Shallow segment rupture associated with the 2025 Kamchatka earthquake inferred from ALOS-2/PALSAR-2 InSAR

Shallow segment rupture associated with the 2025 Kamchatka earthquake inferred from ALOS-2/PALSAR-2 InSAR | 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 Shallow segment rupture associated with the 2025 Kamchatka earthquake inferred from ALOS-2/PALSAR-2 InSAR Yuji Himematsu, Hiroshi Munekane This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8111287/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 14 Apr, 2026 Read the published version in Earth, Planets and Space → Version 1 posted 5 You are reading this latest preprint version Abstract We detected coseismic deformation associated with the 2025 Kamchatka earthquake using Advanced Land Observing Satellite-2 (ALOS-2) ScanSAR (wide-swath) data. The coseismic deformation revealed a predominantly eastward displacement of more than 1.5 m, with smaller vertical movement observed on the southern Kamchatka Peninsula. Our best-fit model prefers a shallow, near-trench slip concentration on the order of 10–15 m at 10–20 km depth and a similar along-strike extent to that of the 1952 M9.0 earthquake inferred from tsunami or historical records in previous studies. Furthermore, the trenchward rupture in 2025 overlapped the seismicity gap observed between 1970 and 2019 as well as areas of strong plate coupling, suggesting a decades-long accumulation of slip deficit. In contrast, the upper-bound of slip deficit budget is insufficient to explain the inferred peak coseismic slip, because the slip deficit reaches at most ~ 6 m, even with full coupling and a convergence rate of about 80 mm/yr since the 1952 event. The imbalance between the slip deficit budget and the coseismic slip may reflect a partial failure to release slip deficit due to the 1952 earthquake or fine-scale slip segmentation in the subduction zone; however, the model resolution is limited to resolve these interpretations uniquely. Megathrust earthquake Ground deformation Kamchatka InSAR Figures Figure 1 Figure 2 Figure 3 Figure 4 1 Introduction Rupture segmentation in subduction zones provides a fundamental framework for understanding megathrust earthquake scenarios (Chlieh et al. 2008 ; Pritchard and Simons, 2006 ; Scholz and Campos, 2012 ). Generally, megathrust earthquakes in subduction zones occur when an accumulated slip deficit is released. Although only a limited number of megathrust earthquakes have been observed by modern geodetic and seismic approaches (e.g., Sumatra 2004 M9.1, Chile 2010 M8.8, Tohoku 2011 M9.0; Lay et al. 2005 ; Vigny et al. 2011 ; Ozawa et al. 2011 ), the approximate rupture extents of historical events have been inferred from tsunami or historical records (Satake and Atwater, 2007 ). Knowledge of the rupture extents of historical earthquakes help to estimate the likelihood of forthcoming earthquakes following a previous rupture (Lay et al. 2010; Rong et al. 2003 ). Plate coupling can be used to investigate the potential for earthquake occurrence. Seismicity during the interseismic period and deformation revealed by geodetic techniques can serve as indicators of coupling strength along plate boundaries (Chlieh et al. 2011 ; Loveless and Meade, 2011 ). Plate coupling is influenced by topographic features, mineralogical conditions, and temperature (Oleskevich et al. 1999 ; van Rijsingen et al. 2019 ; Saffer et al. 2012 ). Clusters of earthquakes are interpreted as weakly coupled or conditionally stable patches, whereas seismicity gaps may indicate aseismic behavior (creep) or a strongly locked asperity accumulating slip deficit. Thus, both seismological and geodetic observations are important for evaluating the coupling conditions of plate boundaries. The Kamchatka Subduction Zone is the northwestern termination of the boundary between the North American Plate and the Pacific Plate. The Pacific Plate subducts northwestward beneath the North American Plate at a rate of ~ 80 mm/yr (DeMets et al. 1990 ). In Kamchatka, historical earthquake catalog indicates roughly two events of magnitude M > 8.8 earthquakes and one event of M 9.0 earthquake in a century on average (Gusev and Shumilina, 2004 ). The interseismic deformation and seismicity help reveal the spatiotemporal characteristics of locked asperities in the Kamchatka Subduction Zone (Bürgmann et al. 2005 ). Interseismic deformations measured using Global Navigation Satellite Systems (GNSS) are consistent with persistent locked patches that overlap historic rupture areas, with weaker coupling north of ~ 53° N and locking extending to ~ 50 km depth. The most recent megathrust earthquake in the Kamchatka Subduction Zone occurred in 1952, with a magnitude of M 9.0 (Kanamori, 1976 ). The 1952 earthquake also generated tsunamis and affected the coasts of the Pan-Pacific region (Macdonald and Wentworth, 1954 ). The ruptured area of the 1952 earthquake extended from the offshore of the central Kamchatka Peninsula to the northern Kuril Islands in the south, as inferred from tsunami waveform inversion and historical records (Johnson and Satake, 1999 ; MacInnes et al. 2010 ). The slip exceeding 10 m may be explained by the release of cumulative slip deficit accumulated since the previous megathrust earthquake in 1737 (Gusev and Shumilina, 2004 ). Seventy-three years after the 1952 M9.0 earthquake, an M8.8 earthquake occurred offshore of the Kamchatka Peninsula on July 29, 2025, at 23:24 UTC (USGS, 2025). The position and depth of the epicenter were similar to those of the 1952 M9.0 earthquake (Fig. 1 ; USGS, 2025). The megathrust earthquake generated a tsunami, with run-up height exceeding 1 m along the coasts of the Pan-Pacific region, including Japan, Hawaii, and Chile (NOAA, 2025). The ~ 70-year recurrence interval and the proximal source regions of the 1952 and 2025 earthquakes raise two questions: (i) how does the slip distribution in 2025 differ from that in 1952, and (ii) how is the extent of rupture in 2025 consistent with the slip-deficit budget and previously reported interseismic plate coupling in the Kamchatka Subduction Zone? We addressed these two questions by investigating coseismic deformation using satellite synthetic aperture radar (SAR) data and by modeling the slip distributions. By comparing the 1952 slip distribution reported in previous studies, we examined the spatial characteristics of the 2025 slip pattern and its correlations with the asperities on the plate boundary and seismicity. 2 Method Coseismic deformation was detected using ALOS-2/PALSAR-2 data in ScanSAR (wide-swath) mode (Fig. 1 ; Table S1 ). The ALOS-2 ScanSAR data cover a width of 350 km, which is greater than the 250 km swath of Sentinel-1 interferometric wide (IW) mode. Wider image coverage is superior for revealing the spatial characteristics of coseismic deformation with fewer data acquisitions and for avoiding the contamination of early-phase post-seismic deformations. Topography-dependent phase changes were corrected using ALOS World 3D-30m (AW3D30) data (Takaku et al. 2014 ). Phase unwrapping was performed by the minimum cost flow algorithm implemented in the SNAPHU software (Chen and Zebker, 2000 ). Ionospheric errors were corrected by a range split spectrum algorithm proposed by Wegmüller et al. ( 2018 ). Using ALOS-2/PALSAR-2 data from Path 010 (descending orbit) and 113 (ascending orbit) for InSAR processing, we decomposed the two independent line-of-sight (LOS) changes into quasi-east–west and quasi-up-down displacement components using the least-squares method. To model the slip distribution, we used the analytical solution of surface deformation caused by dislocations at each rectangular subfault under the assumption of an homogeneous elastic half-space with a Poisson’s ratio of 