit large scatter or multiple modes. We used a series of boxes with a size of 2500 m. We did not plot results with fewer than 10 rays passing through the block. As shown in Fig. 2 , the dominant anisotropy direction throughout the peninsula was NE‒SW. In addition, NW‒SE anisotropy existed locally (e.g., at some grids in the northeastern Peninsula, as in Okada et al. ( 2024a )). A north‒south anisotropy was observed in the central part of the northern peninsula. The near-neighbor method of GMT (Wessel et al., 2019 ) was used to estimate the spatial distribution of the magnitude of the anisotropy. We used a regular grid of 5 km with a 10 km search radius centered on each grid, comparable to the block size, to estimate the average FSOD by using TESSA. From the obtained spatial distribution (Fig. 3 ), regions of relatively large anisotropy (1.0–2.0%, blue to yellow on the color scale) were identified in the eastern and western parts of the northern Noto Peninsula. 3.2 Seismic wave velocity structure Figures 4 and S3 show vertical cross-sections of Vp/Vs and dVs (perturbation of Vs from the average value at each depth) across the focal area. High-Vp/Vs and low-Vs regions were identified beneath or at deeper extensions of the hypocenters, and the assumed or modeled faults in the hypocentral area were at a depth of approximately 18 km. Slightly larger P-wave velocities were obtained in this high-Vp/Vs and low-Vs region than in the surrounding area. Figure 5 shows vertical cross sections of Vp across the focal area. The high-Vp/Vs regions at a depth of approximately 18 km tended to have high Vp values. In the shallow crust, the hanging wall of the fault, determined from the aftershock alignment and the fault plane, had low velocities in most cross sections. However, in F and K, where large coseismic slip occurred, as suggested by a large InSAR displacement, the hanging wall side had a higher velocity than the other cross sections. 4. Discussion The dominant fast direction of anisotropy throughout the peninsula is NE‒SW (Fig. 3 ). This direction generally coincides with the strike direction of faults and folds and may be due to anisotropy caused by theirstructure. In addition, local NW–SE anisotropy (e.g., at some grids in the northeastern Peninsula, as in Okada et al. ( 2024a )) generally coincides with the direction of the axis of maximum horizontal compression (e.g., Terakawa and Matsu'ura, 2010; Matsumoto et al., 2024 ; Tagami and Okada, 2024 ; Takano et al., 2024 ; Tagami et al., 2025). North‒south anisotropy is observed in the central part of the northern peninsula. This spatial variation may be related to the surface geology. The N–S anisotropy is correlated with surface volcanic rocks. This N–S anisotropy may have two possible causes: 1) This anisotropy is due to N–S-striking faults recognized in the surface geology (Fig. 1 ). Some of the N–S-striking faults correspond to transfer faults that originally formed in the Miocene at the time of rifting and normal faulting. These N–S-striking faults are spatially limited in the surface geology, but they may be prevalent in the subsurface. 2) The area of N–S anisotropy corresponds to volcanic rock according to the surface geology. If N–S-striking dykes corresponding to a N–S compressive (E–W extensional) stress were formed during the rifting stage prevalent in the subsurface under volcanic rock, then these groups of dykes could be macroscopically N–S anisotropic media. We observed a heterogeneous distribution of the amount of anisotropy (Fig. 3 ). The large anisotropy region in the eastern part corresponds to the focus of the preceding earthquake swarm and the hypocenter of the 2024 mainshock. The western region of high anisotropy corresponds to the western edge of the focal area of the 2024 earthquake, which also corresponds to the source region of the 2007 M6.9 Noto–Hanto earthquake. These high anisotropy areas could be highly fractured areas with highly pressurized fluids. The temporal change in shear wave splitting due to large earthquakes is under debate (e.g., Graham et al., 2020 ; Okada et al., 2024a ). We examined the temporal difference in shear wave splitting, as shown in Fig. S2 . The orientation of the anisotropy did not significantly or systematically change, but there may have been an increase in the degree of anisotropy along the northeastern part of the peninsula during the 7 months before and 3 months after the large M7.6 earthquake. There may also be an increase near the southwestern part near the 2007 Noto–Hanto earthquake (its hypocenter is shown by the white star). This coseismic increase in anisotropy for the M7.6 earthquake could be spatially correlated with the decrease in shear wave velocity caused by changes in coseismic strain (Paris et al., 2025). However, this change might be apparent due to temporal changes in the hypocenter distribution, i.e., the sampling area (e.g., Okada et al., 2024a ). In future studies, we need to carefully examine the temporal changes in shear wave splitting. In the shallow crust at depths of less than 15 km, the hanging wall of the fault has low velocities. Similar shallow low-velocity areas can be seen in the 2003 northern Miyagi earthquake (Okada et al., 2007a ), the 2004 Niigata–Chuetsu earthquake (Okada et al., 2006 ), and the 2008 Iwate–Miyagi Nairiku earthquake (Okada et al., 2012). This shallow low-velocity zone may be interpreted as the remaining structure of a normal fault that formed during the rifting stage of the Miocene. The low velocity in the shallow part of the hanging wall side of the fault and the anisotropy due to structural properties indicate a complex structure in which the structure of normal faults that developed in this region coexists with a compressional structure due to the present stress field. Note that in cross sections F and K of Fig. 5 , the hanging wall side has a higher velocity than in the other cross sections. In cross sections F and K, large coseismic slip events occurred, as suggested by the large InSAR displacement. This correlation between high seismic velocity and large coseismic slip was reported by Takagi et al. (2025) and supports the hypothesis that a high-velocity area could act as an asperity that can accumulate large strains and result in large coseismic slip (e.g., the 2004 Niigata–Chuetsu earthquake (Okada et al., 2006 ) and the 1995 southern Hyogo earthquake (Okada et al., 2007b)). On the other hand, a region of low Vs, high Vp and high Vp/Vs was identified deep in the hypocentral area, suggesting a relationship with fluids. The values of Vp and Vs in the anomalous region range from approximately 5.4–6.4 km/s and 3.2–3.4 km/s, respectively. One possible interpretation is that the anomaly contains mafic granulite that was caused by a Miocene volcanic intrusion. According to Christensen ( 1996 ), the Vp, Vs, and Vp/Vs of mafic granulite at 600 MPa are 6.94, 3.82, and 1.82, respectively. These values are greater than the observed values but could be decreased by the presence of fluid. According to relations published in Takei ( 2002 ), our measured value of dlnVs/dlnVp = 1.43 corresponds to a pore aspect ratio of approximately 0.02 if we assume that the liquid in the pore is water or 0.04 if we assume a melt at a depth of 18 km. The volume fraction is estimated to be approximately 1% or 0.5% if the pore fluid is water or a melt, respectively. There are two possible sources of fluid: water from cooled and solidified Tertiary magma or upwelling from a deeper part (e.g., the mantle). The first possibility is suggested by a relatively large helium isotope ratio (He3/He4) value of over 1.7 (air-corrected) (Umeda et al., 2009 , 2024 ), and the latter is suggested by the regional seismic velocity tomography, which shows seismic low velocity area beneath the Peninsula in the uppermost mantle (Nakajima, 2023). Tertiary igneous rocks are distributed in the target area, suggesting that the high-Vp/Vs region may represent an old magma reservoir and that fluids released from/through it were involved in the 2024 Noto Peninsula earthquake as well as in the swarm seismic activity that preceded the 2024 earthquake. Various fault models have been proposed for the Noto Peninsula earthquake, but Yamada et al. ( 2024 , 2025) carefully checked the dense GNSS and InSAR displacements by combining the aftershock distributions and proposed an interesting model with a high dip angle in the shallow part (< 5–10 km) of the crust and a low dip angle in the deep part. This depth dependency of the fault dip angle could be interpreted as a balance between the reactivation of a nonoptimally oriented fault with a high dip angle and the formation of a new optimally