Recent studies on the velocity structure beneath the Kanto-Tokai District, Honshu, Japan, pointed out the large lateral heterogeneity of velocity structure. This heterogeneity is large enough to suspect that hypocenters determined by a laterally homogeneous velocity model are systematically deviated from true locations.
Computation of travel times for a laterally inhomogeneous velocity model by three-dimensional (3-D) ray tracing revealed that travel time residuals of -1.0 to 1.2 s are produced if the lateral inhomogeneity is neglected. Residual distribution varied according to hypocenter locations. This indicates that a laterally homogeneous velocity model with station corrections is not sufficient to obtain accurate travel times in this district.
According to a numerical experiment, the resultant deviation in hypocenter location is up to +-5 km and +-8 km for epicenter and focal depth, respectively. For obtaining reliable hypocenter distribution, about 10, 000 hypocenters of earthquakes which were observed by the stations of the National Research Center for Disaster Prevention for the period from 1983 to 1985 were relocated by using the 3-D velocity model. Shape of earthquake clusters became clearer after the relocation. For the computation of hypocenters, we used an approximate method of 3-D ray tracing, which gives a sufficient approximation of the exact ray tracing.

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Basing on the Bayesian statistics, a new method to quantify seismicity is proposed. Earthquake occurrences are assumed to follow a non-stationary Poisson process of which the parameter (Poisson rate) changes both in time and in space Time-space for study is divided into hundreds of segments, and earthquakes are sorted into these segments. The Poisson rate is assumed to be constant in each segment, and the time-space variation of the rate is estimated by using Akaike's Bayesian Information Criterion (ABIC). To demonstrate the effectiveness of this method, the time-space variation in seismicity was studied for the area of the Nemuro-oki earthquake of June 17, 1973(M_JMA=7.4). As a result of the analysis, seismicity in the source area is found to be rather constant for the period of 1927-1972. The precursory seismic quiescence pointed out in earlier works should be attributed to the high seismic activities in the surrounding areas due to aftershocks of the 1952, 1968, and 1969 earthquakes.

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The Fukui Earthquake fault was investigated by seismic methods such as time-term analysis, spectral amplitude, and phase velocity of Love waves in the frequency-space domain, and the CDP analysis in reflection seismology. Amplitude and phase velocity of Love waves were used as a powerful tool for investigating the lateral variation of subsurface structure. Active use of these methods revealed that the fault area has small level gaps of only 2.4 m in the alluvium from which the recurrence time of the Fukui Earthquake in the Recent epoch was estimated to be less than 2, 000 years. It is further inferred that the Tajima River was probably trapped by the faulting topography.

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Aftershock distribution of the Urakawa-oki earthquake (Ms 6.8) of 1982 is investigated. A simultaneous inverse method is applied to P-wave arrival time data from the aftershocks and quarry blasts in order to estimate a three-dimensional velocity structure in the aftershock region. Strong correlations between the obtained velocity structure, geology, and gravity anomalies are observed.
Hypocentral determination of the aftershocks by using the three-dimensional velocity structure shows that the epicentral distribution is a triangle and its area is 790 km². The detailed aftershock distribution indicates that three dipping planes with high aftershock activities appeared in the aftershock region: two of the planes are almost parallel to each other with a dip angle of 50°in a depth range of 3 km to 27 km, and these planes clearly form double aftershock planes, which have been previously supposed to be an identical plane; the other plane is located perpendicularly to the double aftershock planes in a depth range of 13 km to 27 km and seems to be a conjugate plane with the double aftershock planes. The aftershocks occur in a region with P-wave velocities higher than 5 km/s, and the conjugate plane is located along the structural boundary. It is found that the rupture of the main shock took place on the conjugate plane and also might have been caused by a tectonic force derived from a collision between the Kurile and Northern Honshu Japan Arcs.

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Thailand, excepting its northern part, is a stable peninsular region. Release of significant seismic energy is a comparatively rare phenomenon here. But on April 22, 1983 a destructive earthquake (m_B= 5.8) occurred in western Thailand, not very far from Bangkok. The epicentre was located at the upper reach of the big Srinagarind reservoir. Considering the absence of major tectonic stress release in this region, even the present level of seismicity appears significant. In the present research, this particular earthquake along with its many foreshocks and aftershocks have been studied and their focal mechanisms have been determined. Using these solutions, as well as the earlier mechanism solutions in the surrounding region, the stress pattern of this region has been analyzed.
It is inferred that these particular earthquakes of western Thailand have no direct relation to the subduction or the back arc spreading processes operating in the western side of Thailand. They have occurred probably due to the release of accumulated internal strain through a local zone of weakness. Primarily because of the action of the two major faults-Tak and Three Pagoda faults of western Thailand-the basement rock in the epicentral region was in a highly prestressed condition. In such circumstances, the water of the Srinagarind reservoir served as an additional factor to weaken the basement formation, and thereby reduce the frictional resistance. The accumulated strain was finally released causing the present seismicity. The directions of the principal axes (P axis NW-SE directed for the main shock and the largest aftershock) derived from our solution possibly indicate the internal stress pattern of this peninsular crustal block.

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