0.25 (Okada, 1985 ). Small patches at 15 km intervals were set to design the geometry of the plate boundary between the North American and Pacific plates (Iwasaki et al. 2015 ). The slip patches extend over depths ranging from 6 km to 60 km along the modeled plate boundary. The dip angle of the derived slip ranged from less than 10° at a depth of 10 km to ~ 35° at a depth of 60 km (Fig. S1 ). Shallower patches extended up to a depth of 6 km, corresponding to the distribution of aftershocks of the 2025 earthquake (through August 17, 2025) and seismicity during 1970–2019. The ALOS-2/PALSAR-2 InSAR data were subsampled based on a quad-tree algorithm to reduce computational cost while retaining the spatial characteristics of coseismic deformation (Jónsson et al. 2002 ). The slip distribution reproducing the ALOS-2/PALSAR-2 LOS changes in Path 010 and 113 was derived by solving least-squares problems. No constraints were imposed on the slip direction. The Laplacian smoothness for the slip distribution was optimized by minimizing the Akaike’s Bayesian Information Criterion value (Akaike, 1980 ). Each LOS change was weighted based on the variance in the LOS change inferred from the data outside the deformed area. Several slip constraints were applied at the bottom and side edges of the segment by assigning zero values. Although no phase unwrapping errors were confirmed on the Kamchatka Peninsula, it remains unclear how the phase changes on the Kuril Islands, including Paramushir and Shumshu Islands, should be connected to those on the Kamchatka Peninsula (Fig. 2 ). To avoid possible unwrapping errors, we separately unwrapped the phase changes over Kamchatka and the Paramushir–Shumshu Islands, and then simultaneously estimated the offsets of LOS change between two regions and the coseismic slip distribution. This strategy prevents the phase unwrapping errors and also constrains the southern termination of slip on the segment. To evaluate model uncertainty, we applied bootstrap algorithm (Efron, 1979 ; Hartzell et al. 2007 ). In each of 1000 iterations, we randomly extracted 20% of the subsampled observations, added 25% random noise, and then computed the standard deviation of the derived slip distribution. 3 Result ALOS-2/PALSAR-2 InSAR reached a peak of over 1m of LOS change at a peak in the southern Kamchatka Peninsula, approximately 200 km southwest of the hypocenter of the 2025 earthquake (Fig. 2 ). The gradient of wrapped LOS changes on the southern peninsula cross in the northeast–southwest direction, whereas those on the Paramushir and Shumshu Islands are oriented in the north–south direction, suggesting the dominance of east-west ground deformation gradient. The quasi-east–west displacement decomposed from the ascending and descending LOS changes showed over 1.5 m of eastward displacement in the southern part of the Kamchatka Peninsula, while few quasi-up-down displacements were observed (Figs. 2 c and 2 d). Based on coseismic deformation derived from ALOS-2/PALSAR-2 data, we estimated the slip distribution along the plate boundary. The inferred slip distribution indicates a slip concentration in the shallower part of the segment (at depths less than 20 km), with a maximum slip of 16 m observed at 10–20 km depth (Fig. 3 ). Furthermore, a slip greater than 10 m extended to the shallower edge of the slip patches at a depth of 6 km. The released geodetic moment magnitude reached 2.4×10 22 Nm, corresponding to Mw 8.85, assuming a rigidity of 40 GPa. Most of the slip was directed perpendicular to the trench strike (Fig. 3 b). The epicenter was located in the northernmost and deepest portion of the derived slip distribution. The preferred model reproduced the observations with root-mean-square error (RMSE) of 0.047 m and 0.030 m for Path 010 and 113 on Kamchatka, respectively (Figs. S2–S3; Table S2 ). The offsets of LOS change on Shumshu and Paramushir Islands were − 0.41 m and 0.25 m in Path 010 and 113, corresponding to maximum LOS changes of 0.6–0.7 m, respectively (Table S3). A standard deviation of slip exceeding 2 m was identified off the Kuril Islands, as inferred from the bootstrap algorithm (Fig. 3 c). The larger standard deviations may have resulted from insufficient data points on the northern Kuril Islands to constrain slip on the southwestern patches. The RMSE of the offsets of LOS change on the Kuril Islands were 0.13 and 0.10 for Path 010 and 113, respectively (Table S3). The resolution test showed that synthetic slip at a depth of greater than 20 km was well reproduced, whereas synthetic slip at shallower depths ( 20 km) supports the plausibility of the inferred slip, which implies a smaller slip. When comparing the synthetic deformation derived from predefined slip distributions with the observations, the slip concentrated at the shallowest patches minimized the residuals between the observations and the synthetic deformations (Fig. S5). This implies that concentration of trenchward slip improves to fit the observed deformation. The slip distribution inferred using coseismic deformation only at Kamchatka Peninsula exhibited characteristics similar to those of the best-fit slip distribution, except for a reduced slip in the southwestern patches (Fig. S6). The released moment magnitude was 2.2×10 22 Nm (Mw 8.83), which was nearly identical to that obtained from the model including deformation on the Kuril Islands. 4 Discussion 4.1 Comparison with seismicity Seismicity can be used as an indicator to evaluate coupling at the plate boundary. Comparison with the inferred slip distribution shows that the major rupture extent coincides with areas of the sparse seismicity at shallower depths (< 20 km) of the Kamchatka Subduction Zone during 1970–2019 (Fig. 4 ). Instead, several earthquake clusters are distributed northeast and southwest of the 2025 rupture extent. The epicenter of the 2025 event was located near the northeastern cluster of earthquakes. Seismicity at deeper, landward depths (20–60 km) is also sparse off the southern Kamchatka Peninsula, where the main rupture of the 1952 earthquake occurred. The epicenters of historic M > 7 earthquakes since 1900 have also been predominantly distributed landward rather than near the trench (compiled by Johnson and Satake, 1999 ). Vorobieva et al. ( 2019 ) also inferred the spatial variation in earthquake magnitude, however, near-trench parameters off the southern Kamchatka Peninsula were not constrained because of the limited number of earthquakes. Seismicity gaps in subduction zones, in general, suggest aseismic creeping or strong coupling between plate boundaries. Here, we interpret the shallow seismicity gap as consistent with strong plate locking in this region as discussed below. The interseismic deformation detected by GNSS indicated full or partial coupling in the subduction zone offshore of the southern Kamchatka Peninsula (Bürgmann et al. 2005 ). Although Bürgmann et al. ( 2005 ) noted limited model resolution near the trench due to insufficient observation points, the back-slip model was consistent with strong coupling at a depth of 60 km near the coast of the peninsula. These patterns persist under stronger spatial smoothing in the back-slip model. The extent of strong coupling inferred from the back slip model overlaps with the main rupture extent in our model. Therefore, the shallow seismicity gap offshore of the southern Kamchatka Peninsula corresponds to strong plate coupling, which has the potential to accumulate slip deficit over a long period. 