oriented fault with a low dip angle (e.g., Sibson, 2009 ; Sibson, 2012 ). Under pore-fluid pressure, at shallower depths, the reactivation of a nonoptimally oriented fault with a high dip angle would be dominant, whereas at greater depths, the formation of a new optimally oriented (dipping) fault would be dominant. According to Sibson and Ghisetti’s ( 2018 ) assumption, when the pore water pressure is approximately 50% of the lithostatic pressure, a cohesionless old normal fault will be active as a reverse fault on the side shallower than approximately 5–10 km in depth, whereas a new fault will be generated on the deeper side. The high Vp/Vs values observed in the seismic wave velocities are qualitative evidence of such high pore pressures. The faults associated with the 2024 Noto Peninsula earthquake may have been arterial faults driven synchronously by overpressured fluid, as suggested for previous earthquakes (e.g., Sibson, 2019; Okada et al., 2010, 2012, 2014 for the 2008 M7.2 Iwate–Miyagi Nairiku Earthquake; Okada et al., 2024c for the 2016 M7.8 Kaikoura earthquake). Areas with a large degree of anisotropy over 1% were found in the hypocenter of the 2024 Noto Peninsula earthquake and near the focal area of the 2007 Noto–Hanto earthquake (white star in Fig. 3 ). These areas could also be heavily stressed by a high fluid pressure near the focal area and hypocenters, such as in the cases of a sequential earthquake, e.g., the “Amatrice–Visso–Norcia Seismic Sequence” reported by Pastori et al. ( 2019 ). Alternatively, the breaking of rock by the mainshock and aftershocks could have opened cracks that increased the anisotropy, as has been suggested for some other regions (e.g., swarms at Whakaari volcano in New Zealand: Mengesha et al., 2024). From the Vp/Vs cross sections in Fig. 4 , the seismicity seems confined to the blue and green sections, i.e., low and medium Vp/Vs, with very little seismicity in the high-Vp/Vs areas—some of which are immediately under regions of high seismicity. The average value of Vp/Vs at the locations of hypocenters is \(\:1.69\pm\:0.02\) , which is smaller than that for all the area of \(\:1.73\pm\:0.09\) . Large amounts of fluid could make the area more plastic than brittle. For example, in quartz-rich shear zones, the fluid-filled porosity significantly weakens, resulting in plastic deformation (e.g., Okazaki et al., 2020). 5. Conclusions We obtained the fine seismic velocity and seismic anisotropy structure in the inland focal area of the 2024 Noto Peninsula earthquake by using data from dense temporary seismic observations. The lower seismic velocity hanging wall structure caused by ancient normal faulting and faulting-controlled anisotropy suggest that the focal area was highly deformed during a long geological history since the Miocene. Highly pressurized fluid observed in high-Vp/Vs regions may have promoted the occurrence of multifault rupture during the 2024 Noto Peninsula earthquake and the seismic swarm that preceded it. The results suggest that an understanding of the complex structure of faults and the role of pore fluid pressure would enable assessment of fault activity in compressional inversion areas, which is generally difficult (e.g., Ghisetii and Sibson, 2018 ). Abbreviations AIST: National Institute of Advanced Industrial Science and Technology CRT: checkerboard resolution test FSOD: fast shear wave oscillation direction JMA: Japan Meteorological Agency MFAST: Multiple Filter Automatic Splitting Technique (Savage et al., 2010) NIED: National Research Institute for Earth Science and Disaster Resilience SC91: Method of Silver and Chan (1991) SHmax: maximum horizontal compressive stress Declarations Ethics approval and consent to participate Not applicable Consent for publication Not applicable Availability of data and materials The data supporting the findings of this study are available from the corresponding author, Tomomi Okada, upon request. Competing interests The authors declare that they have no conflicts of interest. Funding This work was conducted with Grants-in-Aid for Special Purposes (22K19949 and 23K17482), the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and the Japan Society for the Promotion of Science, Japan. This study was also supported by MEXT of Japan under its Earthquake and Volcano Hazards Observation and Research Program (project numbers THK_02 and THK_07 in 2022–2024 and THK_12 in 2025). Authors' contributions TO: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing - original draft. MKS: Supervision, Writing - review & editing. AT: Conceptualization, Investigation, Writing - review & editing. RF: Conceptualization, Investigation, Writing - review & editing. RT: Conceptualization, Data curation, Supervision, Writing - review & editing. KY: Conceptualization, Data curation, Supervision, Writing - review & editing. SM: Conceptualization, Data curation, Supervision, Writing - review & editing. KE: Conceptualization, Data curation, Supervision, Writing - review & editing. YY: Conceptualization, Data curation, Supervision, Writing - review & editing. KK: Conceptualization, Data curation, Supervision, Writing - review & editing. TM: Conceptualization, Supervision, Writing - review & editing. IM: Conceptualization, Supervision, Writing - review & editing. SS: Conceptualization, Data curation, Supervision, Writing - review & editing. MM: Conceptualization, Data curation, Supervision, Writing - review & editing. HY: Conceptualization, Supervision, Writing - review & editing. KI: Conceptualization, Data curation, Supervision, Writing - review & editing. NU: Conceptualization, Data curation, Supervision, Writing - review & editing. SH: Data curation. SK: Data curation. TT: Conceptualization, Data curation, Supervision, Writing - review & editing. MO: Conceptualization, Data curation, Supervision, Writing - review & editing. T. Shibutani: Conceptualization, Data curation, Supervision, Writing - review & editing. T Shi’ina: Conceptualization, Data curation, Supervision, Writing - review & editing. YH: Project administration, Supervision, Writing - review & editing. Acknowledgments We gratefully acknowledge local governments and landowners, who allowed access to their land and the installation of seismic stations. We used seismological data from the JMA. The program codes of the double-difference tomography series were kindly provided by Prof. Haijiang Zhang and Prof. Clifford H. Thurber. Mr. Kohtaro R. Araragi kindly provided his program code for converting the waveform data. Discussion of compressional inversion with Prof. Richard Sibson and Prof. Francesca Ghisetti benefited the promotion of this study. We are grateful to the referee and to the reviewers for their useful comments. Authors' information TO, Professor at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University MS, Professor at the School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand, AT, Researcher at AIST RF, Graduate student at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University RT, Associate Professor at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University KY, Associate Professor at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University SM, Professor at Kyushu University KM, Associate Professor at Kyushu University YY, Professor at Nagoya University KK, Associate Professor at Hokkaido University TM, Professor at Hirosaki University MI, Lecturer at Yamagata University SS, Professor at the Interfaculty Initiative in Information Studies and Graduate School of Interdisciplinary Information Studies, University of Tokyo MM, Professor at Kyoto University HY, Associate Professor at Kagoshima University KI, Deputy director of AIST NU, Professor at the University of Tokyo SH, Technical Staff at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University SK, Technical Staff at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University TT, Professor at Nagoya University MO, Professor at Hokkaido University T. Shibutani, Professor Emeritus at Kyoto University T. Shiina, Senior Researcher at AIST YH, Professor with the Faculty of Geosciences and Civil Engineering, Institute of Science and Engineering, Kanazawa University Endnotes No endnotes. References Amezawa, Y., Hiramatsu, Y., Miyakawa, A., Imanishi, K., Otsubo, M. (2023) Long‐Living Earthquake Swarm and Intermittent Seismicity in the Northeastern Tip of the Noto Peninsula, Japan. Geophys. Res. Lett. 50: doi:10.1029/2022GL102670 Aster, R. C., Shearer, P. M., & Berger, J. (1990) Quantitative measurements of shear wave polarizations at the Anza seismic network, southern California: implications for shear wave splitting and earthquake prediction. Journal of Geophysical Research, 95(B8): 12,449-412,473. Boness, N.L., Zoback, M.D., 2006. Mapping stress and structurally controlled crustal shear velocity anisotropy in California. Geology 34: 825. doi:10.1130/G22309.1 Christensen, N.I. (1996) Poisson’s ratio and crustal seismology. J. Geophys. 