We have derived the asymptotic expressions for high-frequency spectral amplitude of acceleration due to unsteadily propagating two-dimensional semi-infinite cracks both in antiplane strain and in plane strain. Property of generalized functions is employed in the derivation, by which mathematical analysis of high-frequency radiation will become rather simple.
It is shown, as Madariaga showed for rather simple crack models, that the abrupt change of crack tip velocity highly contributes to the generation of high-frequency waves. In the present paper near-source geometrical attenuation of high-frequency acceleration is specifically studied. We employ, as a measure of the intensity of acceleration, the envelope-amplitude of high-frequency spectral-amplitude of acceleration. If the crack tip velocity makes an abrupt change during its propagation, the envelope-amplitude is frequency-independent in the high-frequency range. In the calculation the crack surface is divided into segments, and the crack tip velocity is assumed to take a constant value in each segment.
It is shown that the intensity of acceleration is larger when the mean value and/or the variance of the distribution of crack tip velocity are higher. It is also shown that the near-source geometrical attenuation of envelope-amplitude is in harmony with the observed attenuation of peak acceleration associated with earthquake ruptures. This may suggest that seismic high-frequency waves are attributed to the abrupt changes of earthquake rupture velocity.

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The crustal structure in the Kinki district of Japan has been investigated by means of Phinney's spectral-ratio method and the travel-time analysis of microearthquakes. The crustal thicknesses at KYOTO and HIKONE, which are situated on the west and east sides of Lake Biwa, respectively, are determined as about 32km and 38km, espectively, by means of the spectral-ratio method. The value of 32km at KYOTO agrees well with the crustal thickness determined by travel times of the Kurayosi explosions, so that the crust is almost horizontally layered in the northwestern Kinki district, and the P_n velocity is 7.6km/sec. The value of 7.6km/sec is similar to that derived in the inland area of northeastern Japan, and a few percent lower than those at the coastal regions of the Pacific side of southwestern Japan. On the other hand, the crustal thickness in the northeastern Kinki district is determined as 40km by analysing microearthquakes near the Moho discontinuity beneath Lake Biwa. Therefore, the Moho-offset amounting to about 8km seems to exist at the western coastal region of Lake Biwa. This structure can explain the pattern of Bouguer anomalies in the northern part of the Kinki district very well.

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The uppermost-mantle structure in the Kinki district has been investigated using apparent velocities of subcrustal earthquakes in the Kii peninsula observed at the Tottori and Hokuriku Microearthquake Observatories. Apparent velocities determined by the least-squares method increase with depths from 7.55km/sec at 25km to 8.2km/sec at 65km in the Hokuriku network for the epicentral distances of 160-260km. They concentrate on two values, or about 7.8 and 8.0km/sec, in a depth range of 45-60km. Since no reverse profile exists, the uppermost-mantle structure has been constructed assuming a horizontally layered model. Three layers, of which velocities are about 7.6, 7.8, and 8.0km/sec, exist below the Moho. Top depths of them are 40, 50, and 60km, respectively. On the other hand, we have applied the same method to the data in the Tottori network for the epicentral distances of 130-200km. The Pn velocity has been determined as 7.6km/sec when the Moho is 32km in depth, and an about 8.0km/sec layer lies at a depth of about 60km. Therefore, the layer of about 8.0km/sec exists generally at a depth of about 60km in the Kinki district. We cannot tell, however, whether this layer is an aseismic continuation of the Philippine Sea plate or not.

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The earthquake sequence which occurred off the east coast of the Izu Peninsula, Japan, in 1980 is investigated in detail on the basis of a homogeneous hypocenter file (M≥2.9) carefully constructed for this purpose. Absolute and relative precisions of hypocenters are improved to be 1-2km and several hundred meters, respectively, by applying two kinds of correction to arrival times. Furthermore spectra of seismic waves recorded at OYM (Δ-50km) are examined to detect possible changes in spectral features in the course of the development of the sequence. The present sequence is the superposition of two types of activities, the swarm activity (M≤5.1, M_JMA≤4.9) and the mainshock (M6.5, M_JMA 6.7)-aftershock sequence. The swarm activity consisted of 28 bursts of activity, each of which lasted for about 1 to 2hr after a quiescent period of six to ten-several hours. Such swarm activity occurred within a narrow area of 6km by 2km throughout the whole sequence. The depth was also limited from 9 to 11km. Each burst of activity moved from place to place in this limited area without any significant relation to the largest shock of the sequence, occupying an area of about 1-2km square each time. It only seems to show the local fracture strength in the swarm area. The estimated fault of the largest shock (M_JMA 6.7) is a vertical plane of 20km in length and 8-10km in width with a strike of N20°W. Its location agrees well with an evident cliff of the submarine topography. The hypocenter of the largest shock is 3km south of the swarm area. This place had been quiescent during the early stage of the sequence, but about 20hr before the largest shock, an M4.0 (M_JMA 3.9) shock occurred there.

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