4.2 Slip deficit budget since the 1952 earthquake The inferred slip distribution for the 2025 earthquake indicates a slip concentration offshore of the southern Kamchatka Peninsula at a shallower depth of 20 km, extending toward the trench (Fig. 2 ). Aftershocks were mainly distributed around 20 km depth but were also observed closer to the trench (Fig. 4 ). In the Kamchatka Subduction Zone, the along-strike rupture extent of the 2025 earthquake may be comparable to that of the M9.0 earthquake in 1952 (Johnson and Satake, 1999 ; MacInnes et al. 2010 ). In contrast, the preferred slip distribution in this study exceeds the budget of the cumulative slip deficit assuming that the 1952 earthquake occurred on the similar portion of the plate boundary. Even assuming full coupling, a simple slip budget estimate based on a plate convergence rate of ~ 80 mm/yr yields less than 6 m of an average slip in the source region. Therefore, the time elapsed between the 1952 and 2025 events is insufficient to account for the inferred peak slip of ~ 10–15 m of coseismic slip for the 2025 event. Such an apparent imbalance suggests that slip deficit in the area ruptured in 2025 may have begun to accumulate prior to 1952. The excess of coseismic slip relative to the expected slip deficit budget is known as negative slip deficit (e.g., Moreno et al. 2012 ). The slip distribution inferred from tsunami waveform inversion yields better resolution near the trench than landward (Romano et al. 2010 ). In contrast, onshore geodetic inversion has limited resolution near the trench (e.g., Cruz-Atienza et al. 2025 ). For the 2011 Tohoku-oki earthquake, a near-trench slip was required to explain tide gauge data (Hossen et al. 2015 ; Satake et al. 2013 ), whereas onshore geodetic data were reproduced by the dominance of downward slip rather than ruptures near the trench (Romano et al. 2012 ; Simons et al. 2011 ). Although the coseismic deformation is best explained by the trenchward slip concentration as shown in Fig. S5, the resolution in our model is also coarse, especially at the shallower portion of the plate boundary. 4.3 Possible interpretations of negative slip deficit We discuss the possible interpretations for the imbalance between the slip deficit budget and coseismic slip of the 2025 earthquake. One possible interpretation is the partial failure of slip deficit to be released due to the 1952 earthquake, which was not fully released during the 1952 rupture, partially slipped in 2025. Similar imbalances between slip deficit budget and coseismic slip have been reported for the 2010 Maule earthquake (e.g., Moreno et al. 2012 ). In the simple framework, we expect megathrust earthquakes to release most of slip deficit budget accumulated since the previous event, but such imbalances can be accommodated by contributions from aseismic slip during the postseismic period, aftershocks, or apparent local overshoot. Megathrust earthquakes can induce rapid early afterslip on and around the ruptured area of the mainshock (e.g., Ozawa et al., 2011 ), which has a potential to release a significant fraction of the cumulative deficit. In our case, the slip distribution is inferred solely from coseismic deformation observed by ALOS-2/PALSAR-2 data acquired within 3 days of the mainshock, thus contamination from postseismic deformations should be limited. The largest aftershock before the acquisition of secondary ALOS-2/PALSAR-2 data was an Mw 6.9 event occurring 45 minutes after the mainshock (USGS, 2025), whose additional contribution is unlikely to dominate the large-scale deformation pattern in our model. Although there is no further information to support interpretations that previous studies suggested, we cannot rule out the residual of a partial release of the slip deficit associated with the 1952 earthquake to explain the excess of coseismic slip in 2025. Another possible interpretation is a fine-scale slip segmentation in the subduction zone. The slip segmentation in both along-strike and along-dip directions is a key characteristic for inferring the slip deficit budget and facilitating discussions about the earthquake cycle. The along-strike heterogeneity of interseismic coupling indicates cumulative slip deficit, the recurrence likelihood, and asperity distribution (Chlieh et al. 2008 ; Loveless and Meade, 2011 ; Moreno et al. 2010 ). The along-strike variation in morphology and seismic behavior is mainly controlled by pre-existing crustal structures of plates, which correspond to gravity anomalies and residual bathymetry (Bassett and Watts, 2015 ). Global comparisons and analog modeling suggest a correlation between high seismic coupling and relatively low plate boundary roughness, but this correlation is not universal (Lallemand et al. 2018 ; van Rijsingen et al. 2019 ). Along-dip segmentation is typically characterized by a depth-dependent variation in the rupture process: aseismic creeping, slow slip, and tremor in the conditionally stable shallow portion; unstable, seismogenic behavior at intermediate depths; and slow slip and tremor in the deeper transient region (Cruz-Atienza et al. 2021 ; Hirose et al. 2010 ). In general, the shallowest portion of the subduction zone (accretionary wedge) is a conditionally stable region that tends to have long duration transients, such as slow slip or tremors. Coseismic stress transfer from the seismogenic zone can produce coseismic strengthening in the shallowest accretionary region, which tends to inhibit trench breaching ruptures if the strengthening exceeds a critical threshold (Hu and Wang, 2008 ). However, an up-dip rupture nucleating at greater depths can potentially propagate to the shallowest portion, as observed in the 2011 Tohoku earthquake. This represents an additional consideration—and a possible exception—when evaluating the slip deficit (Lay et al. 2012 ). Yomogida et al. ( 2011 ) suggested along-dip segmentation in the Japan trench and the Aleutian-Alaska Subduction Zone, based on contrasts in background seismicity and rupture areas of the 2011 Tohoku-oki earthquake and the 1964 Alaska earthquake. In both regions, the shallow seismicity gap near the trench coincides with the main rupture extent at shallower depths. In contrast, such features are not evident in the Chilean or Sumatran subduction zones, suggesting that they are not universally present. While the study did not analyze the characteristics of the Kamchatka Subduction Zone, the seismicity gap at the shallower depths during 1970–2019 and the rupture area in 2025 are similarly overlapped (Fig. 4 ). The slip segmentation may be a possible interpretation for explaining the imbalance between the slip deficit budget and the peak coseismic slip; however, it is impossible to resolve the fine-scale segmentation because of limited modeling resolution as we show in Fig. S4. 5 Conclusion The slip distribution associated with the M8.8 Kamchatka earthquake in 2025 indicated a peak slip on the order of 10–15 m in shallow, near-trench portions. Although the model resolution is limited in the shallow near-trench region, the observed coseismic deformation detected by ALOS-2 InSAR implies concentrations of apparent trenchward slip. The main ruptured area overlapped with the shallow seismicity gap and the region of strong interseismic plate coupling that has persisted for decades, suggesting long-term slip deficit accumulation prior to the 1952 event. Even if the 1952 event ruptured at similar extents, the temporal interval between 1952 and 2025 is insufficient to accumulate the slip deficit budget for explaining the inferred slip distribution. The significant imbalance of the slip deficit budget and the coseismic slip may reflect partial failure of slip deficit left by the 1952 earthquake or fine-scale slip segmentation in subduction zones. However, such interpretations cannot yet be uniquely resolved due to the limited model resolution. Abbreviations ALOS-2 Advanced Land Observation Satellite-2 PALSAR-2 Phased-Array L-band Synthetic Aperture Radar-2 SAR