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Gondwana Res. 33: 24-43. doi:10.1016/j.gr.2015.05.008 Supplementary Files GraphicalAbstractnoto2250423.jpg supfigurenoto2mks250526mksTOnotrack.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Takuto","middleName":"","lastName":"Maeda","suffix":""},{"id":469127655,"identity":"6ebf69a2-d2b4-4be3-9df3-33d76f780029","order_by":11,"name":"Motoko Ishise","email":"","orcid":"","institution":"Yamagata University: Yamagata Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Motoko","middleName":"","lastName":"Ishise","suffix":""},{"id":469127656,"identity":"12178301-bbba-496f-9b42-08d9c3e2e73b","order_by":12,"name":"Shin'ichi Sakai","email":"","orcid":"","institution":"University of Tokyo: Tokyo Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Shin'ichi","middleName":"","lastName":"Sakai","suffix":""},{"id":469127657,"identity":"8a0ab793-efaf-43e9-9c37-cfffe49ad9ad","order_by":13,"name":"Masatoshi Miyazawa","email":"","orcid":"","institution":"Kyoto University: Kyoto Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Masatoshi","middleName":"","lastName":"Miyazawa","suffix":""},{"id":469127658,"identity":"b9711930-2430-4439-b016-b51b188b0227","order_by":14,"name":"Hiroshi Yakiwara","email":"","orcid":"","institution":"Kagoshima University: Kagoshima Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Hiroshi","middleName":"","lastName":"Yakiwara","suffix":""},{"id":469127659,"identity":"1df2a237-8738-4aa6-9204-a268a86471eb","order_by":15,"name":"Kazutoshi Imanishi","email":"","orcid":"","institution":"National Institute of Advanced Industrial Science and Technology Tsukuba Center: Tokai Daigaku Kaiyo Gakubu Daigakuin Kaiyogaku Kenkyuka","correspondingAuthor":false,"prefix":"","firstName":"Kazutoshi","middleName":"","lastName":"Imanishi","suffix":""},{"id":469127660,"identity":"b7a14673-f7b7-4b3c-9435-f3466bd7d19b","order_by":16,"name":"Naoki Uchida","email":"","orcid":"","institution":"University of Tokyo: Tokyo Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Naoki","middleName":"","lastName":"Uchida","suffix":""},{"id":469127661,"identity":"dd544c72-2582-4f7b-a46e-6346091f93a6","order_by":17,"name":"Satoshi Hirahara","email":"","orcid":"","institution":"Tohoku University: Tohoku Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Satoshi","middleName":"","lastName":"Hirahara","suffix":""},{"id":469127662,"identity":"f179652f-b605-4eab-95fc-da67b6260194","order_by":18,"name":"Shu'utoku Kimura","email":"","orcid":"","institution":"Tohoku University: Tohoku Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Shu'utoku","middleName":"","lastName":"Kimura","suffix":""},{"id":469127663,"identity":"9da96ba8-861f-42ec-9fe7-1b9f80f1a5a1","order_by":19,"name":"Toshiko Terakawa","email":"","orcid":"","institution":"Nagoya University: Nagoya Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Toshiko","middleName":"","lastName":"Terakawa","suffix":""},{"id":469127664,"identity":"2bb6e4f8-3fdb-41c4-b24b-db5209e1b0d9","order_by":20,"name":"Mako Ohzono","email":"","orcid":"","institution":"Hokkaido University: Hokkaido Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Mako","middleName":"","lastName":"Ohzono","suffix":""},{"id":469127665,"identity":"354dc4d5-8fbf-4142-9560-ec016a9398da","order_by":21,"name":"Takuo Shibutani","email":"","orcid":"","institution":"Kyoto University: Kyoto Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Takuo","middleName":"","lastName":"Shibutani","suffix":""},{"id":469127666,"identity":"46def825-b62f-41f4-ae21-315a49ff2a28","order_by":22,"name":"Takahiro Shiina","email":"","orcid":"","institution":"National Institute of Advanced Industrial Science and Technology Tsukuba Center: Tokai Daigaku Kaiyo Gakubu Daigakuin Kaiyogaku Kenkyuka","correspondingAuthor":false,"prefix":"","firstName":"Takahiro","middleName":"","lastName":"Shiina","suffix":""},{"id":469127667,"identity":"ee0c8f21-e784-459e-8eae-a6fb7e4ec9c7","order_by":23,"name":"Yoshihiro Hiramatsu","email":"","orcid":"","institution":"Kanazawa University: Kanazawa Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Yoshihiro","middleName":"","lastName":"Hiramatsu","suffix":""}],"badges":[],"createdAt":"2025-05-02 08:47:21","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6576798/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6576798/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":84452329,"identity":"e4b3c928-4552-4bcd-9986-f122bd8ffd17","added_by":"auto","created_at":"2025-06-12 07:03:52","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":374793,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Geological map from Sawada et al. (2012). The F-net moment tensor solution for the 2024 Noto Peninsula earthquake (\u003ca href=\"https://www.fnet.bosai.go.jp/event/tdmt.php?_id=20240101070900\u0026amp;LANG=ja\"\u003ehttps://www.fnet.bosai.go.jp/event/tdmt.php?_id=20240101070900\u0026amp;LANG=ja\u003c/a\u003e) is also shown. The broken red ellipse is a possible Miocene caldera (Sawada and Hiramatsu, 2022; Okada et al., 2024). The red lines denote the faults from Sawada et al. (2012) and the Geological Survey of Japan (2025). Segments M (Monzen-oki), Sa (Saruyama-oki), Wa (Wajima-oki), and Su (Suzu-oki) are approximately bounded by broken black lines. (b) Epicenter map from 12/01/2020–02/08/2024 (modified from JMA, 2024). The events before and after the M7.6 earthquakes are shown in black and red, respectively. The focal mechanisms of some major earthquakes are also shown. The bottom is a magnitude–time plot for region a.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6576798/v1/2f4a458d53b81b0bb456ae02.png"},{"id":84452330,"identity":"d7d6257e-255f-431c-84f2-38e0b5332786","added_by":"auto","created_at":"2025-06-12 07:03:52","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":340118,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Hypocenter distribution. The hypocenters are indicated by colored circles in the map view. The panels above and to the side of the map are the E‒W and N‒S cross sections, respectively. The color and size of the circles denote the earthquake depth and magnitude. (b) Station distribution along with the tomography grid. The stations are plotted as squares in a map view. Red, blue, and open squares denote the temporary stations operated by Sakai et al. (2022), temporary stations operated by Okada et al. (2024), and permanent stations (routinely operated by Kyoto University, University of Tokyo, AIST, NIED and JMA), respectively.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6576798/v1/0c21ef189c4e1bf022041352.png"},{"id":84452332,"identity":"d8d711e8-ee7d-4472-90cf-261e696c9978","added_by":"auto","created_at":"2025-06-12 07:03:52","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":378157,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Spatial average of the polarization of the fast shear wave from shear wave splitting calculated by inverse weighting proportional to the square of the distance from the station, using the TESSA spatial averaging code (Johnson et al., 2011). The red rose diagram is normalized, and the yellow bars represent the average polarization. Rose diagrams are plotted at the center of each block; blocks with fewer data than the threshold are not plotted. The background gray represents the shaded topography. The rose diagram of all the FSODs is also shown at the top left. (b) Spatial average of the degree of anisotropy. The solid, open large, and open small stars denote the epicenters of the 2007 M6.9 Noto–Hanto, 2024 M7.6 Noto Peninsula, and 2023 M6.5 earthquakes, respectively.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6576798/v1/a737a063edd488feb830a77b.png"},{"id":84452331,"identity":"f5be6e1f-b15b-4148-842d-8e254466d9fe","added_by":"auto","created_at":"2025-06-12 07:03:52","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":462302,"visible":true,"origin":"","legend":"\u003cp\u003eResults of seismic velocity tomography: cross-sections along lines A to L for Vp/Vs with two fault models: red is the assumed fault model based on the topography and subsurface shallow geological structure from the Nihon-kai Earthquake and Tsunami Research Project (2015), and blue is the observed fault model from Yamada et al. (2024) of the fault planes of the Jan. 1st, 2024, mainshock. The black and red stars in D denote the M5.5 earthquake at 16:06 on January 1st, 2024, and the M7.6 earthquake at 16:10 on January 1st, 2024, respectively. The white stars and the black stars denote the earthquakes with magnitude greater than or equal to M5.4, before the M7.6 earthquake and after the M7.6 earthquake, respectively. The white dots and the black dots in all panels denote the earthquakes with magnitudes smaller than M5.4, from 2019 before the M7.6 earthquake and after the M7.6 earthquake, respectively. The bottom right figure shows the seismic displacement obtained by InSAR analysis (green and red indicate no uplift and approximately 4 m uplift, respectively) (Geospatial Information Authority of Japan, 2024).