The 1923 Kanto earthquake (M7.9), the most disastrous earthquake in Japan, is the first earthquake which left abundant seismometrical data of the main shock and aftershocks. It provides us with one of the best opportunities of studying aftershock activity and long term change in seismicity after an interplate earthquake. However, due to an uncertainty of data quality of that day and difficulty in integrating the available data, detailed examination of aftershocks has been remained untouched except several preliminary investigations carried out by the Central Meteorological Observatory and universities immediately after the earthquake in term of the current knowledge of seismology.
In this study, we made a complete integration of the data existing separately. The intensive data of half year after the 1923 event ensures statistical treatments such as making of S-P histograms and daily frequency of earthquakes observed at each station which may approximately indicate the extent of aftershock zone and time variation of the aftershock activity. By using S-P times mainly and P and S arrivals independently in some cases, we locate about 400 earthquakes. Although each epicenter may include location error of an order of 20km, relatively large extent of aftershock area makes the aftershock distribution show a good correspondence with the faulting models and the seismicity pattern of today.
A high concentration of aftershocks is recognized in the border of Yamanashi and Kanagawa Prefectures, the western boundary of faulting area where the main shock originated. Another concentration of aftershocks suggests that eastern boundary of the faulting zone exists in the middle of the Boso Peninsula. While northern boundary of aftershock region is defined rather clearly near the border of Kanagawa and Tokyo Prefectures, southern limit of aftershock zone is ambiguous because poor location accuracy.
Compared to the periphery of the aftershock area, two inactive areas of aftershock activity are recognized at the middle of faulting region which corresponds to low seismicity areas found with recent microearthquake studies and the nucleation zone of the main shock where large dislocation of the faulting is estimated by the dynamic source process studies. This result suggests that basic seismicity pattern of present day in the southern Kanto region including the 1923 focal region has been created from its aftershock activity. Seismicity pattern before 1923 might be almost similar to those of today except middle of the Boso Peninsula. Relatively high seismic activity is recognized there immediately after the 1923 earthquake and it has taken ten to twenty years the seismicity to recover the normal level.
Magnitude frequency relation of the aftershocks was compared with those of four interplate earthquakes of similar large size in Japan. It shows that the Kanto earthquake is accompanied with larger number of aftershock of M>6. Prominence of aftershock activity may be attributed to the complexity of tectonic setting of the focal region which is located near the collision boundary of the Philippine Sea plate and the Eurasian plate.

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In an attempt to examine the characteristic behavior of fault asperities (large slip areas), we comparatively studied two large earthquakes: the Tokachi-oki earthquake (M 7.9) of May 16, 1968 and the Sanriku-oki earthquake (M 7.5) of December 28, 1994, which have a partially common source area. Both the strong motion records at a regional network and the teleseismic body waves at global networks were analyzed to determine the detailed spatio-temporal distribution of moment release. The aftershock distribution, which may provide us with a more reliable location of asperity, was also re-examined using the same underground structure and the same algorithm for both events.
The total seismic moment, M_o, and the source duration, T are obtained as: M_o=3.5×10²¹Nm; T=90s for the 1968 event, and M_o=4.4×10²⁰Nm; T=60s for the 1994 event. It is also shown that the 1968 event consists of more than two asperities, one of which took a role of asperity again for the 1994 event. The distribution of relocated aftershocks, which fringe the major asperities, strongly supports this fact. A simple calculation indicates that the seismic coupling is almost perfect (100%) in this common asperity. We thus propose that there exist characteristic sites for asperities where fault slip occurs only as a seismic event, and that the individual asperities usually manifest M 7 class earthquakes but sometimes synchronize to cause M 8 class earthquakes.