Synthetic Aperture Radar InSAR Interferometric Synthetic Aperture Radar RMSE Root-mean-square error GNSS Global Navigation Satellite System USGS United States Geological Survey NOAA The National Oceanic and Atmospheric Administration IW Interferometric Wide LOS Line-of-sight Declarations Ethics approval and consent to participate Not applicable. Consent for publication Not applicable. Availability of data and materials The processed data in this study are available from an institutional repository (doi: 10.57499/XXX). Bathymetry data is available from GEBCO (https://download.gebco.net/). The earthquake catalog is available on the United States Geological Survey website (https://earthquake.usgs.gov/earthquakes/search/). The original ALOS-2/PALSAR-2 can be purchased from either RESTEC (https://www.restec.or.jp/en/) or PASCO (http://en.alos-pasco.com). Competing interests The authors declare that they have no competing interests. Funding Not applicable. Authors' contributions YH: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft. HM: Interpretation, Discussion, Writing – review & editing. Acknowledgements ALOS-2/PALSAR-2 data were provided by the Japan Aerospace Exploration Agency (JAXA) under a cooperative agreement between the Geospatial Information Authority of Japan (GSI) and JAXA. 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Earth Planets and Space 63:34. 10.5047/eps.2011.06.003 Supplementary Files SuppleEPSKamchatkaEQ2025.docx graphicalabstractlow.png Graphical Abstract Cite Share Download PDF Status: Published Journal Publication published 14 Apr, 2026 Read the published version in Earth, Planets and Space → Version 1 posted Editorial decision: Minor Revision 29 Jan, 2026 Reviewers agreed at journal 25 Nov, 2025 Reviewers invited by journal 25 Nov, 2025 Editor assigned by journal 15 Nov, 2025 First submitted to journal 13 Nov, 2025 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. 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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-8111287","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":550325899,"identity":"7b6bfc19-6c25-4810-b90d-6e22dc74b2f5","order_by":0,"name":"Yuji Himematsu","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0002-1724-8497","institution":"Geospatial Information Authority of Japan","correspondingAuthor":true,"prefix":"","firstName":"Yuji","middleName":"","lastName":"Himematsu","suffix":""},{"id":550325900,"identity":"b916991c-5338-404b-ae62-67d45f3d1e80","order_by":1,"name":"Hiroshi Munekane","email":"","orcid":"","institution":"Geospatial Information Authority of 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1","display":"","copyAsset":false,"role":"figure","size":294868,"visible":true,"origin":"","legend":"\u003cp\u003eStudy area. (a) Bathymetry map with plate boundary contours every 20 km (Iwasaki et al. 2015). Gray circles show aftershock hypocenters through August 17, 2025 (USGS). Bathymetry data are derived from GEBCO website.\u003c/p\u003e\n\u003cp\u003e(b) A broader map of the study area.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8111287/v1/3ce4005c61c4366500539492.png"},{"id":97388637,"identity":"4b74ed2c-0124-4dd8-93a0-a1ba96ad97d5","added_by":"auto","created_at":"2025-12-03 21:22:20","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":395632,"visible":true,"origin":"","legend":"\u003cp\u003eCoseismic deformation derived from ALOS-2/PALSAR-2. (a, b) Wrapped line-of-sight (LOS) change. Positive LOS changes indicate distance changes away from the satellite (increase in distance). The details of the dataset are listed in Table S1. (c, d) Quasi-east-west (QEW) and quasi-up-down (QUD) displacement decomposed from two LOS changes. Black cross marks the mainshock hypocenter. Solid black lines: plate interface depth at 20 km intervals; dashed lines: auxiliary 10 km contours.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8111287/v1/7a0ca40e7f427d647c4efbd2.png"},{"id":97664950,"identity":"6132d5df-ad5a-4690-9718-1c8d4b7e60d1","added_by":"auto","created_at":"2025-12-08 09:15:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":348755,"visible":true,"origin":"","legend":"\u003cp\u003eCoseismic slip distribution. (a) Slip distribution inferred from the ALOS-2/PALSAR-2 InSAR. (b) An enlarged map around the major rupture area with vectors indicating slip directions (gray arrows). (c) Standard deviation of slip inferred from bootstrap algorithm (see Methods). Black cross marks the mainshock hypocenter. Solid black lines: plate interface depth with 20 km intervals, dashed lines: auxiliary 10 km contours.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8111287/v1/fa4747eeccde346d34de1de4.png"},{"id":97388641,"identity":"7b068bf5-58df-4860-b13c-bfb25974c9f5","added_by":"auto","created_at":"2025-12-03 21:22:21","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":400603,"visible":true,"origin":"","legend":"\u003cp\u003ePreferred slip distribution and seismicity. (a, b) Slip distribution with hypocenters. Colored circles indicate the hypocenters of aftershocks through August 17, 2025, and background seismicity (M\u0026gt;4) during 1970–2019, respectively. Coloring of circles indicates the epicenter depth. Contours indicate the preferred slip distributions with 2 m intervals. (c, d) Slip distribution with seismicity heatmap. Comparison of slip distribution with heatmap of aftershocks until August 17, 2025, and background seismicity (M\u0026gt;4) in 1970–2019, respectively.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8111287/v1/445ef28daeafc83863936093.png"},{"id":107351728,"identity":"8d4607ac-5deb-4333-a366-f1421a17c9c6","added_by":"auto","created_at":"2026-04-20 16:11:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1559671,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8111287/v1/4c7da5c5-5e6f-49ba-81ab-6a3c134c6178.pdf"},{"id":97388655,"identity":"77346395-1d14-4e8a-91b2-6ded7d29d2d9","added_by":"auto","created_at":"2025-12-03 21:22:21","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1349477,"visible":true,"origin":"","legend":"","description":"","filename":"SuppleEPSKamchatkaEQ2025.docx","url":"https://assets-eu.researchsquare.com/files/rs-8111287/v1/849b36d4e0903e9a6ac15bb2.docx"},{"id":97664943,"identity":"f5ac40ac-0125-4c54-a642-f446bc63dcb6","added_by":"auto","created_at":"2025-12-08 09:15:34","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":96772,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGraphical Abstract\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"graphicalabstractlow.png","url":"https://assets-eu.researchsquare.com/files/rs-8111287/v1/7c1a00c98b9dabaa772bead1.png"}],"financialInterests":"","formattedTitle":"Shallow segment rupture associated with the 2025 Kamchatka earthquake inferred from ALOS-2/PALSAR-2 InSAR","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eRupture segmentation in subduction zones provides a fundamental framework for understanding megathrust earthquake scenarios (Chlieh et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Pritchard and Simons, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Scholz and Campos, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Generally, megathrust earthquakes in subduction zones occur when an accumulated slip deficit is released. Although only a limited number of megathrust earthquakes have been observed by modern geodetic and seismic approaches (e.g., Sumatra 2004 M9.1, Chile 2010 M8.8, Tohoku 2011 M9.0; Lay et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Vigny et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Ozawa et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), the approximate rupture extents of historical events have been inferred from tsunami or historical records (Satake and Atwater, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Knowledge of the rupture extents of historical earthquakes help to estimate the likelihood of forthcoming earthquakes following a previous rupture (Lay et al. 2010; Rong et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2003\u003c/span\u003e).