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6576798/v1/3fe54428b0deb634ba5f0b1d.png"},{"id":84452333,"identity":"fb51c27c-1900-4488-b4d2-aa8913f67b93","added_by":"auto","created_at":"2025-06-12 07:03:52","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":486324,"visible":true,"origin":"","legend":"\u003cp\u003eResults of seismic velocity tomography: cross-sections along lines A to L for Vp with two fault models: red is the fault model from the Nihon-kai Earthquake and Tsunami Research Project (2015), and blue is the fault model from Yamada et al. (2024). The black and red stars in D denote the M5.5 earthquake at 16:06 on January 1st, 2024, and the M7.6 earthquake at 16:10 on January 1st, 2024, respectively. The white stars and the black stars denote the earthquakes with magnitude greater than or equal to M5.4, before the M7.6 earthquake and after the M7.6 earthquake, respectively. \u0026nbsp;The gray dots and the black dots in all panels denote earthquakes with magnitudes smaller than M5.4, from 2019 before the M7.6 earthquake and after the M7.6 earthquake, respectively. The bottom right figure shows the seismic displacement obtained by InSAR analysis (green and red indicate no uplift and approximately 4 m uplift, respectively) (Geospatial Information Authority of Japan, 2024).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6576798/v1/30323ffff10397a2a47e4170.png"},{"id":85918077,"identity":"709bd0c9-d03c-4155-bff0-e5912ac30f31","added_by":"auto","created_at":"2025-07-03 07:19:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2638466,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6576798/v1/5654e239-27ef-4938-99bb-4bcad0c8f748.pdf"},{"id":84453025,"identity":"9726e824-499e-4017-91b1-8e70d9d895b5","added_by":"auto","created_at":"2025-06-12 07:11:52","extension":"jpg","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":44828,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstractnoto2250423.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6576798/v1/2877e799f523765f1a6860b8.jpg"},{"id":84452345,"identity":"97348cd6-1e95-4a2e-b519-a37b6e040c23","added_by":"auto","created_at":"2025-06-12 07:03:52","extension":"docx","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":990813,"visible":true,"origin":"","legend":"","description":"","filename":"supfigurenoto2mks250526mksTOnotrack.docx","url":"https://assets-eu.researchsquare.com/files/rs-6576798/v1/f73284e0573ce44302d219ee.docx"}],"financialInterests":"","formattedTitle":"Shear wave splitting and seismic velocity structure in the onshore focal area of the 2024 Noto Peninsula earthquake, central Japan, as a fluid-promoted multifault rupture in a compressional inversion area","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe M7.6 Noto Peninsula earthquake occurred on 1 January 2024 in the Noto region of Ishikawa Prefecture (e.g., Fujii and Satake, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Okuwaki et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Seismic activity in the area has increased since approximately August 2020. Possible causes of this increased seismic activity include stress changes due to crustal deformation and increased fluid pressure. These changes have been inferred from regional-scale seismic tomography (e.g., Nakajima et al., 2022) and electrical resistivity structures (e.g., Yoshimura et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), and spatio-temporal changes in\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ecrustal deformation (Nishimura et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2023\u003c/span\u003e),: hypocenters (e.g., Amezawa et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Kato, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Yoshida et al., \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e, 2023b, \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), static stress drops (Fukuoka et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), focal mechanisms, and local stress fields (Takano et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Matsumoto et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn our previous studies at the eastern end of the Noto Peninsula (Okada et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e; Okada and Joint inland aftershock observation group for the 2024 Noto Peninsula Earthquake, 2024), we found a high-Vp/Vs body with high Vp and low Vs beneath a low gravity (Bouguer) anomaly (Honda et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Sawada et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Sawada and Hiramatsu, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) at the bottom of the swarm seismicity and interpreted it as an ancient magma body.\u003c/p\u003e \u003cp\u003eSeismic anisotropy could provide clues for understanding the stress or fault structure of a focal area. Under differential stress, the preferred alignment of an opened planar crack, in which the plane is aligned with the maximum horizontal stress orientation (SHmax), could cause stress-induced anisotropy (e.g., Nur and Simmons, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1969\u003c/span\u003e; Nur, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1971\u003c/span\u003e; Crampin, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Aster et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Savage et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Gledhill, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Aster and Shearer, 1992; Okada et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Hiramatsu et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Savage et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Hiramatsu and Iidaka, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Fault parallel alignment of fault fabrics and minerals could cause structure-controlled anisotropy (e.g., Savage et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Sayers, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Boness and Zoback, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Furthermore, an anticline folding structure could be manifested as macroscopic anisotropy for seismic waves whose wavelengths are longer than the characteristic length of the structure (Okaya et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). We observed seismic anisotropy using shear wave splitting (e.g., Savage, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). In shear wave splitting, two split shear waves are observed, and the fast shear wave oscillation direction (FSOD) tends to be parallel to the SHmax orientation or the strike of the fault and anticline axis. In the eastern part of the Noto Peninsula (Okada et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e), where SHmax is oriented NW\u0026ndash;SE (e.g., Terakawa and Matsu\u0026rsquo;ura, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Matsumoto et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Tagami and Okada, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Takano et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Tagami et al., 2025), we found stress-induced anisotropy with an FSOD of NW\u0026ndash;SE in the southern part of the swarm seismicity and structurally controlled anisotropy with an FSOD of E‒W in the north, where the swarm was more active.\u003c/p\u003e \u003cp\u003eIn the focal area, Tertiary sedimentary and volcanic rocks are widely distributed (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Sawada et al., 2013). This finding suggests that the focal area was ina rift zone in the Miocene when the Japan Sea opened (e.g., Okamura et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Okamura, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). At present, faulting and deformation are thought to have occurred in the tectonic inversion stage, and old originally formed unfavorably oriented normal faults and optimally oriented reverse faults coexist (e.g., Sibson and Ghisetti, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). In the case of the 2003 northern Miyagi earthquake (M6.4), NE Japan, Okada et al. (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2007a\u003c/span\u003e) reported that the hanging wall of the fault has a lower seismic velocity than the footwall. In particular, this situation is considered to reflect the existence of a thick sedimentary layer formed on the hanging wall.