We investigate the rupture process of the M9.0 Tohoku‐Oki mega‐thrust earthquake using the relatively low‐frequency strong‐motion records (0.01–0.125 Hz) observed at 36 K‐NET and KiK‐net stations, the epicentral distances of which range from 120 km to 400 km. The fault model is a rectangular plane, the length and width of which are 510 km along the Japan Trench and 210 km along subducting direction of the Pacific Plate, respectively. We perform the multi‐time‐window inversion analysis with a 30 × 30 km2 subfault. The derived slip model has one large slip area. This area extends from the region around the hypocenter to the shallow part of the fault plane and further to the north and south along the trench axis, located far off southern Iwate, Miyagi, and northern Fukushima prefectures. The seismic moment is 4.42 × 1022 Nm (Mw 9.0) and the maximum slip is 48 m. The slips near the coast are relatively small, except off Miyagi prefecture, which experienced a slip greater than 5 m. The shallow large slip area, which continuously ruptured from 60 s to 100 s after the initial break, radiated seismic waves rich in very‐low‐frequency content (<0.02 Hz). The rupture after 100 s propagating to the southern fault area, contributes to the distinct phases observed for Fukushima and Ibaraki prefectures. The relationship between the proposed rupture model and the feature of the acceleration waveforms is not straightforward and suggests the frequency dependency of the seismic wave radiation.

The detailed source rupture process of the M 7.3 event (April 16, 2016, 01:25, JST) of the 2016 Kumamoto, Japan, earthquakes was derived from strong-motion waveforms using multiple-time-window linear waveform inversion. Based on the observations of surface ruptures, the spatial distribution of aftershocks, and the geodetic data, a realistic curved fault model was developed for source-process analysis of this event. The seismic moment and maximum slip were estimated as 5.5 × 1019 Nm (M w 7.1) and 3.8 m, respectively. The source model of the M 7.3 event had two significant ruptures. One rupture propagated toward the northeastern shallow region at 4 s after rupture initiation and continued with large slips to approximately 16 s. This rupture caused a large slip region 10-30 km northeast of the hypocenter that reached the caldera of Mt. Aso. Another rupture propagated toward the surface from the hypocenter at 2-6 s and then propagated toward the northeast along the near surface at 6-10 s. A comparison with the result of using a single fault plane model demonstrated that the use of the curved fault model led to improved waveform fit at the stations south of the fault. The source process of the M 6.5 event (April 14, 2016, 21:26, JST) was also estimated. In the source model obtained for the M 6.5 event, the seismic moment was 1.7 × 10 18 Nm (M w 6.1), and the rupture with large slips propagated from the hypocenter to the surface along the north-northeast direction at 1-6 s. The results in this study are consistent with observations of the surface ruptures.

The source geometry and slip distribution at rupture termination of the 1995 Hyogo-ken Nanbu earthquake were investigated using waveform inversion on the assumption of fault branching in the northeastern part of the rupture model. Possible branching of the Okamoto fault is suggested both by the static-displacement distribution and damage extension east of Kobe (Nishinomiya area). To exclude data contaminated by the basin-edge-diffracted wave in the waveform-inversion process, we examined the spatio-temporal variation of its influence and from a comparison with a flat model, we determined windows appropriate for the data. Three subevents were identified, as was reported in previous works. The largest was in the shallow part on the Awaji side, whereas the smaller two occurred at great depths (Ͼ7km) on the Kobe side. We found a smaller subevent in the deep part of the branch. Total variance reduction was larger, and the ABIC value was smaller when we assumed the branch than when we did not, which shows the superiority of the branching fault model. Resolution checks showed that the slips on proposed branched portions are physical and not caused by random-data noise or systematic errors in Green's functions that arise from misestimation of velocity structures. Calculation of the relative static displacement between two leveling observation stations near the branching fault also showed the greater utility of the branching fault model. The effect of slip on the branched fault has been shown in near-source ground-motion simulation using the 3D finite-difference method (Iwata et al., 1999). The characteristic distribution of ground motion in the near-source region indicated by the damage distribution is well reproduced by the modeling of both the source process and wave propagation in the realistic 3D velocity structure. Slip on the branched fault affected the ground motion in eastern Kobe (Nada and Higashi-Nada wards), Ashiya, and Nishinomiya cities, but its contribution is not dominant even in those regions; about 30 to 50% maximum velocity in the frequency range of 0.1 to 1.0 Hz.We conclude that branching rupture of the Okamoto fault during the Hyogo-ken Nanbu earthquake is the preferable interpretation on the source process of this earthquake. This supports the idea of a geometrical barrier suggested by geological observations.

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[1] The spatial and temporal distribution of the stress on the fault plane of the 1999 Chi-Chi, Taiwan, earthquake is calculated from kinematic inversion results using a three-dimensional finite difference method for solving the elastodynamic equations. We analyze the relations between stress and slip for all grid positions on the fault, and use these relations to infer the friction law for the rupture. The dynamic source parameters were also determined. Our results show that for most of the points on the fault, the relation between stress and slip was consistent with the slip-weakening law during the rupture process, especially for those points with large slip. However, consistency with the velocity-weakening law is not clear from the observed relation between stress and slip velocity. The distributions of the dynamic parameters on the fault are very heterogeneous. The peak value of the static stress drop is $35 MPa. In general, high stress drop occurred in the areas with large slip. The estimated strength excesses are generally small suggesting that the tectonic shear stress had reached the level of the fault strength before the main shock. The slip-weakening distance D c and the fracture energy G c are proportional to the final slip. The aftershock activity correlates with the spatial distribution of dynamic source parameters. Usually, the aftershocks near the fault plane are concentrated in regions with small or negative static stress drop, and there were few aftershocks in regions which had large values of critical slip-weakening distance D c .

A great earthquake, named the 2003 Tokachi-oki earthquake, occurred in the southern Kuril subduction zone on 26th September 2003, 4:50 JST (41.7797 • N, 144.0795• E, 42 km depth; Japan Meteorological Agency). Its ground motion was recoreded at 655 stations of the nationwide strong motion networks, K-NET and KiK-net. A maximum peak ground acceleration of 988 cm/s 2 (gal) was observed at station HKD100 and amplitudes greater than 200 cm/s 2 were observed over a wide area of eastern Hokkaido. We used a multi-line linear waveform inversion method to estimate the rupture process from the strong motion data of supplied by 15 stations. We assumed a fault plane model of 140 km × 160 km with strike and dip angles of our fault model are N246• E and 18• , respectively, placed on the estimated upper boundary of the subducting Pacific Plate. The estimated total slip distribution consisted of three major slip areas; (a) around the hypocenter, (b) the northwest part of the fault with the maximum slip of 5.9 m, and (c) the northeast edge of the fault plane. The major asperity (b) was composed of two large slip areas with different slip rate functions: the duration of moment release in the sourtheast part is longer than 15 sec, but in contrast most of the seismic moment of the northwest part was released in a short period of less than 10 sec. Our estimation of the total seismic moment was 2.9 × 10 21 N·m which corresponded to Mw = 8.2.

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