\u003c/p\u003e\u003cp\u003ePlate coupling can be used to investigate the potential for earthquake occurrence. Seismicity during the interseismic period and deformation revealed by geodetic techniques can serve as indicators of coupling strength along plate boundaries (Chlieh et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Loveless and Meade, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Plate coupling is influenced by topographic features, mineralogical conditions, and temperature (Oleskevich et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; van Rijsingen et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Saffer et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Clusters of earthquakes are interpreted as weakly coupled or conditionally stable patches, whereas seismicity gaps may indicate aseismic behavior (creep) or a strongly locked asperity accumulating slip deficit. Thus, both seismological and geodetic observations are important for evaluating the coupling conditions of plate boundaries.\u003c/p\u003e\u003cp\u003eThe Kamchatka Subduction Zone is the northwestern termination of the boundary between the North American Plate and the Pacific Plate. The Pacific Plate subducts northwestward beneath the North American Plate at a rate of ~\u0026thinsp;80 mm/yr (DeMets et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). In Kamchatka, historical earthquake catalog indicates roughly two events of magnitude M\u0026thinsp;\u0026gt;\u0026thinsp;8.8 earthquakes and one event of M 9.0 earthquake in a century on average (Gusev and Shumilina, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The interseismic deformation and seismicity help reveal the spatiotemporal characteristics of locked asperities in the Kamchatka Subduction Zone (B\u0026uuml;rgmann et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Interseismic deformations measured using Global Navigation Satellite Systems (GNSS) are consistent with persistent locked patches that overlap historic rupture areas, with weaker coupling north of ~\u0026thinsp;53\u0026deg; N and locking extending to ~\u0026thinsp;50 km depth.\u003c/p\u003e\u003cp\u003eThe most recent megathrust earthquake in the Kamchatka Subduction Zone occurred in 1952, with a magnitude of M 9.0 (Kanamori, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1976\u003c/span\u003e). The 1952 earthquake also generated tsunamis and affected the coasts of the Pan-Pacific region (Macdonald and Wentworth, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1954\u003c/span\u003e). The ruptured area of the 1952 earthquake extended from the offshore of the central Kamchatka Peninsula to the northern Kuril Islands in the south, as inferred from tsunami waveform inversion and historical records (Johnson and Satake, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; MacInnes et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The slip exceeding 10 m may be explained by the release of cumulative slip deficit accumulated since the previous megathrust earthquake in 1737 (Gusev and Shumilina, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2004\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eSeventy-three years after the 1952 M9.0 earthquake, an M8.8 earthquake occurred offshore of the Kamchatka Peninsula on July 29, 2025, at 23:24 UTC (USGS, 2025). The position and depth of the epicenter were similar to those of the 1952 M9.0 earthquake (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; USGS, 2025). The megathrust earthquake generated a tsunami, with run-up height exceeding 1 m along the coasts of the Pan-Pacific region, including Japan, Hawaii, and Chile (NOAA, 2025).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe ~\u0026thinsp;70-year recurrence interval and the proximal source regions of the 1952 and 2025 earthquakes raise two questions: (i) how does the slip distribution in 2025 differ from that in 1952, and (ii) how is the extent of rupture in 2025 consistent with the slip-deficit budget and previously reported interseismic plate coupling in the Kamchatka Subduction Zone? We addressed these two questions by investigating coseismic deformation using satellite synthetic aperture radar (SAR) data and by modeling the slip distributions. By comparing the 1952 slip distribution reported in previous studies, we examined the spatial characteristics of the 2025 slip pattern and its correlations with the asperities on the plate boundary and seismicity.\u003c/p\u003e"},{"header":"2 Method","content":"\u003cp\u003eCoseismic deformation was detected using ALOS-2/PALSAR-2 data in ScanSAR (wide-swath) mode (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). The ALOS-2 ScanSAR data cover a width of 350 km, which is greater than the 250 km swath of Sentinel-1 interferometric wide (IW) mode. Wider image coverage is superior for revealing the spatial characteristics of coseismic deformation with fewer data acquisitions and for avoiding the contamination of early-phase post-seismic deformations.\u003c/p\u003e\u003cp\u003eTopography-dependent phase changes were corrected using ALOS World 3D-30m (AW3D30) data (Takaku et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Phase unwrapping was performed by the minimum cost flow algorithm implemented in the SNAPHU software (Chen and Zebker, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). Ionospheric errors were corrected by a range split spectrum algorithm proposed by Wegm\u0026uuml;ller et al. (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Using ALOS-2/PALSAR-2 data from Path 010 (descending orbit) and 113 (ascending orbit) for InSAR processing, we decomposed the two independent line-of-sight (LOS) changes into quasi-east\u0026ndash;west and quasi-up-down displacement components using the least-squares method.\u003c/p\u003e\u003cp\u003eTo model the slip distribution, we used the analytical solution of surface deformation caused by dislocations at each rectangular subfault under the assumption of an homogeneous elastic half-space with a Poisson\u0026rsquo;s ratio of 0.25 (Okada, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1985\u003c/span\u003e). Small patches at 15 km intervals were set to design the geometry of the plate boundary between the North American and Pacific plates (Iwasaki et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The slip patches extend over depths ranging from 6 km to 60 km along the modeled plate boundary. The dip angle of the derived slip ranged from less than 10\u0026deg; at a depth of 10 km to ~\u0026thinsp;35\u0026deg; at a depth of 60 km (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Shallower patches extended up to a depth of 6 km, corresponding to the distribution of aftershocks of the 2025 earthquake (through August 17, 2025) and seismicity during 1970\u0026ndash;2019.\u003c/p\u003e\u003cp\u003eThe ALOS-2/PALSAR-2 InSAR data were subsampled based on a quad-tree algorithm to reduce computational cost while retaining the spatial characteristics of coseismic deformation (J\u0026oacute;nsson et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). The slip distribution reproducing the ALOS-2/PALSAR-2 LOS changes in Path 010 and 113 was derived by solving least-squares problems. No constraints were imposed on the slip direction. The Laplacian smoothness for the slip distribution was optimized by minimizing the Akaike\u0026rsquo;s Bayesian Information Criterion value (Akaike, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1980\u003c/span\u003e). Each LOS change was weighted based on the variance in the LOS change inferred from the data outside the deformed area. Several slip constraints were applied at the bottom and side edges of the segment by assigning zero values.