\u003c/p\u003e \u003cp\u003eMajor faults exist along the northern coastline of the peninsula. There are four segments from east to west, Suzu-oki, Wajima-oki, Saruyama-oki, and Monzen-oki, inferred from the continuity of each fault (Inoue and Okamura, 2010). Some studies have suggested that the 2007 Noto-Hanto earthquake (M6.9) corresponded to the Monzen-oki segment (e.g., Hiramatsu et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and that the Wajima-oki segment ruptured during a historical earthquake in 1729 (Hamada et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSeveral faults successively ruptured during the 2024 Noto Peninsula earthquake (e.g., Fujii and Satake, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Okuwaki et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Tagami and Okada (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) and Tagami et al. (2025) reported that most of the 2024 Noto Peninsula earthquake faults had a high slip tendency toward the stress field before the 2024 earthquake and thus interpreted the faults as being under conditions that could cause multiple fault ruptures (e.g., Matsuno et al., 2020 for the 2016 M7.8 Kaikoura earthquake).\u003c/p\u003e \u003cp\u003eIn this study, following an analysis of the seismic swarm area northeast of the Noto Peninsula (Okada et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e), S-wave splitting was investigated using temporary and permanent stations in the epicentral area. Seismic wave velocity tomography analysis was also conducted to investigate the average isotropic velocity structure in the onshore source region. The key questions are as follows: 1) Can we observe a preexisting structure related to inversion tectonics in the focal area? 2) How did fluid affect the Noto Peninsula earthquake at the hypocenter and along the focal area of the multifault rupture?\u003c/p\u003e"},{"header":"2. Data and methods","content":"\u003cp\u003eWaveform data were collected from 20 temporary stations in the source region from the end of June 2022 to April 2024 (Sakai et al., 2022; Okada et al., 2024b) and from 70 permanent stations of Kyoto University, University of Tokyo, Nagoya University, AIST, NIED and JMA. Seismographs were short-period (natural frequency: 1 Hz or 2 Hz) at most temporary and permanent stations and broadband at one permanent station. The sampling frequency was 100 Hz or 250 Hz.\u003c/p\u003e\n\u003cp\u003eFor S-wave splitting, MFAST (Savage et al., 2010), was used. \u0026nbsp; It is based on Silver and Chan (1991), and performs a cluster analysis of the results of analyses with time windows of various locations and lengths. The measurements were graded from A (best) to D (worst) depending on the consistency between the measurements in different time windows (Savage et al., 2010 for details). This approach is expected to provide a stable and fast estimation of the S-wave anisotropy. We used MFAST on waveform data from permanent and temporary stations in the epicenter area from January 1st to March 15th, 2024. A previous study in the same area (Okada et al., 2024a) examined data from June 2022 to May 8th, 2023. Figure S1 shows the measurements from 2019 through 2024. The coverage of data becomes better because the aftershock area becomes wider.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Waveforms with an incident angle greater than 35 degrees at the surface were not used in the analysis because of phase shifts of the waveform due to S-to-P conversion at the free surface. The incident angles were calculated with a velocity structure (Ueno et al., 2002) that had an S-wave velocity of 2.84 km/s at the surface. We examined 10162 waveforms (event‒station pairs), of which 2217 were grade A, and used in the analysis.\u003c/p\u003e\n\u003cp\u003eThe double-difference tomography method was used for the seismic wave velocity structure (Zhang and Thurber, 2003, 2006); 2282 earthquakes from February 2020 to March 15th, 2024, were used in the analysis (Fig. 2). We excluded all offshore events that had a ray coverage of less than 180 degrees. The initial hypocenter locations, structure, and arrival time data were obtained from the Japan Meteorological Agency (JMA) catalog with the JMA one-dimensional velocity model (Ueno et al., 2002), and the arrival times at temporary stations were obtained via manual picking for M \u0026gt; 1.5 events and the automated arrival time picking system of Horiuchi et al. (2014) for smaller-magnitude events. Fig. 2 shows the hypocenter and station map.\u0026nbsp;We used a grid with horizontal\u0026nbsp;intervals\u0026nbsp;of 5.6 km\u0026nbsp;at N340°E\u0026nbsp;and 4.3 km at N70°E\u0026nbsp;and depth nodes at\u0026nbsp;depths of\u0026nbsp;0, 3, 6, 12, 18, 24, 30, 36, and 48 km. Compared\u0026nbsp;with\u0026nbsp;the initial model, the root-mean-square of the arrival time residual in the final tomographic model decreased from 0.246 seconds to 0.088 seconds. We estimated that the spatial resolution of the obtained model ranged from one grid (approximately 6 km) in the area of dense seismicity to two grids (approximately\u0026nbsp;12 km) in the surrounding area based on a checkerboard resolution test (CRT) (Fig. S2). The hypocenter location error, estimated through the CRT, was\u0026nbsp;approximately\u0026nbsp;100 m horizontally and 280 m vertically.\u003c/p\u003e\n\u003cp\u003eAn isotropic velocity was assumed in this seismic velocity tomography, which can be obtained when there is good recovery of the checkerboard pattern (c.f., Zhao et al. 2016).\u003c/p\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Spatial distribution of the S-wave polarization anisotropy\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the spatial distribution of the S-wave polarization anisotropy. To characterize the spatial distribution of the anisotropy, we used the TESSA (Johnson et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) method to obtain a two-dimensional spatial average of the anisotropy direction, weighting the FSODs by the inverse of the distance (from the station to the grid cell) squared and assigning them to each grid block that each ray passed through. We plot rose diagram of the FSODs in grids and indicate the mean direction (computed using circular statistics) for each grid block only if the standard deviation of the data is less than 30\u0026deg; and the standard error of the mean is less than 10\u0026deg;. These criteria allow us to exclude blocks that exhibit large scatter or multiple modes. We used a series of boxes with a size of 2500 m. We did not plot results with fewer than 10 rays passing through the block.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the dominant anisotropy direction throughout the peninsula was NE‒SW. In addition, NW‒SE anisotropy existed locally (e.g., at some grids in the northeastern Peninsula, as in Okada et al. (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e)). A north‒south anisotropy was observed in the central part of the northern peninsula.\u003c/p\u003e \u003cp\u003eThe near-neighbor method of GMT (Wessel et al., \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) was used to estimate the spatial distribution of the magnitude of the anisotropy. We used a regular grid of 5 km with a 10 km search radius centered on each grid, comparable to the block size, to estimate the average FSOD by using TESSA. From the obtained spatial distribution (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), regions of relatively large anisotropy (1.0\u0026ndash;2.0%, blue to yellow on the color scale) were identified in the eastern and western parts of the northern Noto Peninsula.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Seismic wave velocity structure\u003c/h2\u003e \u003cp\u003eFigures\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and S3 show vertical cross-sections of Vp/Vs and dVs (perturbation of Vs from the average value at each depth) across the focal area. High-Vp/Vs and low-Vs regions were identified beneath or at deeper extensions of the hypocenters, and the assumed or modeled faults in the hypocentral area were at a depth of approximately 18 km. Slightly larger P-wave velocities were obtained in this high-Vp/Vs and low-Vs region than in the surrounding area.