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The Hokkaido Eastern Iburi Earthquake (M = 6.7) occurred on Sep. 6, 2018 in the southern part of Central Hokkaido, Japan. Since Paleogene, this region has experienced the dextral oblique transpression between the Eurasia and North American (Okhotsk) Plates and the subsequent collision between the Northeast Japan Arc and the Kuril Arc due to the oblique subduction of the Pacific Plate. This earthquake occurred beneath the foreland fold-and-thrust belt of the Hidaka Collision zone developed by the collision process, and is characterized by its deep focal depth (~ 37 km) and complicated rupture process. The reanalyses of controlled source seismic data collected in the 1998–2000 Hokkaido Transect Project revealed the detailed structure beneath the fold-and-thrust belt, and its relationship with the aftershock activity of this earthquake. Our reflection processing using the CRS/MDRS stacking method imaged for the first time the lower crust and uppermost mantle structures of the Northeast Japan Arc underthrust beneath a thick (~ 5–10 km) sedimentary package of the fold-and-thrust belt. Based on the analysis of the refraction/wide-angle reflection data, the total thickness of this Northeast Japan Arc crust is only 16–22 km. The Moho is at depths of 26–28 km in the source region of the Hokkaido Eastern Iburi Earthquake. Our hypocenter determination using a 3D structure model shows that most of the aftershocks are distributed in a depth range of 7–45 km with steep geometry facing to the east. The seismic activity is quite low within the thick sediments of the fold–thrust belt, from which we find no indication on the relationship of this event with the shallow (< 10–15 km) and rather flat active faults developed in the fold-and-thrust belt. On the other hand, a number of aftershocks are distributed below the Moho. This high activity may be caused by the cold crust delaminated from the Kuril Arc side by the arc–arc collision, which prevents the thermal circulation and cools the forearc uppermost mantle to generate an environment more favorable for brittle fracture.

We deployed 30 temporary seismic stations around the source region of the 2024 Mw 7.5 Noto Peninsula earthquake to investigate the relationship between mainshock rupture and fault geometry. Using machine learning techniques, we detected and precisely located 46,252 aftershocks, revealing several fault planes corresponding to active faults ruptured during the mainshock. Two subparallel landward‐dipping planar structures were identified near the mainshock hypocenter, converging westward to the Wajima‐Oki segment. This geometry suggests that complex rupture episodes near the hypocenter resulted from successive ruptures on adjacent fault planes rather than slip on a single plane. At the western edge of the 2024 rupture area, aftershocks extend close to but rarely occur on the fault that ruptured during the 2007 Mw 6.7 earthquake, suggesting that the 2024 earthquake did not re‐rupture the same faults of the 2007 earthquake.

To clarify the relationship between earthquake occurrence and fluid, we analyzed data from a dense aftershock-observation network with 69 high-gain short-period seismographs installed immediately after the mainshock occurrence (October 21) in the aftershock area of the 2016 Central Tottori Prefecture earthquake. We determined the hypocenters and focal mechanisms of the aftershocks very precisely in the period from October 22 to December 15. We then investigated the temporal changes in the spatial distributions of hypocenters and T-axis azimuths of focal mechanisms. The distributions of aftershock hypocenters and T-axis azimuths are basically temporally stable, except those in limited portions in the shallow layer near the western edge of the aftershock area, where rapid decrease of aftershocks with T-axis azimuths of WSW to west was observed. If fluid rises from the lower crust due to fault rupture, the locations of aftershocks and focal mechanisms may change over time, especially in the deepest part of the aftershock region. However, the temporal change in these parameters was not apparent at depth. These observations suggest that the aftershock activity of the Central Tottori Prefecture earthquake was controlled mainly by stress concentration rather than strength reduction due to high fluid pressure.

In the original publication of this article (Iwasaki et al. 2019), the author name ‘Matsubara Makoto’ in author list is not correct. The correct name should be ‘Makoto Matsubara’.

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

After a large earthquake, many small earthquakes, called aftershocks, ensue. Additional large earthquakes typically do not occur, despite the fact that the large static stress near the edges of the fault is expected to trigger further large earthquakes at these locations. Here we analyse ~10,000 highly accurate focal mechanism solutions of aftershocks of the 2016 Mw 6.2 Central Tottori earthquake in Japan. We determine the location of the horizontal edges of the mainshock fault relative to the aftershock hypocentres, with an accuracy of approximately 200 m. We find that aftershocks rarely occur near the horizontal edges and extensions of the fault. We propose that the mainshock rupture was arrested within areas characterised by substantial stress relaxation prior to the main earthquake. This stress relaxation along fault edges could explain why mainshocks are rarely followed by further large earthquakes.