\u003c/p\u003e\u003cp\u003eAlthough no phase unwrapping errors were confirmed on the Kamchatka Peninsula, it remains unclear how the phase changes on the Kuril Islands, including Paramushir and Shumshu Islands, should be connected to those on the Kamchatka Peninsula (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). To avoid possible unwrapping errors, we separately unwrapped the phase changes over Kamchatka and the Paramushir\u0026ndash;Shumshu Islands, and then simultaneously estimated the offsets of LOS change between two regions and the coseismic slip distribution. This strategy prevents the phase unwrapping errors and also constrains the southern termination of slip on the segment.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo evaluate model uncertainty, we applied bootstrap algorithm (Efron, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1979\u003c/span\u003e; Hartzell et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). In each of 1000 iterations, we randomly extracted 20% of the subsampled observations, added 25% random noise, and then computed the standard deviation of the derived slip distribution.\u003c/p\u003e"},{"header":"3 Result","content":"\u003cp\u003eALOS-2/PALSAR-2 InSAR reached a peak of over 1m of LOS change at a peak in the southern Kamchatka Peninsula, approximately 200 km southwest of the hypocenter of the 2025 earthquake (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The gradient of wrapped LOS changes on the southern peninsula cross in the northeast\u0026ndash;southwest direction, whereas those on the Paramushir and Shumshu Islands are oriented in the north\u0026ndash;south direction, suggesting the dominance of east-west ground deformation gradient. The quasi-east\u0026ndash;west displacement decomposed from the ascending and descending LOS changes showed over 1.5 m of eastward displacement in the southern part of the Kamchatka Peninsula, while few quasi-up-down displacements were observed (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed).\u003c/p\u003e\u003cp\u003eBased on coseismic deformation derived from ALOS-2/PALSAR-2 data, we estimated the slip distribution along the plate boundary. The inferred slip distribution indicates a slip concentration in the shallower part of the segment (at depths less than 20 km), with a maximum slip of 16 m observed at 10\u0026ndash;20 km depth (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Furthermore, a slip greater than 10 m extended to the shallower edge of the slip patches at a depth of 6 km. The released geodetic moment magnitude reached 2.4\u0026times;10\u003csup\u003e22\u003c/sup\u003e Nm, corresponding to Mw 8.85, assuming a rigidity of 40 GPa. Most of the slip was directed perpendicular to the trench strike (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The epicenter was located in the northernmost and deepest portion of the derived slip distribution.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe preferred model reproduced the observations with root-mean-square error (RMSE) of 0.047 m and 0.030 m for Path 010 and 113 on Kamchatka, respectively (Figs. S2\u0026ndash;S3; Table \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e). The offsets of LOS change on Shumshu and Paramushir Islands were \u0026minus;\u0026thinsp;0.41 m and 0.25 m in Path 010 and 113, corresponding to maximum LOS changes of 0.6\u0026ndash;0.7 m, respectively (Table S3).\u003c/p\u003e\u003cp\u003eA standard deviation of slip exceeding 2 m was identified off the Kuril Islands, as inferred from the bootstrap algorithm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). The larger standard deviations may have resulted from insufficient data points on the northern Kuril Islands to constrain slip on the southwestern patches. The RMSE of the offsets of LOS change on the Kuril Islands were 0.13 and 0.10 for Path 010 and 113, respectively (Table S3).\u003c/p\u003e\u003cp\u003eThe resolution test showed that synthetic slip at a depth of greater than 20 km was well reproduced, whereas synthetic slip at shallower depths (\u0026lt;\u0026thinsp;20 km) was less well resolved, suggesting a possibly coarse resolution (Fig. S4). Despite lower sensitivity at shallower depths in our preferred model, higher sensitivity at deeper (landward, \u0026gt;\u0026thinsp;20 km) supports the plausibility of the inferred slip, which implies a smaller slip. When comparing the synthetic deformation derived from predefined slip distributions with the observations, the slip concentrated at the shallowest patches minimized the residuals between the observations and the synthetic deformations (Fig. S5). This implies that concentration of trenchward slip improves to fit the observed deformation.\u003c/p\u003e\u003cp\u003eThe slip distribution inferred using coseismic deformation only at Kamchatka Peninsula exhibited characteristics similar to those of the best-fit slip distribution, except for a reduced slip in the southwestern patches (Fig. S6). The released moment magnitude was 2.2\u0026times;10\u003csup\u003e22\u003c/sup\u003e Nm (Mw 8.83), which was nearly identical to that obtained from the model including deformation on the Kuril Islands.\u003c/p\u003e"},{"header":"4 Discussion","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Comparison with seismicity\u003c/h2\u003e\u003cp\u003eSeismicity can be used as an indicator to evaluate coupling at the plate boundary. Comparison with the inferred slip distribution shows that the major rupture extent coincides with areas of the sparse seismicity at shallower depths (\u0026lt;\u0026thinsp;20 km) of the Kamchatka Subduction Zone during 1970\u0026ndash;2019 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Instead, several earthquake clusters are distributed northeast and southwest of the 2025 rupture extent. The epicenter of the 2025 event was located near the northeastern cluster of earthquakes. Seismicity at deeper, landward depths (20\u0026ndash;60 km) is also sparse off the southern Kamchatka Peninsula, where the main rupture of the 1952 earthquake occurred. The epicenters of historic M\u0026thinsp;\u0026gt;\u0026thinsp;7 earthquakes since 1900 have also been predominantly distributed landward rather than near the trench (compiled by Johnson and Satake, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Vorobieva et al. (\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) also inferred the spatial variation in earthquake magnitude, however, near-trench parameters off the southern Kamchatka Peninsula were not constrained because of the limited number of earthquakes. Seismicity gaps in subduction zones, in general, suggest aseismic creeping or strong coupling between plate boundaries. Here, we interpret the shallow seismicity gap as consistent with strong plate locking in this region as discussed below.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe interseismic deformation detected by GNSS indicated full or partial coupling in the subduction zone offshore of the southern Kamchatka Peninsula (B\u0026uuml;rgmann et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Although B\u0026uuml;rgmann et al. (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) noted limited model resolution near the trench due to insufficient observation points, the back-slip model was consistent with strong coupling at a depth of 60 km near the coast of the peninsula. These patterns persist under stronger spatial smoothing in the back-slip model. The extent of strong coupling inferred from the back slip model overlaps with the main rupture extent in our model. Therefore, the shallow seismicity gap offshore of the southern Kamchatka Peninsula corresponds to strong plate coupling, which has the potential to accumulate slip deficit over a long period.