The mechanism of seismic wave generation by the weight-dropping method was investigated experimentally as well as theoretically with an object to improve the source apparatus used for high-resolution shallow surveys. Through a series of field experiments, we have studied factors which influence the characteristics of the generated seismic waves. We have drawn conclusions on the following subjects: (1) the dependence of the amplitude of seismic waves on the momentum of weight; (2) the dependence of the relative energy efficiency on the weight mass and the impact velocity; (3) the optimum selection of the mass of coupler (embedding body at the impact point) for a given weight mass in order to increase the amplitude of seismic waves. We propose simplified dynamic models for the weight-dropping source, and discuss their features briefly by comparing the observed waveforms with the synthesized ones.

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The problem of excitation of Love-type waves in an inhomogeneous layer lying between two half-spaces is studied. Using the Fourier transform and Green's function method, the dispersion relation for propagation of such waves is derived. Finally, the transmitted wave in the layer is presented.

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More than 8, 000 earthquakes have been relocated to derive regional variations of the seismic-aseismic boundary in the mid-crust of the northern Kinki district of Japan. The boundary depths are estimated as 13-15 and 18-20 km in the southwestern and northeastern parts of the study area, respectively. The relationship between the seismic cutoff depth and the cause of large intraplate earthquakes is studied, making use of the present observations and other available data of seismicity and surface heat-flow, on the basis of the brittle-ductile transition of rock deformation. The regional variations in the cutoff depth of seismicity appear to be well correlated with the thermal structure of the crust. The cutoff depths in various heat-flow provinces in Japan and other countries are found to be inversely proportional to the surface heat-flow values, with the depths roughly corresponding to isotherms of 200-400°C. The shape of the depth-frequency distribution of earthquakes calculated from high-quality data is quite similar to that of the shear resistance calculated using a simple brittle-ductile transition model. Large intraplate earthquakes appear to originate at the peak or just below the peak in the depth-frequency distribution, which also corresponds to the deepest portion of the seismogenic layer. Furthermore, in the source area of large earthquakes, rupture seems to start where sharp changes occur in the cutoff depth of seismicity. Thus, the seismic-aseismic boundary is closely related to large intraplate earthquakes and, in turn, to the tectonics of island arcs.

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We reexamine toroidal oscillations of a two-layered sphere which is composed of a homogeneous mantle and a soft homogeneous core. The rigidity of the core is much smaller than that of the mantle. The spectrum seems to be split by multiple modes around the existing mode of a fluid core model, if a certain conditions is satisfied.

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