Seismic activity of the subcrustal earthquakes was studied to determine the tectonic properties of the southeastern part of the Kinki district. Depth contours deduced from the hypocentral distribution distinguished the seismic zone situated to the south of Lake Biwa from the seismic zone in the northern part of Kii Peninsula. One of the seismic zones corresponding to the marked activity in the Kii Peninsula was found to exhibit the inclination dipping toward N45°W with a dip angle of about 40° and to reach a depth of about 90 km beneath the east coast of Osaka Bay, whereas the zone to the south of Lake Biwa was found to be descending almost westward with a dip angle of 30°.
Focal mechanism solutions of 43 subcrustal earthquakes also indicated that the two regions were distinguished by the tectonic features of P and T axes and the fault types. The tectonic stress system of the parallel extension along the Nankai trough was verified in the north of Kii Peninsula. But the reverse faults caused by the E-W compression were found to be conspicuous in the region to the south of Lake Biwa.
A tectonic line, called "hinge line, " was supposed at the boundary of the two seismic zones. The Yoshino earthquake (1952, M=7.0, h=70 km) occurred around this area, and one of its focal planes, the right-lateral component of fault slip, lay almost in the E-W direction, parallel to this line. These facts suggest that the areas along the supposed tectonic line are capable of generating large earthquakes, though the present seismicity is quiescent thereabouts.

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In October-November, 1989, five M≥6 earthquakes, including an M7.1 earthquake, sequentially took place off Sanriku, northeastern Japan, and the earthquake activity became very high in this region. Since the seismically active zone was far from the land observation network, it was difficult to obtain accurate hypocenter locations. We carried out ocean bottom seismographic observation in order to locate the hypocenters precisely and investigate characteristics of the activity.
Combining the OBS and land station data for hypocenter determination, we obtained two main conclusions. (1) Almost all epicenters of the present activity distribute landward from the 4 km isobath. Some earthquake clusters were observed including an M5.6 earthquake and its aftershocks. (2) Focal depths of the earthquakes are shallower than 20 km and the activity is interpreted to take place around the boundary between the subducting Pacific plate and the landward plate. Taking account of the mechanisms of the larger earthquakes in this active period, this activity is a thrust fault type and is different from the 1933 Sanriku earthquake which was a normal fault type and might have broken the entire oceanic lithosphere.

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We present the generalized representation of the equations of motion for an elastic solid in a concise vector form with arbitrary orthogonal curvilinear coordinates and no assumption on symmetry of elastic moduli. For the particular case of the Cartesian coordinates, this representation leads to the generalized y-method for eigenvalues of surface wave dispersion in an anisotropic plane-stratified medium. The generalized y-method is the integration method to find eigenvalues of surface wave dispersion by iterating numerical depth-integration of first-order ordinary differential equations that are derived from the generalized representation of the equations of motion and Hookean law. Based on the generalized y-method, we present some numerical results for dispersion curves and azimuthal variations of surface wave velocities to fully display Rayleigh-Love wave coupling in Kawasaki's (1986) azimuthally anisotropic model for the upper mantle beneath the Pacific ocean.
When Rayleigh-Love wave coupling takes place in a particular period range between a pair of nearby modes, surface waves display the following distinct singularities: (1) a difference of phase velocities of the pair of the modes is smaller than about 0.5 km/s, (2) a pair of dispersion curves of group velocities cross each other, (3) polarizations of particle motion directions are twisted along the pair of dispersion curves from Rayleigh- to Love-types and from Love- to Rayleigh-types. These sigularities are dependent on the relative depth- and azimuthal-distribution of the upper mantle anisotropy. When Rayleigh-Love wave coupling does not take place, the first-order perturbation theory works well.

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