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Slip deficit budget since the 1952 earthquake\u003c/h2\u003e\u003cp\u003eThe inferred slip distribution for the 2025 earthquake indicates a slip concentration offshore of the southern Kamchatka Peninsula at a shallower depth of 20 km, extending toward the trench (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Aftershocks were mainly distributed around 20 km depth but were also observed closer to the trench (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn the Kamchatka Subduction Zone, the along-strike rupture extent of the 2025 earthquake may be comparable to that of the M9.0 earthquake in 1952 (Johnson and Satake, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; MacInnes et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). In contrast, the preferred slip distribution in this study exceeds the budget of the cumulative slip deficit assuming that the 1952 earthquake occurred on the similar portion of the plate boundary. Even assuming full coupling, a simple slip budget estimate based on a plate convergence rate of ~\u0026thinsp;80 mm/yr yields less than 6 m of an average slip in the source region. Therefore, the time elapsed between the 1952 and 2025 events is insufficient to account for the inferred peak slip of ~\u0026thinsp;10\u0026ndash;15 m of coseismic slip for the 2025 event. Such an apparent imbalance suggests that slip deficit in the area ruptured in 2025 may have begun to accumulate prior to 1952. The excess of coseismic slip relative to the expected slip deficit budget is known as negative slip deficit (e.g., Moreno et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe slip distribution inferred from tsunami waveform inversion yields better resolution near the trench than landward (Romano et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). In contrast, onshore geodetic inversion has limited resolution near the trench (e.g., Cruz-Atienza et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). For the 2011 Tohoku-oki earthquake, a near-trench slip was required to explain tide gauge data (Hossen et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Satake et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), whereas onshore geodetic data were reproduced by the dominance of downward slip rather than ruptures near the trench (Romano et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Simons et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Although the coseismic deformation is best explained by the trenchward slip concentration as shown in Fig. S5, the resolution in our model is also coarse, especially at the shallower portion of the plate boundary.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e4.3 Possible interpretations of negative slip deficit\u003c/h2\u003e\u003cp\u003eWe discuss the possible interpretations for the imbalance between the slip deficit budget and coseismic slip of the 2025 earthquake. One possible interpretation is the partial failure of slip deficit to be released due to the 1952 earthquake, which was not fully released during the 1952 rupture, partially slipped in 2025. Similar imbalances between slip deficit budget and coseismic slip have been reported for the 2010 Maule earthquake (e.g., Moreno et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). In the simple framework, we expect megathrust earthquakes to release most of slip deficit budget accumulated since the previous event, but such imbalances can be accommodated by contributions from aseismic slip during the postseismic period, aftershocks, or apparent local overshoot.\u003c/p\u003e\u003cp\u003eMegathrust earthquakes can induce rapid early afterslip on and around the ruptured area of the mainshock (e.g., Ozawa et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), which has a potential to release a significant fraction of the cumulative deficit. In our case, the slip distribution is inferred solely from coseismic deformation observed by ALOS-2/PALSAR-2 data acquired within 3 days of the mainshock, thus contamination from postseismic deformations should be limited. The largest aftershock before the acquisition of secondary ALOS-2/PALSAR-2 data was an Mw 6.9 event occurring 45 minutes after the mainshock (USGS, 2025), whose additional contribution is unlikely to dominate the large-scale deformation pattern in our model. Although there is no further information to support interpretations that previous studies suggested, we cannot rule out the residual of a partial release of the slip deficit associated with the 1952 earthquake to explain the excess of coseismic slip in 2025.\u003c/p\u003e\u003cp\u003eAnother possible interpretation is a fine-scale slip segmentation in the subduction zone. The slip segmentation in both along-strike and along-dip directions is a key characteristic for inferring the slip deficit budget and facilitating discussions about the earthquake cycle. The along-strike heterogeneity of interseismic coupling indicates cumulative slip deficit, the recurrence likelihood, and asperity distribution (Chlieh et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Loveless and Meade, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Moreno et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The along-strike variation in morphology and seismic behavior is mainly controlled by pre-existing crustal structures of plates, which correspond to gravity anomalies and residual bathymetry (Bassett and Watts, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Global comparisons and analog modeling suggest a correlation between high seismic coupling and relatively low plate boundary roughness, but this correlation is not universal (Lallemand et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; van Rijsingen et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAlong-dip segmentation is typically characterized by a depth-dependent variation in the rupture process: aseismic creeping, slow slip, and tremor in the conditionally stable shallow portion; unstable, seismogenic behavior at intermediate depths; and slow slip and tremor in the deeper transient region (Cruz-Atienza et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Hirose et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). In general, the shallowest portion of the subduction zone (accretionary wedge) is a conditionally stable region that tends to have long duration transients, such as slow slip or tremors. Coseismic stress transfer from the seismogenic zone can produce coseismic strengthening in the shallowest accretionary region, which tends to inhibit trench breaching ruptures if the strengthening exceeds a critical threshold (Hu and Wang, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). However, an up-dip rupture nucleating at greater depths can potentially propagate to the shallowest portion, as observed in the 2011 Tohoku earthquake. This represents an additional consideration\u0026mdash;and a possible exception\u0026mdash;when evaluating the slip deficit (Lay et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Yomogida et al. (\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) suggested along-dip segmentation in the Japan trench and the Aleutian-Alaska Subduction Zone, based on contrasts in background seismicity and rupture areas of the 2011 Tohoku-oki earthquake and the 1964 Alaska earthquake. In both regions, the shallow seismicity gap near the trench coincides with the main rupture extent at shallower depths. In contrast, such features are not evident in the Chilean or Sumatran subduction zones, suggesting that they are not universally present. While the study did not analyze the characteristics of the Kamchatka Subduction Zone, the seismicity gap at the shallower depths during 1970\u0026ndash;2019 and the rupture area in 2025 are similarly overlapped (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The slip segmentation may be a possible interpretation for explaining the imbalance between the slip deficit budget and the peak coseismic slip; however, it is impossible to resolve the fine-scale segmentation because of limited modeling resolution as we show in Fig. S4.