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The Mw 7.5 Noto Peninsula earthquake, which occurred on January 1, 2024, was considerably hazardous to the peninsula and surrounding regions owing to a strong motion, large-scale crustal deformation, and subsequent tsunami. Significant surface displacement was observed by the dense global navigation satellite system (GNSS) stations, including universities and SoftBank corporation sites, and synthetic aperture radar (SAR). To estimate reliable coseismic slip distribution and its uncertainties for this event, we used the dense GNSS and the line-of-sight displacements from the SAR based on the Bayesian optimization framework. Considering the listric fault structure of this source fault, we validated the fault dip angles using the grid-search approach in the slip estimation. The acquired models indicated reverse fault motion, including a right-lateral slip component, and two slip peaks were estimated in the eastern and western regions of the fault in the central peninsula, independent of the assumed dip angles. These locations correspond to regions of significant uplift and westward displacement. The dip angle assumption affects the horizontal and vertical component of the calculated displacements: a higher dip angle model (≥ 45°) well reproduces vertical components of GNSS and synthetic aperture radar displacements, whereas a lower dip angle model (< 45°) well reproduces horizontal displacements. Overall, a fault dip of 45° is plausible, although it is not consistent with the listric structure suggested by the seismic reflection survey and the aftershock distribution below the central part of the peninsula. To test such a listric fault model, we conducted a coseismic slip estimation assuming a relatively high (60°) and low (25°) dip angles for the shallow and deep sections of the fault, respectively. Even in this case, we acquired a slip model similar to that of a plain fault, which reasonably reproduced each component of the surface displacements as well as the simple plane fault models. These results suggest that listric geometry is also acceptable for the source faults of this event, although the flat geometry similarly explains the observations. Graphical Abstract

The Noto Peninsula has experienced seismic swarms accompanied by transient crustal deformation since November 2020, followed by two major earthquakes (M6.5 on May 5, 2023, and M7.6 on Jan. 1, 2024). Previous studies have suggested that fluids are involved in a series of activities. Most evidence on fluids constrains only their existence, and quantitative information on dynamic fluid migration remains scarce. Past precise gravity measurements in volcanic areas captured changes at the μGal scale (10 –8  m/s 2 ) due to magma movement. Here, we report the gravity difference caused by the M6.5 earthquake that was obtained via a similar method of measurement. Most of the observed gravity change can be explained by a fault slip model determined from the geodetic inversion of GNSS data. However, a significant change of approximately 10 μGal remains unexplainable in the northern coastal area of the northeastern tip of the Noto Peninsula. To explain this change, we estimate environmental effects, such as groundwater and sea-level variations. These environmental effects are too small to fully explain the change unless large local groundwater changes that are not represented in the groundwater model are considered. Instead, adding a fluid-fed fault that opens above the coseismic fault could reasonably explain both the GNSS and gravity data. The inferred volume of fluids is approximately 10% of the volume to have accumulated in a deeper fault by June 2022, as estimated from GNSS data. This result suggests that fluids migrating to shallower areas may have increased the risk of the M7.6 earthquake. The relatively shallow seismic velocity anomalies inferred by seismic tomography might indicate that such an upward migration process due to large earthquakes has been repeated in the past. Graphical Abstract

Seismic activity in the Noto region of Ishikawa Prefecture, central Japan, has increased since August 2020 and has continued as of August 2023. Stress changes due to subsurface sources and increases in fluid pressure have been discussed as the causes of the seismic activity increase. In this study, S-wave polarization anisotropy was investigated by S-wave splitting analysis using temporary and permanent stations located in the epicenter area. We also investigated the seismic wave velocity structure in the source region by analyzing seismic wave velocity tomography. The fast orientations of anisotropy (fast shear wave oscillation direction, FSOD) were generally NW–SE in the southern part of the focal area and east–west in the northern part. The NW–SE anisotropy generally coincides with the direction of the maximum horizontal compression axis, both near the surface and at earthquake depths. Therefore, stress-induced anisotropy can be the cause of the observed NW–SE anisotropy. On the other hand, faults with strike directions generally east–west have been identified, and structural anisotropy may be the cause of the observed east–west anisotropy. We examined the time variation of anisotropy at N.SUZH, one of the permanent stations. No significant time variation was observed in the FSOD. Larger anisotropy was observed, particularly for the activity in the western part of the focal area, from about June–September 2021 compared to the previous period. A high Vp/Vs region was identified beneath the focal area, at a depth of 18 km. This high Vp/Vs region has slightly larger P-wave velocities than the surrounding area. Since Tertiary igneous rocks are distributed in the target area, the high Vp/Vs region may represent a Tertiary magma reservoir, suggesting that fluids released through the old magma reservoir are involved in this seismic swarm. This seismic activity started in the southern part of the area, where relatively immature fault structure exists, where stress-induced anisotropy is distributed, and where high Vp/Vs regions suggestive of fluid at depth are identified. Subsequently, seismicity became more active in the northern part, where structural anisotropy with well-developed fault structures is distributed. Graphical Abstract

