Explosion seismic data in South Kyushu are reexamined and a model of the upper crustal structure is presented. A layer of P-wave velocity V_P= 4.9km/sec is widely distributed over the profile. Below the layer, lateral variations in P-wave velocity are observed: V_P=6.1km/sec in the western part of the profile, V_P=5.9km/sec in the eastern part and V_P=5.7km/sec in the central part. A depression of this layer is confirmed in the central part, or in the low velocity region. The depth of this layer increases abruptly by 1-2km at the west boundary of the region and decreases gradually toward the east. This layer subsides again by about 2km to the west of Miyakonojo City in the Osumi Peninsula. This is suggested by geological evidence as well. These variations in the depth of the layer can be interpreted well using gravity data.
Decreases in wave velocity are ascertainable in the region under the Sakurajima Volcano and the Aira Caldera where seismic waves are highly attenuated.

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The stability and instability of two collinear shear cracks were investigated theoretically. When instability occurs in two pre-existing cracks, the cracks coalesce spontaneously and a large-scale crack is formed. Many seismological observations suggest the occurrence of this kind of rupture in the earth. Specific fracture energy or stress-drop is assumed to be inhomogeneously distributed. It is shown that the mode of coalescence has great diversity and is sensitive to the distribution of stress-drop or specific fracture energy. This diversity will be closely related to the diversity of earthquake rupture processes.
In this paper we specifically investigate the mechanism of generation of slow earthquakes and fault creeps along the San Andreas fault system. Our source model developed in the present paper satisfactorily explains observations for the above rupture phenomena. Our source model also explains, in a unified fashion, the existence of two distinct types of fault creeps, that is, stable continuous creep and propagating episodic creep, along the San Andreas fault system.

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The precursory swarm hypothesis as developed in New Zealand has been generalised on the basis of a systematic study of multiple earthquake events at moderate to large magnitudes in the Japan region. In the generalised hypothesis the precursory seismic sequence consists of one or more swarms followed by one or more mainshock events in the same sub-region. The predictor is the characteristic swarm magnitude, defined as the average magnitude of the three largest earthquakes in the swarm or swarms. The observed value of the predictor yields estimates of the magnitude and time of occurrence of the largest mainshock in the sequence; these parameters are estimated as soon as the first swarm occurs, and are revised on the occurrence of successive swarms in the sequence. Further statistical analysis of secondary mainshock events is required, after which a "score-sheet" of past successes, failures and false-alarms can be compiled, and the hypothesis can then be rigorously tested against future earthquake activity in the region.

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P and S arrival times of crustal and subcrustal earthquakes, obtained from three-component wide-band seismographs of the 10-station telemetered network of Nagoya University, were inverted by a damped least squares method to obtain the P and S velocity structure of the crust and the sub-Moho mantle in the Chubu district, Honshu, Japan. The hypocentral parameters and the P and S station anomalies were determined simultaneously. The solutions showed satisfactory stability and resolution. The crust and the sub-Moho mantle were divided into four layers, based on results of refraction studies. The uppermost layer of 4km thickness is the "sedimentary layer" which was not incorporated into the inversion. The second layer with a thickness of about 20km is the "upper crust" for which P and S velocities of 6.1 and 3.6km/sec were obtained with a V_P/V_S ratio of 1.69. This P velocity is consistent with the results of refraction studies. When this layer was divided into two parts at a depth of 12km, the sums of squared residuals were reduced dramatically, suggesting a subdivision of the upper curst at this depth. The upper portion possesses P and S velocities of 6.0 and 3.55km/sec and the lower portion, of 6.3 and 3.7km/sec, respectively. Both portions are characterized by a V_P/V_S ratio of 1.69. The third layer of about 10km thickness includes the "lower crust" there in, for which P and S velocities of 6.9-7.0 and 3.8-3.95km/sec were obtained. The corresponding V_P/V_S ratio is 1.78-1.81, in marked contrast to the upper crustal V_P/V_S ratio. The fourth layer is the "sub-Moho mantle" for which we obtained P and S velocities of 7.7-7.8 and 4.4km/sec that give a V_P/V_S ratio of 1.76-1.77, a value similar to that for the lower crust. The inverted sub-Moho P velocity is in agreement with the results of refraction studies. The Moho boundary probably lies at a depth of around 33km. The boundary between the upper and lower crust is not well constrained by the data. The consistency of our P velocity structure with those derived from explosion seismic studies indicates the feasibility of the present method for obtaining an S velocity structure. Poisson's ratio changes significantly across the boundary between the upper and lower crust but not across the Moho.

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