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows vertical cross sections of Vp across the focal area. The high-Vp/Vs regions at a depth of approximately 18 km tended to have high Vp values. In the shallow crust, the hanging wall of the fault, determined from the aftershock alignment and the fault plane, had low velocities in most cross sections. However, in F and K, where large coseismic slip occurred, as suggested by a large InSAR displacement, the hanging wall side had a higher velocity than the other cross sections.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe dominant fast direction of anisotropy throughout the peninsula is NE‒SW (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This direction generally coincides with the strike direction of faults and folds and may be due to anisotropy caused by theirstructure. In addition, local NW\u0026ndash;SE anisotropy (e.g., at some grids in the northeastern Peninsula, as in Okada et al. (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e)) generally coincides with the direction of the axis of maximum horizontal compression (e.g., Terakawa and Matsu'ura, 2010; Matsumoto et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Tagami and Okada, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Takano et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Tagami et al., 2025). North‒south anisotropy is observed in the central part of the northern peninsula.\u003c/p\u003e \u003cp\u003eThis spatial variation may be related to the surface geology. The N\u0026ndash;S anisotropy is correlated with surface volcanic rocks. This N\u0026ndash;S anisotropy may have two possible causes: 1) This anisotropy is due to N\u0026ndash;S-striking faults recognized in the surface geology (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Some of the N\u0026ndash;S-striking faults correspond to transfer faults that originally formed in the Miocene at the time of rifting and normal faulting. These N\u0026ndash;S-striking faults are spatially limited in the surface geology, but they may be prevalent in the subsurface. 2) The area of N\u0026ndash;S anisotropy corresponds to volcanic rock according to the surface geology. If N\u0026ndash;S-striking dykes corresponding to a N\u0026ndash;S compressive (E\u0026ndash;W extensional) stress were formed during the rifting stage prevalent in the subsurface under volcanic rock, then these groups of dykes could be macroscopically N\u0026ndash;S anisotropic media.\u003c/p\u003e \u003cp\u003eWe observed a heterogeneous distribution of the amount of anisotropy (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The large anisotropy region in the eastern part corresponds to the focus of the preceding earthquake swarm and the hypocenter of the 2024 mainshock. The western region of high anisotropy corresponds to the western edge of the focal area of the 2024 earthquake, which also corresponds to the source region of the 2007 M6.9 Noto\u0026ndash;Hanto earthquake. These high anisotropy areas could be highly fractured areas with highly pressurized fluids.\u003c/p\u003e \u003cp\u003eThe temporal change in shear wave splitting due to large earthquakes is under debate (e.g., Graham et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Okada et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e). We examined the temporal difference in shear wave splitting, as shown in Fig. \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e. The orientation of the anisotropy did not significantly or systematically change, but there may have been an increase in the degree of anisotropy along the northeastern part of the peninsula during the 7 months before and 3 months after the large M7.6 earthquake. There may also be an increase near the southwestern part near the 2007 Noto\u0026ndash;Hanto earthquake (its hypocenter is shown by the white star). This coseismic increase in anisotropy for the M7.6 earthquake could be spatially correlated with the decrease in shear wave velocity caused by changes in coseismic strain (Paris et al., 2025). However, this change might be apparent due to temporal changes in the hypocenter distribution, i.e., the sampling area (e.g., Okada et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e). In future studies, we need to carefully examine the temporal changes in shear wave splitting.\u003c/p\u003e \u003cp\u003eIn the shallow crust at depths of less than 15 km, the hanging wall of the fault has low velocities. Similar shallow low-velocity areas can be seen in the 2003 northern Miyagi earthquake (Okada et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2007a\u003c/span\u003e), the 2004 Niigata\u0026ndash;Chuetsu earthquake (Okada et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), and the 2008 Iwate\u0026ndash;Miyagi Nairiku earthquake (Okada et al., 2012). This shallow low-velocity zone may be interpreted as the remaining structure of a normal fault that formed during the rifting stage of the Miocene.\u003c/p\u003e \u003cp\u003eThe low velocity in the shallow part of the hanging wall side of the fault and the anisotropy due to structural properties indicate a complex structure in which the structure of normal faults that developed in this region coexists with a compressional structure due to the present stress field.\u003c/p\u003e \u003cp\u003eNote that in cross sections F and K of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the hanging wall side has a higher velocity than in the other cross sections. In cross sections F and K, large coseismic slip events occurred, as suggested by the large InSAR displacement. This correlation between high seismic velocity and large coseismic slip was reported by Takagi et al. (2025) and supports the hypothesis that a high-velocity area could act as an asperity that can accumulate large strains and result in large coseismic slip (e.g., the 2004 Niigata\u0026ndash;Chuetsu earthquake (Okada et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and the 1995 southern Hyogo earthquake (Okada et al., 2007b)).\u003c/p\u003e \u003cp\u003eOn the other hand, a region of low Vs, high Vp and high Vp/Vs was identified deep in the hypocentral area, suggesting a relationship with fluids. The values of Vp and Vs in the anomalous region range from approximately 5.4\u0026ndash;6.4 km/s and 3.2\u0026ndash;3.4 km/s, respectively. One possible interpretation is that the anomaly contains mafic granulite that was caused by a Miocene volcanic intrusion. According to Christensen (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1996\u003c/span\u003e), the Vp, Vs, and Vp/Vs of mafic granulite at 600 MPa are 6.94, 3.82, and 1.82, respectively. These values are greater than the observed values but could be decreased by the presence of fluid. According to relations published in Takei (\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), our measured value of dlnVs/dlnVp\u0026thinsp;=\u0026thinsp;1.43 corresponds to a pore aspect ratio of approximately 0.02 if we assume that the liquid in the pore is water or 0.04 if we assume a melt at a depth of 18 km. The volume fraction is estimated to be approximately 1% or 0.5% if the pore fluid is water or a melt, respectively. There are two possible sources of fluid: water from cooled and solidified Tertiary magma or upwelling from a deeper part (e.g., the mantle). The first possibility is suggested by a relatively large helium isotope ratio (He3/He4) value of over 1.7 (air-corrected) (Umeda et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), and the latter is suggested by the regional seismic velocity tomography, which shows seismic low velocity area beneath the Peninsula in the uppermost mantle (Nakajima, 2023).\u003c/p\u003e \u003cp\u003eTertiary igneous rocks are distributed in the target area, suggesting that the high-Vp/Vs region may represent an old magma reservoir and that fluids released from/through it were involved in the 2024 Noto Peninsula earthquake as well as in the swarm seismic activity that preceded the 2024 earthquake.\u003c/p\u003e \u003cp\u003eVarious fault models have been proposed for the Noto Peninsula earthquake, but Yamada et al. (\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2024\u003c/span\u003e, 2025) carefully checked the dense GNSS and InSAR displacements by combining the aftershock distributions and proposed an interesting model with a high dip angle in the shallow part (\u0026lt;\u0026thinsp;5\u0026ndash;10 km) of the crust and a low dip angle in the deep part.