The northern Fossa Magna (NFM) basin is a Miocene rift system formed in the final stage of the opening of the Japan Sea. The northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna. In order to understand the active tectonics in these areas, it is essential to explain the seismic velocity structures, deep structures of active faults, and microseismicity near the active faults. In the autumn of 2002, we conducted a seismic array observation across the northern part of the ISTL and the NFM to obtain a structural image beneath the NFM. Arrival times of local earthquakes and explosive shots were used in a joint inversion for earthquake locations and 3-D V p and Vp/V s structures. P- and S-wave arrival time data were obtained from 73 events including 4 explosive shots, and 3809 P- and 2659 S-wave arrival times were used for the inversion analysis. We obtained a seismic velocity model revealing good correlations with the surface geology along the profile. In particular, we found thick low-velocity zones beneath the NFM and the Komoro basin and a high-velocity zone beneath the Central Uplift Zone. Beneath the NFM, a low-velocity zone with low-to-moderate V p/V s extends to a depth of approximately 10 km. The low-velocity suggests the existence of aqueous fluid-filled pores with high aspect ratios.

A seismic experiment was carried out in east Shikoku, Japan, to detect deep reflections across the Median Tectonic Line (MTL), which juxtaposes low-P/T metamorphic rocks with high-P/T metamorphic rocks. Our experiment employed an unconventional technique: sparse shot spacing, a strong energy source (dynamite) and a dense array of seismometers. The above specifications produce only single fold coverage without common midpoint (CMP) stacking. Nevertheless, the reflection profile provides essential information on the deep structure of the MTL, of other major faults, and of the Moho in east Shikoku. On the MTL, this profile is the first to delineate the MTL from the surface to about 12 km depth. The following three factors were essential to the success of our experiment. First, the receiver interval was sufficiently small to provide horizontal resolution that was able to detect deep reflectors. Second, the simple crustal structure does not require CMP stacking to enhance data quality. Third, a thin weathering layer at the surface reduced the attenuation of seismic waves and minimized the generation of the surface waves that often obscure deep reflectors. In these conditions, the technique can be an effective means of probing the deep crust while substantially reducing survey costs.

The velocity structure and accurate aftershock distributions of the Noto Hanto Earthquake in 2007 (thrust type) are elucidated by inverting the arrival times from 917 aftershocks using double-difference tomography. P -wave velocity ( V p ) of the hanging wall in the southeast appears to be higher than that of the footwall in the northwest, and the high- V p body of the hanging wall has a relatively high V p / V s ratio. Conversely, the low- V p body in the footwall appears to have a low V p / V s ratio at depths greater than 3 km. Aftershocks associated with the mainshock fault are roughly distributed along this velocity boundary between the hanging wall and footwall. Near-surface thin layers with significantly low V p and high V p / V s are imaged in a northwest direction from the mainshock epicenter. A likely explanation is that the mainshock fault plane was reactivated as a reverse fault in terms of the inversion tectonics due to the crustal shortening which initiated from the late Miocene. Both the mainshock hypocenter and the vertical alignment of aftershocks beneath it are located in the low- V p and low- V p / V s zones, indicating the potential presence of water-filled pores. Crustal stretching and shortening in and around the Noto Peninsula have created complex structures, including weak high-dip angle faults, almost vertical faults, and low velocity zones, which can potentially affect the seismic activities around the source region.

The velocity structure and accurate aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake (thrust type) are obtained by inverting the arrival times from 848 aftershocks observed by a dense seismic network deployed immediately after the mainshock (8 h later). Both the detailed velocity structure and the accurate aftershock distribution show lateral heterogeneity along the fault strike. In the northeast area, aftershocks are aligned along both the NW- and SE-dipping planes. These planes are conjugate to each other. The mainshock hypocenter is located close to the bottom of an approximately 50° NW-dipping plane, which indicates that the mainshock rupture could have initiated on the NW-dipping plane. The high- V p body beneath this aftershock alignment shows a convex upward shape. In contrast, from the center to the southwest area, most of the aftershocks are aligned along SE-dipping planes. The high- V p body beneath this aftershock alignment shows a convex downward shape. Based on these results, we suggest that the crustal structure in the source region is divided into two segments by a boundary zone situated between the northeast and southwest areas. It should be noted that this segment boundary zone is coincident with the complex aftershock zone where numerous conjugate fault planes exist. We propose that the mainshock rupture initiated near the bottom of the NW-dipping fault plane and ran to the southwest, then transferred at the segment boundary zone which has numerous conjugate fault planes to the SE-dipping plane.