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Lateral heterogeneities of S-wave velocity structures in and around Ashigara valley in the west of Kanagawa prefecture, Japan were discussed using the phase velocity dispersion of Love waves. The phase velocities in the frequency range of 0.1 to 0.5Hz were determined using the strong motion records of over 40 in and around Ashigara valley from the east off Izu peninsula earthquake (M_j=5.7) of May 3, 1998. We applied a semblance analysis for determining phase velocities by dividing the observation area into several sub-arrays, which consist of 4 or 5 sites. A clear difference of phase velocity dispersions in a wide frequency range was found between the valley region and the mountain/hill sites that are surrounding the valley. S-wave velocity structure models beneath the sub-arrays were estimated by an inversion technique of the genetic algorithm. The structure models show a laterally complex heterogeneity and two distinctive features. The depth to the basement in the east side of Kouzu-Matsuda fault is shallower than that of the west side. The depth to the layer with S-wave velocity of 1.5km/s in the south of Ashigara valley is deeper than that in the north, however, it is opposite for the depth to the layer with S-wave velocity of 2.4km/s, that is, the south of the valley is shallower than that in the north.

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Seismic intensity at Shikine-jima during the earthquake swarm of Izu-Islands in 2000 is usually one or two ranks higher than that at neighbor island, though epicentral distances to these islands are almost similar. In order to investigate the cause of anomalous large seismic intensity, three portable strong motion instruments have been installed at the island.
The spectral ratios of observed S waveforms at sedimentary stations at Shikine-jima demonstrate severe site amplification of 2 to 10 within a wide frequency range between 1 to 10Hz relative to rock site. The experiments based on a GA inversion indicate unusual subsurface structure with very low (V_S=31-427m/s) and high attenuation (Q_S=15-148) superficial layer overlying a bedrock (V_S=1000m/s, Q_S=200) with large contrast at the interface is a main cause of high seismic intensities.
The influence of nonlinearity was also found near a coast of Shikine-jima when the ground acceleration level exceeds 300cm/s². The nonlinear ground response yielded a considerable reduction of the ground motions, so that the observed peak accelerations during the severe earthquakes were only about 50% of that predicted by linear ground response.

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In order for better understanding of ground motion intensity in the near-field region, the damage survey of tombstones and questionnaire seismic intensity survey are conducted at Wakago and Honson areas in Niijima Island for the earthquake on July 15, 2000 (M_w6.0). The relations of damage indexes on overturned tombstones and questionnaire seismic intensity to ground motion intensities are examined. As a result, the ground motion intensity in Wakago area is estimated as the J. M. A. seismic intensity of 6.1, peak ground acceleration of about 800cm/s² and peak ground velocity of about 80-100cm/s. The synthesized seismograms calculated from the existing fault model are compared with the observed strong motion records, and the agreement of calculations and observations is not good. Therefore the fault parameters are modified from the existing fault model. The modified fault model is closer to Niijima Island as compared to the earlier one. The synthesized seismograms calculated from this model show better agreements with the observed ground motion records. According to the synthesized seismogram calculated fromm the modified fault model, in Wakago area, the ground motion around the period of 1sec is dominant in north-south direction. This may be mainly due to the effect of the fault rupture.

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Tono Research Institute of Earthquake Science (TRIES) carried out a seismic reflection survey passing through an “over-step” part of the Enasan fault, Tono area, central Japan. The fault geometry in the over step part was, however, not clear, because the survey was done along a single line on a granite exposure zone.
Then, we performed a precise gravity survey to delineate the shape of basement. The feature of basement topography suggests that the Enasan fault is not continuous under the over step part. This result agrees with the recent geomorphologic/geologic investigations that the activities of crustal blocks at the east side and the west side of step-over are different in age (the east block acted lately). Furthermore, we found the trend of a low gravity anomaly belt, or a characteristic concave feature of the basement locating north of the fault, to be deviated from the fault strike. If it is a remnant of the past faulting, then it can be suggested that the regional stress field was NE-SW compression, when the concave feature was formed. Also, it is consistent with the stress field when the Ena cauldron was formed.

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