\u003c/p\u003e \u003cp\u003eThis depth dependency of the fault dip angle could be interpreted as a balance between the reactivation of a nonoptimally oriented fault with a high dip angle and the formation of a new optimally oriented fault with a low dip angle (e.g., Sibson, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Sibson, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Under pore-fluid pressure, at shallower depths, the reactivation of a nonoptimally oriented fault with a high dip angle would be dominant, whereas at greater depths, the formation of a new optimally oriented (dipping) fault would be dominant. According to Sibson and Ghisetti\u0026rsquo;s (\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) assumption, when the pore water pressure is approximately 50% of the lithostatic pressure, a cohesionless old normal fault will be active as a reverse fault on the side shallower than approximately 5\u0026ndash;10 km in depth, whereas a new fault will be generated on the deeper side. The high Vp/Vs values observed in the seismic wave velocities are qualitative evidence of such high pore pressures. The faults associated with the 2024 Noto Peninsula earthquake may have been arterial faults driven synchronously by overpressured fluid, as suggested for previous earthquakes (e.g., Sibson, 2019; Okada et al., 2010, 2012, 2014 for the 2008 M7.2 Iwate\u0026ndash;Miyagi Nairiku Earthquake; Okada et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2024c\u003c/span\u003e for the 2016 M7.8 Kaikoura earthquake). Areas with a large degree of anisotropy over 1% were found in the hypocenter of the 2024 Noto Peninsula earthquake and near the focal area of the 2007 Noto\u0026ndash;Hanto earthquake (white star in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). These areas could also be heavily stressed by a high fluid pressure near the focal area and hypocenters, such as in the cases of a sequential earthquake, e.g., the \u0026ldquo;Amatrice\u0026ndash;Visso\u0026ndash;Norcia Seismic Sequence\u0026rdquo; reported by Pastori et al. (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Alternatively, the breaking of rock by the mainshock and aftershocks could have opened cracks that increased the anisotropy, as has been suggested for some other regions (e.g., swarms at Whakaari volcano in New Zealand: Mengesha et al., 2024).\u003c/p\u003e \u003cp\u003eFrom the Vp/Vs cross sections in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the seismicity seems confined to the blue and green sections, i.e., low and medium Vp/Vs, with very little seismicity in the high-Vp/Vs areas\u0026mdash;some of which are immediately under regions of high seismicity. The average value of Vp/Vs at the locations of hypocenters is \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:1.69\\pm\\:0.02\\)\u003c/span\u003e\u003c/span\u003e, which is smaller than that for all the area of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:1.73\\pm\\:0.09\\)\u003c/span\u003e\u003c/span\u003e. Large amounts of fluid could make the area more plastic than brittle. For example, in quartz-rich shear zones, the fluid-filled porosity significantly weakens, resulting in plastic deformation (e.g., Okazaki et al., 2020).\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eWe obtained the fine seismic velocity and seismic anisotropy structure in the inland focal area of the 2024 Noto Peninsula earthquake by using data from dense temporary seismic observations. The lower seismic velocity hanging wall structure caused by ancient normal faulting and faulting-controlled anisotropy suggest that the focal area was highly deformed during a long geological history since the Miocene. Highly pressurized fluid observed in high-Vp/Vs regions may have promoted the occurrence of multifault rupture during the 2024 Noto Peninsula earthquake and the seismic swarm that preceded it. The results suggest that an understanding of the complex structure of faults and the role of pore fluid pressure would enable assessment of fault activity in compressional inversion areas, which is generally difficult (e.g., Ghisetii and Sibson, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eAIST:\u0026nbsp;National Institute of Advanced Industrial Science and Technology\u003c/p\u003e\n\u003cp\u003eCRT: checkerboard resolution test\u003c/p\u003e\n\u003cp\u003eFSOD: fast shear wave oscillation direction\u003c/p\u003e\n\u003cp\u003eJMA: Japan Meteorological Agency\u003c/p\u003e\n\u003cp\u003eMFAST: Multiple Filter Automatic Splitting Technique (Savage et al., 2010)\u003c/p\u003e\n\u003cp\u003eNIED: National Research Institute for Earth Science and Disaster Resilience\u003c/p\u003e\n\u003cp\u003eSC91: Method of Silver and Chan (1991)\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSHmax: maximum horizontal compressive stress\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eNot applicable\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eNot applicable\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data supporting the findings of this study are available from the corresponding author, Tomomi Okada, upon request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was conducted with Grants-in-Aid for Special Purposes (22K19949 and 23K17482), the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and the Japan Society for the Promotion of Science, Japan.\u003c/p\u003e\n\u003cp\u003eThis study was also supported by MEXT of Japan under its Earthquake and Volcano Hazards Observation and Research Program (project numbers THK_02 and THK_07 in 2022\u0026ndash;2024 and THK_12 in 2025).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTO: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing - original draft. MKS: Supervision, Writing - review \u0026amp; editing. AT: Conceptualization, Investigation, Writing - review \u0026amp; editing. RF: Conceptualization, Investigation, Writing - review \u0026amp; editing. RT: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. KY: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. SM: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. KE: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. YY: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. KK: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. TM: Conceptualization, Supervision, Writing - review \u0026amp; editing. IM: Conceptualization, Supervision, Writing - review \u0026amp; editing. SS: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. MM: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. HY: Conceptualization, Supervision, Writing - review \u0026amp; editing. KI: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. NU: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. SH: Data curation. SK: Data curation. TT: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. MO: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. T. Shibutani: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. T Shi\u0026rsquo;ina: Conceptualization, Data curation, Supervision, Writing - review \u0026amp; editing. YH: Project administration, Supervision, Writing - review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe gratefully acknowledge local governments and landowners, who allowed access to their land and the installation of seismic stations. We used seismological data from the JMA. The program codes of the double-difference tomography series were kindly provided by Prof. Haijiang Zhang and Prof. Clifford H. Thurber. Mr. Kohtaro R. Araragi kindly provided his program code for converting the waveform data. Discussion of compressional inversion with Prof. Richard Sibson and Prof. Francesca Ghisetti benefited the promotion of this study. We are grateful to the referee and to the reviewers for their useful comments.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTO, Professor at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University\u003c/p\u003e\n\u003cp\u003eMS, Professor at the School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand,\u003c/p\u003e\n\u003cp\u003eAT, Researcher at AIST\u003c/p\u003e\n\u003cp\u003eRF, Graduate student at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University\u003c/p\u003e\n\u003cp\u003eRT, Associate Professor at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University\u003c/p\u003e\n\u003cp\u003eKY, Associate Professor at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University\u003c/p\u003e\n\u003cp\u003eSM, Professor at Kyushu University\u003c/p\u003e\n\u003cp\u003eKM, Associate Professor at Kyushu University\u003c/p\u003e\n\u003cp\u003eYY, Professor at Nagoya University\u003c/p\u003e\n\u003cp\u003eKK, Associate Professor at Hokkaido University\u003c/p\u003e\n\u003cp\u003eTM,\u0026nbsp;Professor at Hirosaki University\u003c/p\u003e\n\u003cp\u003eMI, Lecturer at Yamagata University\u003c/p\u003e\n\u003cp\u003eSS, Professor at the Interfaculty Initiative in Information Studies and Graduate School of Interdisciplinary Information Studies, University of Tokyo\u003c/p\u003e\n\u003cp\u003eMM, Professor at Kyoto University\u003c/p\u003e\n\u003cp\u003eHY, Associate Professor at Kagoshima University\u003c/p\u003e\n\u003cp\u003eKI, Deputy director of AIST\u003c/p\u003e\n\u003cp\u003eNU, Professor at the University of Tokyo\u003c/p\u003e\n\u003cp\u003eSH, Technical Staff at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University\u003c/p\u003e\n\u003cp\u003eSK, Technical Staff at the Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University\u003c/p\u003e\n\u003cp\u003eTT, Professor at Nagoya University\u003c/p\u003e\n\u003cp\u003eMO, Professor at Hokkaido University\u003c/p\u003e\n\u003cp\u003eT. Shibutani, Professor Emeritus at Kyoto University\u003c/p\u003e\n\u003cp\u003eT. Shiina, Senior Researcher at AIST\u003c/p\u003e\n\u003cp\u003eYH, Professor with the Faculty of Geosciences and Civil Engineering, Institute of Science and Engineering, Kanazawa University\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEndnotes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo endnotes.