\u003c/p\u003e\u003c/div\u003e"},{"header":"5 Conclusion","content":"\u003cp\u003eThe slip distribution associated with the M8.8 Kamchatka earthquake in 2025 indicated a peak slip on the order of 10\u0026ndash;15 m in shallow, near-trench portions. Although the model resolution is limited in the shallow near-trench region, the observed coseismic deformation detected by ALOS-2 InSAR implies concentrations of apparent trenchward slip. The main ruptured area overlapped with the shallow seismicity gap and the region of strong interseismic plate coupling that has persisted for decades, suggesting long-term slip deficit accumulation prior to the 1952 event. Even if the 1952 event ruptured at similar extents, the temporal interval between 1952 and 2025 is insufficient to accumulate the slip deficit budget for explaining the inferred slip distribution. The significant imbalance of the slip deficit budget and the coseismic slip may reflect partial failure of slip deficit left by the 1952 earthquake or fine-scale slip segmentation in subduction zones. However, such interpretations cannot yet be uniquely resolved due to the limited model resolution.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eALOS-2\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eAdvanced Land Observation Satellite-2\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003ePALSAR-2\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003ePhased-Array L-band Synthetic Aperture Radar-2\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eSAR\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eSynthetic Aperture Radar\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eInSAR\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eInterferometric Synthetic Aperture Radar\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eRMSE\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eRoot-mean-square error\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eGNSS\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eGlobal Navigation Satellite System\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eUSGS\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eUnited States Geological Survey\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eNOAA\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eThe National Oceanic and Atmospheric Administration\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eIW\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eInterferometric Wide\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eLOS\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eLine-of-sight\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe processed data in this study are available from an institutional repository (doi: 10.57499/XXX). Bathymetry data is available from GEBCO (https://download.gebco.net/). The earthquake catalog is available on the United States Geological Survey website (https://earthquake.usgs.gov/earthquakes/search/). The original ALOS-2/PALSAR-2 can be purchased from either RESTEC (https://www.restec.or.jp/en/) or PASCO (http://en.alos-pasco.com).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eYH: Conceptualization, Methodology, Formal analysis, Investigation, Writing \u0026ndash; original draft. HM: Interpretation, Discussion, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eALOS-2/PALSAR-2 data were provided by the Japan Aerospace Exploration Agency (JAXA) under a cooperative agreement between the Geospatial Information Authority of Japan (GSI) and JAXA. All figures were generated using Generic Mapping Tools (Wessel et al. 2013).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAkaike H (1980) Likelihood and the Bayes procedure. 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Earth Planets and Space 63:34. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.5047/eps.2011.06.003\u003c/span\u003e\u003cspan address=\"10.5047/eps.2011.06.003\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\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":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Megathrust earthquake, Ground deformation, Kamchatka, InSAR","lastPublishedDoi":"10.21203/rs.3.rs-8111287/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8111287/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe detected coseismic deformation associated with the 2025 Kamchatka earthquake using Advanced Land Observing Satellite-2 (ALOS-2) ScanSAR (wide-swath) data. The coseismic deformation revealed a predominantly eastward displacement of more than 1.5 m, with smaller vertical movement observed on the southern Kamchatka Peninsula. Our best-fit model prefers a shallow, near-trench slip concentration on the order of 10\u0026ndash;15 m at 10\u0026ndash;20 km depth and a similar along-strike extent to that of the 1952 M9.0 earthquake inferred from tsunami or historical records in previous studies. Furthermore, the trenchward rupture in 2025 overlapped the seismicity gap observed between 1970 and 2019 as well as areas of strong plate coupling, suggesting a decades-long accumulation of slip deficit. In contrast, the upper-bound of slip deficit budget is insufficient to explain the inferred peak coseismic slip, because the slip deficit reaches at most\u0026thinsp;~\u0026thinsp;6 m, even with full coupling and a convergence rate of about 80 mm/yr since the 1952 event. The imbalance between the slip deficit budget and the coseismic slip may reflect a partial failure to release slip deficit due to the 1952 earthquake or fine-scale slip segmentation in the subduction zone; however, the model resolution is limited to resolve these interpretations uniquely.\u003c/p\u003e","manuscriptTitle":"Shallow segment rupture associated with the 2025 Kamchatka earthquake inferred from ALOS-2/PALSAR-2 InSAR","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-03 21:22:16","doi":"10.21203/rs.3.rs-8111287/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Minor Revision","date":"2026-01-30T00:32:55+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-11-25T05:59:20+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-25T05:49:51+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-15T14:50:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Earth, Planets and Space","date":"2025-11-14T01:22:39+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"652d7d1a-b583-4d36-acd9-9c862ccfc428","owner":[],"postedDate":"December 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-04-20T16:08:11+00:00","versionOfRecord":{"articleIdentity":"rs-8111287","link":"https://doi.org/10.1186/s40623-026-02437-4","journal":{"identity":"earth-planets-and-space","isVorOnly":false,"title":"Earth, Planets and Space"},"publishedOn":"2026-04-14 15:57:45","publishedOnDateReadable":"April 14th, 2026"},"versionCreatedAt":"2025-12-03 21:22:16","video":"","vorDoi":"10.1186/s40623-026-02437-4","vorDoiUrl":"https://doi.org/10.1186/s40623-026-02437-4","workflowStages":[]},"version":"v1","identity":"rs-8111287","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8111287","identity":"rs-8111287","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}