An examination has been made of the distribution of seismic intensities by the earthquakes which occurred in the uppermost mantle beneath the Kanto district, Japan, during the period between 1949 and 1978. Anomalously high seismic intensities were observed on the eastern side of the Izu Peninsula (Ajiro (AJI) and Oshima (OSH)) for the earthquakes at depths between 60 and 70km beneath northern and central Tokyo Bay. High seismic intensities were also observed along the western shore of Tokyo Bay (Tokyo (TOK) and Yokohama (YOK)) and on the eastern side of the Izu Peninsula (Ajiro and Oshima) for the earthquakes in the depth range from 60 to 80km beneath northern and central Chiba. The high seismic intensities observed at Tokyo and Yokohama must have been due to an amplification of ground motion by the thick alluvial deposits along the western shore of Tokyo Bay. It is in our opinion that these anomalously high seismic intensities observed at Ajiro and Oshima are caused by the practically unattenuated transmission of seismic waves through the descending Philippine Sea plate that dips northeastwards from the Sagami trough beneath the southern Kanto district. The existence of this plate had been suggested from seismicity and earthquake mechanism studies, but not confirmed from seismic wave transmission. High seismic intensities on the eastern side of the Izu Peninsula were also observed from the earthquakes at depths between 70 and 80km beneath southern Ibaraki. The earthquakes in the depth range from 30 to 60km beneath southwestern Ibaraki show normal concentric distribution of seismic intensities. These characteristic distributions of seismic intensities can be explained by the locations of earthquakes and the geometry of the high-Q plates.
A comment is made on a possible systematic error of epicenter locations of the historically earlier earthquakes along the western shore of Tokyo Bay (Tokyo-Yokohama area).

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Seismic waves from some shallow earthquakes in and near Japan are characterized by appreciable lack of high-frequency components. The size of felt areas for these earthquakes (low-frequency earthquakes) is relatively small for their magnitudes determined from amplitudes of medium to long period seismic waves. About 800 shallow earthquakes of M≥6.0 which occurred during the last 76 years have been classified into three groups, N (normal), L (low-frequency), and VL (very low-frequency) events by the use of the relation between the size of felt area and the magnitude. The proportion of L and VL events to N events varies spatially in a systematic manner. The pattern of epicenter distribution of N, L and VL events changes also with time. In the active seismic zone along the plate boundary, this pattern seems to be related to the occurrence of major earthquakes of magnitude about 71/2 or more. Few earthquakes of L and VL classes with M≥6.0 occur for several to more than 10 years just prior to the occurrence of major interplate earthquakes within or close to their future focal regions. The cause of low-frequency earthquakes is primarily attributed to the weak radiation of high-frequency waves from the source due to slow faulting or some unknown mechanisms. The attenuation of high-frequency waves during the transmission through a low-Q zone is also responsible for the low-frequency nature of crustal earthquakes on the marginal sea side.

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This study investigates the generation of waves due to the incidence of the Rayleigh wave upon the corner of an elastic quarter space. The Rayleigh wave is incident from infinity and travels along one surface of a right-angled wedge. Integral equations are derived by use of the Fourier transform technique and are solved by deforming their integration path along which the integrands vary smoothly in magnitude. Expressions for the energy fluxes of the Rayleigh waves along two free surfaces and the scattered body waves are obtained. Partition of energy fluxes and directivities of the scattered P and S waves are discussed. The scattered S waves are then found to be composed primarily of four kinds of waves as if they were generated from different wave sources. The orbital motions of the particle of the elastic medium are depicted along two free surfaces. The orbit form on the second surface tends to that of the Rayleigh wave more rapidly than on the first surface where the incident Rayleigh wave exists. The comparison with experimental results previously obtained by several authors shows considerably good agreement.

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For a layered structure sandwiched by two half-spaces, the dynamic properties of head waves propagated along some interface which interfere with reflected waves and/or head waves along other interfaces are investigated by calculating their theoretical seismograms. The methods adopted here to obtain the waveforms are the new method without ray expansion recently proposed by the present authors in a previous paper and the method with ray expansion. The former method gives accurate waveforms for a medium with thin layers, while in the latter method individual rays are calculated and the interference waves are interpreted as the superposition of many rays.
Head waves for three structural models having a single-interface, an intervenient layer and a transition layer are studied. The head wave from a single interface is separated from a reflected wave and becomes a pure head wave in the far-field. Near the critical distance, there exist interference waves, whose nature is investigated in detail. The intervenient layer whose thickness is smaller than a wave-length does not have much influence on the pure head wave. The effects of the transition layer whose thickness is larger than a wave-length depend on the structure of the transition layer.

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