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAmezawa, Y., Hiramatsu, Y., Miyakawa, A., Imanishi, K., Otsubo, M. (2023) Long‐Living Earthquake Swarm and Intermittent Seismicity in the Northeastern Tip of the Noto Peninsula, Japan. Geophys. Res. Lett. 50: doi:10.1029/2022GL102670\u003c/li\u003e\n \u003cli\u003eAster, R. C., Shearer, P. M., \u0026amp; Berger, J. (1990) Quantitative measurements of shear wave polarizations at the Anza seismic network, southern California: implications for shear wave splitting and earthquake prediction. 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Lett. 51: https://doi.org/10.1029/2024GL109224\u003c/li\u003e\n \u003cli\u003eParis, N., Itoh, Y., Brenguier, F., Wang, Q.-Y., Sheng, Y., Okada, T., Uchida, N., Higueret, Q., Takagi, R., Sakai, S., Hirahara, S., Kimura, S. (2025) Coseismic crustal seismic velocity changes associated with the 2024 Mw 7.5 Noto earthquake, Japan. Earth, Planets, Sp. 77 (1): 55, https://doi.org/10.1186/s40623-025-02177-x\u003c/li\u003e\n \u003cli\u003ePastori, M., Baccheschi, P., Margheriti, L. (2019) Shear Wave Splitting Evidence and Relations With Stress Field and Major Faults From the \u0026ldquo;Amatrice‐Visso‐Norcia Seismic Sequence.\u0026rdquo; Tectonics 38: 3351\u0026ndash;3372. https://doi.org/10.1029/2018TC005478\u003c/li\u003e\n \u003cli\u003eSakai, S., Kurashimo, E., Iidaka, T., Uchida, N., Yoshida, K., Okada, T. (2022) Seismic Data from Temporary Seismic Observation in the Northeast Noto Peninsula, Central Japan. Zenodo. doi:10.5281/ZENODO.6767362\u003c/li\u003e\n \u003cli\u003eSavage, M.K. 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(2024) The role of fluids in earthquake swarms in northeastern Noto Peninsula, central Japan: insights from source mechanisms, Earth Planets Space, 76:151. https://doi.org/10.1186/s40623-024-02099-0\u003c/li\u003e\n \u003cli\u003eTakei, Y (2002) Effect of pore geometry on V P / V S : From equilibrium geometry to crack. J Geophys Res 107: 2043. doi: 10.1029/2001JB000522.\u003c/li\u003e\n \u003cli\u003eTerakawa, T., Matsu\u0026rsquo;ura, M. (2010) The 3-D tectonic stress fields in and around Japan inverted from centroid moment tensor data of seismic events. Tectonics 29: doi:10.1029/2009TC002626\u003c/li\u003e\n \u003cli\u003eUeno, H., Hatakeyama, S., Aketagawa, T., Funasaki, J., Hamada, N. (2002) Improvement of hypocenter determination procedures in the Japan Meteorological Agency. J. Seismol. Volcanol. Relat. Eng. 65: 123\u0026ndash;134.\u003c/li\u003e\n \u003cli\u003eUmeda, K., Ninomiya, A., Negi, T. (2009) Heat source for an amagmatic hydrothermal system, Noto Peninsula, Central Japan. J. Geophys. Res. Solid Earth 114: doi:10.1029/2008JB005812\u003c/li\u003e\n \u003cli\u003eUmeda, K., Yamazaki, Y., Sumino, H. (2024) Geochemical Signature of Deep Fluids Triggering Earthquake Swarm in the Noto Peninsula, Central Japan. Geophys. Res. Lett. 51: https://doi.org/10.1029/2024GL108581\u003c/li\u003e\n \u003cli\u003eWessel, P., Luis, J. F., Uieda, L., Scharroo, R., Wobbe, F., Smith, W. H. F., \u0026amp; Tian, D. (2019) The Generic Mapping Tools Version 6. Geochemistry, Geophysics, Geosystems, 20 (11): 5556\u0026ndash;5564. https://doi.org/10.1029/2019GC008515\u003c/li\u003e\n \u003cli\u003eYamada T., Yusaku Ohta, Takuya Nishimura, Yoshihiro Hiramatsu, (2024) Estimation of slip distribution of the 2024 Noto Peninsula earthquake based on dense GNSS observation network, MTT37-P02, 2024 JpGU annual meeting.\u003c/li\u003e\n \u003cli\u003eYamada, T., Ohta, Y., Nishimura, T., Yoshida, K., Hiramatsu, Y., Kinoshita, Y. (2025) Coseismic slip distribution of the 2024 Noto Peninsula earthquake deduced from dense global navigation satellite system network and interferometric synthetic aperture radar data: effect of assumed dip angle. Earth, Planets Sp. 77: 19. https://doi.org/10.1186/s40623-025-02154-4\u003c/li\u003e\n \u003cli\u003eYoshida, K., Uno, M., Matsuzawa, T., Yukutake, Y., Mukuhira, Y., Sato, H., Yoshida, T. 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Geophys. 163: 373\u0026ndash;403. https://doi.org/10.1007/s00024-005-0021-y\u003c/li\u003e\n \u003cli\u003eZhao, D., Yu, S., Liu, X. (2016) Seismic anisotropy tomography: New insight into subduction dynamics. Gondwana Res. 33: 24-43. doi:10.1016/j.gr.2015.05.008\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Shear wave splitting, seismic velocity structure, earthquake swarm","lastPublishedDoi":"10.21203/rs.3.rs-6576798/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6576798/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe M7.6 Noto Peninsula earthquake occurred on 1 January 2024 in the Noto region of Ishikawa Prefecture. Several faults successively ruptured during the 2024 Noto Peninsula earthquake. Seismic activity in the area has increased since approximately August 2020. Possible causes of this increased seismic activity include stress changes due to crustal deformation and increased fluid pressure. In this study, following an analysis of the seismic swarm area northeast of the Noto Peninsula, S-wave splitting in the epicentral area was investigated using a dense seismic network of temporary and permanent stations. Seismic wave velocity tomography analysis was also conducted to investigate the isotropic seismic wave velocity structure in the onshore source region.\u003c/p\u003e\n\u003cp\u003eThe predominant fast direction of anisotropy throughout the peninsula was NE‒SW. This direction generally coincided with the strike direction of faults and folds and may be due to structural anisotropy.. In addition, NW‒SE anisotropy, whose direction generally coincided with the axis of maximum horizontal compression, and north‒southanisotropy were also observed in some areas.\u003c/p\u003e\n\u003cp\u003eIn the shallow crust, the hanging wall of the fault has low velocities. This shallow low-velocity hanging wall may be associated with structures that developed during past normal faulting that formed in this region. The low velocity in the shallow part of the hanging wall side of the fault and the anisotropy due to structural properties indicate a complex structure in which the structure of normal faults that developed in this region coexists with a compressional structure due to the present stress field.\u003c/p\u003e\n\u003cp\u003eLow-Vs and high-Vp/Vs regions were identified beneath or at deeper extensions of hypocenters and faults at a depth of 18 km. Slightly larger P-wave velocities were obtained in this region than in the surrounding area. Tertiary igneous rocks are distributed in the target area, suggesting that the low-Vs and high-Vp/Vs regions may represent an old magma reservoir and that fluids released from/through this reservoir were involved in the 2024 Noto Peninsula earthquake and preceded seismic swarm activity that occurredbefore the 2024 earthquake.\u003c/p\u003e","manuscriptTitle":"Shear wave splitting and seismic velocity structure in the onshore focal area of the 2024 Noto Peninsula earthquake, central Japan, as a fluid-promoted multifault rupture in a compressional inversion area","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-12 07:03:47","doi":"10.21203/rs.3.rs-6576798/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a8c70ed2-92a7-4a4c-958d-7948559ed2cd","owner":[],"postedDate":"June 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-07-03T07:11:19+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-12 07:03:47","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6576798","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6576798","identity":"rs-6576798","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}