We investigated the characteristics and mechanism of postseismic deformation following the 2024 Noto Peninsula earthquake (M7.6), which occurred on January 1, 2024, in the Noto Peninsula, Japan. The observed postseismic deformation for 10 months after the earthquake indicated that the magnitude of the horizontal deformation near the fault (Noto Peninsula) and farther away (Niigata Prefecture and Toyama Prefecture) was nearly the same, ranging from 2 to 4 cm toward the northwest. Regarding vertical deformation, subsidence of several centimeters was observed in the Noto Peninsula, while uplift of a few centimeters was observed from Niigata Prefecture to Toyama Prefecture. No single mechanism, poroelastic rebound, afterslip, or viscoelastic relaxation, could explain all the observed deformations. The key to understanding multiple mechanisms lied in the vertical deformation particularly, the deformation at the Hegura Island station, the only station located northwest of the focal region. Based on the constraint that the Hegura Island station subsides, we constrained the parameters governing viscoelastic relaxation and then estimated afterslip based on the data that removed the deformation caused by viscoelastic relaxation. This approach allowed us to explain the observed horizontal and vertical deformations. The estimated optimal thickness of the elastic layer and viscosity were 40 km and 4 × 10 18  Pa·s, respectively. Afterslip was estimated in the northern part of the Noto Peninsula. The postseismic deformation for 10 months after the earthquake was dominated by viscoelastic relaxation, and the effects of the afterslip were confined to the northern part of the Noto Peninsula. The slip distribution differed significantly when considering only afterslip compared to when considering multiple mechanisms. Focusing on a single mechanism can lead to misinterpretations. Finally, since postseismic deformation of several mm/yr is expected to continue for decades or longer, it must be remembered that postseismic deformation is included in the monitoring of crustal deformation in this region. Graphical Abstract

The Noto Peninsula earthquake (Mj7.6), which occurred on New Year’s Day of 2024, had two characteristic features: multiple tremors at the initiation of the rupture and a long fault rupture exceeding 100 km. The source process included three significant tremors for 15 s: Mj ~ 3 event, Mj 5.9 event, and Mj 7.6 event. The rupture started at the tip of the Noto Peninsula and propagated bilaterally in northeast and southwest directions. We evaluated the performance of the Japanese earthquake early warning (EEW) issued to the public. The source determination process of the EEW was triggered by the preceding Mj ~ 3 event and the warning threshold was exceeded by the Mj 5.9 event, so there was at least a 13-s lead time before the S-arrival of the Mj 7.6 event, allowing many residents to take protective measures. The first warning was issued to only the Northern part of the Ishikawa prefecture. However, the second warning that was distributed to as far as a few hundred kilometers was issued 27.1 s after the first warning, which was longer than expected. This is because the magnitude was underestimated during the rupture process and the warning was issued based on the shaking observation of the Mj7.6 event. We recomputed the shaking estimation from the Integrated Particle Filter (IPF) method and the Propagation of Local Undamped Motion (PLUM) method used in the Japanese EEW, and additionally, the XYtracker method to evaluate the effect of fault finiteness. At the initial part of the rupture, the fault finiteness is difficult to capture, and the finite-source approach produced a similar shaking estimation to the point-source approach. As the rupture propagates, shakings in the western area near the fault were significantly underestimated by the point-source approach. For large earthquakes, considering fault finiteness may be able to capture the rupture directivity and improve the accuracy of shaking estimation. Graphical Abstract

