Aftershocks of the 1983 Nihonkai-chubu (Japan Sea) earthquake (MJMA 7.7) were relocated using an inclined-layer velocity model, a one-layer crust in dipping contact with underlying half-space mantle. We dealt with aftershocks which occurred from June 5 through June 23, 1983, when the seismographic stations of Hirosaki University were most densely distributed in northern Tohoku, Honshu, Japan. The parameters of the velocity model were determined by applying a method of simultaneous estimation of velocity model parameters and hypocenter parameters to an ensemble of P-wave arrival times from selected aftershocks. The determined Moho interface dips nearly eastward with an angle of 6°-8°, being placed at a depth of 18 km at the western end of the aftershock area and 28 km on the Japan Sea coast of Akita and Aomori Prefectures. The relocated hypocenters concentrated intensively on an easterly dipping plane, thus clearly suggesting the geometry of the main-shock fault. The fault mapped by the aftershock distribution changes its strike halfway along the fault length, striking in N15°-20°E in the southern part and N10°-15°W in the northern part. This change in fault strike agrees with a change in the general trend of geology in the aftershock area. The dip of the fault, which had previously been uncertain, was reliably estimated to be dipping in an easterly direction with an angle of about 20° for both the northern and southern parts. The bottom of the fault is probably limited within the crust because the relocated aftershocks are confined to a depth range shallower than 20 km. When compared with hypocenters of aftershocks determined for the inclined-layer velocity model, those determined for laterally homogeneous velocity models were much more scattered. Reduction of the scatter of hypocenters has been accomplished by the introduction of the inclined-layer velocity model which adequately accounts for the lateral variation of velocity structure in and around the aftershock area.

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Some indications of the possibility of a large earthquake occurrence around Kyoto in the near future are to be found in three kinds of data: auto-correlation curve of destructive earthquake occurrence around Kyoto since the 7th century, annual sums of accelerations caused by felt earthquakes near Kyoto since 1891 and results from precise levelings along a route from Fukuchiyama to Kyoto since 1887. To examine the possibility of occurrence of the said earthquake, data obtained by observations of crustal linear strains and tilts performed at Osakayama since 1951, some observations having been augmented in the period from 1961 to 1970, were analyzed. Average secular changes were estimated by fitting a typical formula to monthly values from moving averages of 13 months, and deviations of these monthly values from the formula-fitted average secular changes (anomalies) were compared with changes in ground-water level and annual frequency of felt earthquakes. Partial-correlation between the annual means of the observed anomalies in crustal movement at Osakayama and the annual frequency of felt earthquakes is remarkable in most parts of the observations. From the anomalies of crustal deformation, no definite conclusion could be drawn concerning an approaching large earthquake.

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We developed an inversion method to infer the earthquake rupture process from far-field body waves. The earthquake source is modeled by a space-time dependent slip on a fault plane. The slip motion at a point on the fault plane is assumed to be a ramp function in time, which is specified by the final dislocation, the rise time, and the time when the rupture front reaches the point. The fault plane is divided into many subfaults. The final dislocation and the rise time are assumed to be constant within each subfault. The rupture front motion is determined by the rupture starting time at the subfault corners using an interpolation formula. Thus the model parameters are the final dislocation, the rise time, and the rupture starting time assigned to each subfault. An algorithm to invert waveforms for the source model is formulated on the basis of a general approach of nonlinear inversion under prior constraints. Numerical experiments using simulated data show that the inversion method gives correct solutions. We also attempted a different approach using a constant rupture velocity model and found that we cannot estimate the true dislocation distribution under the assumption of constant rupture velocity.

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The response of a borehole volume strainmeter to various kinds of disturbances is investigated. The effects of atmospheric pressure, rainfall, changes in the water table, seismic waves and earth tides are analyzed. The secular strain changes and the strain steps including co-seismic steps which may contribute to earthquake prediction and earthquake source studies are also discussed. Rock properties around the strainmeter's sites are taken into consideration. Two groups of the sites are discernible, depending on the sensitivities to atmospheric pressure changes and seismic waves. The difference in the site characteristics may be partly due to the various mechanical conditions of the ground and partly due to its coupling with the strainmeter. The absolute sensitivity of the strainmeter is determined for each site independently by observing both the Rayleigh waves from a distant earthquake and the P waves from a relatively nearby earthquake. In determining the sensitivity, seismograms are utilized to estimate the in situ strain. This sensitivity can be applied to a range of periods comprising those of seismic waves and earth tides. Comparison of the secular strain change observed by the Omaezaki strainmeter with those obtained from geodetic measurements shows that the sensitivity to secular strain change can be different from those of shorter-period disturbances and that the difference in the nature of the data should be kept in mind. The sensitivity for the co-seismic step is also different from the other ones. However, its magnitude recorded at selected sites can be used for estimating the source moment.

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