Major intraplate earthquakes pose a substantial threat to nearby inhabited regions, but their rupture characteristics are often unclear due to limited observations. The 2024 Mw 7.5 Noto Peninsula earthquake in Japan, recorded by numerous near-fault strong-motion seismometers, high-rate GNSS, and satellite data, presents a unique opportunity to investigate fault rupture evolution and the resulting strong ground motions in detail. Using kinematic rupture modeling, we developed a source model that reproduces SAR-based and GNSS data, as well as near-fault velocity and displacement waveforms with periods longer than 4 s. Our approach integrates 3D velocity and inelastic attenuation models for Japan, incorporating regional topography and bathymetry. To reduce the number of unknown parameters, we used an a priori fault slip model derived from SAR and GNSS data and fixed the fault geometry and final slip distribution, adjusting only the rupture timing and rise time of individual fault segments. The preferred source model reveals multiple slip episodes and intricate rupture evolution, including a backward-propagating rupture toward the mainshock hypocenter likely triggered by abrupt changes in local fault geometry. The mainshock hypocenter and subsequent rupture initiations occur in areas of increased shear stresses along the periphery of the preceding swarm activity. These subsequent ruptures propagated bilaterally along southwestern and northeastern fault segments with rupture speeds ranging from 1.4 to 2.1 km/s, slower than those of other intraplate thrust earthquakes of similar magnitude. The southwestward rupture broke large slip asperities (up to ∼ 10 m) on non-planar fault segments offshore Monzen, where the coseismic uplift was ∼ 4 m. Our results suggest that the 2024 Noto Peninsula earthquake is a remarkable example of a complex intraplate earthquake involving multi-segment rupture with multiple slip episodes, providing important insights into the physics of rupture propagation and the resulting ground motions. Graphical Abstract

On January 1st, 2024, a moment magnitude ( M w ) 7.5 earthquake occurred on an active reverse fault in the northern part of Noto Peninsula, being one of the largest intraplate events recorded in Japan. In previous studies, the dynamic triggering of seismicity in Japan following some large remote earthquakes has been well documented, such as in the case of the 2011 M w 9.0 Tohoku–Oki earthquake, the 2016 M w 7.1 Kumamoto earthquake, and other large teleseismic events. In this study, we investigate the remote triggering of microearthquakes by the 2024 Noto earthquake and their characteristics. We analyze waveform data recorded at high-sensitivity seismic stations in Japan, before and after the occurrence of the Noto mainshock. Local earthquakes are detected on high-pass filtered three-component seismograms. Low-pass filtered waveforms are used for visualizing the mainshock surface waves and estimating dynamic stresses. Our results show a relatively widespread activation of small earthquakes—none of them listed in the Japan Meteorological Agency (JMA) earthquake catalog—that were triggered by the passage of the mainshock surface waves in many regions of Japan. These include Hokkaido and Tohoku in northeastern Japan, Kanto in central Japan, and Kyushu in southern Japan. The triggering is mostly observed in volcanic regions, supporting the hypothesis that such places are relatively easy to be activated dynamically, likely due to the excitation of fluids by the passage of mainshock surface waves. The calculated dynamic stress changes estimated from peak ground velocities, which triggered the earthquakes after the Noto mainshock, are in the range 12.8–102.6 kPa. We also report potential, less well-constrained dynamic triggering by the M w 5.3 Noto foreshock, which occurred ~ 4 min before the mainshock, at levels of stress about 100 times smaller. The analysis of a longer-term (1 month) seismicity pattern, based on the JMA catalog, revealed a statistically significant increase of seismicity in the remote Akita–Yakeyama (Tohoku region) volcanic area, following the Noto earthquake. Graphical Abstract

A crustal earthquake of the 2024 Noto Peninsula earthquake with a moment magnitude of 7.5 occurred on January 1, 2024, and was followed by many aftershocks distributed in both onshore and offshore regions. The mainshock was characterized as a reverse fault with NW–SE pressure- (P-) axes. We conducted centroid moment tensor (CMT) inversions for the aftershocks using a three-dimensional seismic velocity structure model to capture the detailed stress state and fault geometries around the source region. CMT solutions were obtained for 221 aftershocks with moment magnitudes of 3.2–6.1 at depths shallower than 15 km. Our approach showed substantial improvement in depth determination of CMT solutions, compared with that of the hypocenter determination using P- and S-wave arrival times, even for the early aftershock period when no seismic station was available close to the earthquake source region. Our CMT solutions were characterized as follows: (1) reverse faults with an NW–SE P-axis, which is consistent with that of the mainshock mechanism; (2) strike-slip faults in predominantly shallower regions compared with those of type 1; (3) reverse faults with ENE-WSW P-axes, possibly activated following the mainshock in the shallow southwestern aftershock region; and (4) earthquakes predominantly featuring normal and strike-slip faults localizing at a depth of approximately 5 km around the dip transition zone in geologically constructed fault models. Additionally, we conducted the same CMT inversion for earthquakes that occurred between 2007 and 2023 to further understand the effect of the mainshock on the aftershock dynamics. We confirmed that CMT solutions of types 3 and 4 appeared as new earthquake categories after the mainshock, suggesting that the mainshock could have triggered them. Our results provide a deeper understanding of complex stress fields and fault geometries in the source region. Graphical Abstract

The Noto Peninsula earthquake occurred on January 1, 2024, with an M J of 7.6. This earthquake was triggered by active submarine faults previously mapped off the northern coast of the Noto Peninsula and generated a tsunami. We mapped the tsunami inundation areas and tsunami runup and inundation heights associated with the 2024 earthquake along the coast of the entire Noto Peninsula based on high-resolution aerial photographs and field surveys. The tsunami inundation area was widely distributed along the coast of the Noto Peninsula, Hegurajima, and Notojima Islands. The tsunami inundation area was 3.7 km 2 in total. The tsunami inundation areas were distributed continuously along the west and east coasts of the Noto Peninsula. In contrast, these were discontinuous along the northern coast of the Noto Peninsula. The characteristics of the regional patterns of the tsunami inundation were broadly consistent with the tsunami inundation assumptions made for tsunami hazard maps before the 2024 earthquake. The tsunami height was different between the east and west coasts of the Noto Peninsula, with a peak of 11.3 m in elevation at Kuroshima. The tsunami on the west coast was higher than the one on the east coast. This was due to the location of the earthquake source fault and the distribution of the slip amount of the fault. The distribution of tsunami heights is also influenced by tsunami propagation characteristics, such as reflection and refraction. The tsunami runup and inundation height of the 2024 earthquake was the largest along the coast of the northern Noto Peninsula since the twentieth century compared with the past earthquakes. In contrast to the tsunami height, damages induced by the tsunami were smaller along the west coast and larger along the east coast, which was attributed to the location of the settlements and the presence of coastal structures. This study will contribute to future tsunami disaster prevention along the Sea of Japan coastlines crucial for improving future response strategies by accurately determining tsunami height and potential damage. Graphical Abstract

The Noto Peninsula, extending northward into the Sea of Japan, features a narrow, elongated shape, complex coastal topography, and numerous active faults along its coastline. Since December 2020, intense earthquake swarms accompanied by crustal deformation have occurred in the northeastern peninsula, likely caused by fluid upwelling from deep underground. The largest event, a Magnitude 7.6 earthquake, struck on January 1, 2024, with aftershock distributions indicating multiple faults ruptured over approximately 150 km. This study aimed to clarify the temporal and spatial variation in seismic wave radiation and investigate the source process of the M7.6 event using the back-projection method. This method estimates the origin of wave packets recorded by a seismic array. In Japan, seismic networks operated by local governments often include densely distributed stations to evaluate seismic intensity. We used these dense sites as a seismic array complemented by strong ground motion data from NIED K-NET and KiK-net. The analysis assumed three fault planes, based on previous studies. Velocity waveforms in two frequency bands (0.05–2.0 Hz and 0.5–5.0 Hz) were used to estimate areas of strong radiation intensity, representing the sources of seismic waves. In the low-frequency band, strong radiation intensity was observed near the rupture initiation point and in shallow regions of the northern Noto Peninsula, corresponding to large fault slips that caused the uplift of the coastline. In contrast, no strong radiation intensity was detected off the northeast coast of the Noto Peninsula in the low-frequency band, suggesting the absence of a significant slip. High-frequency analysis revealed distributions of strong radiation intensities complementary to those in the low-frequency band. A subevent occurring around 20 s after the rupture initiation was found to originate near the northern coast of the Noto Peninsula. Graphical Abstract