We examined the spatial distribution of earthquake hypocenters and the three-dimensional velocity structure of P-wave for the Kanto-Tokai District, central Japan, in order to investigate their relation. About 5, 000 earthquakes which occurred during the period from January 1980 to December 1981 were relocated. The double-planed structure of the intermediate-depth seismic zone is observed clearly beneath the Kanto District. The seismic zone underthrusting from Suruga trough toward the west and from Sagami trough toward the east were clearly traced throughout the troughs. Their dip angles are found to be steeper at the northern end compared with the southern part. These inclined seismic zones, however, could not be traced through the inland region to the north of the Izu Peninsula.
The three-dimensional P-wave velocity structure was investigated by applying an inversion method for local earthquake data. The velocity struc-ture between 0 and 145 km in depth was modeled by six layers. Each layer was divided into 10 (latitude)×6 (longitude) blocks, the size of which is 28×26 km2. A high velocity zone appears in and around the Izu Peninsula (northernmost tip of the Philippine Sea plate) in the first layer (0-20 km) and to the east of the Izu Peninsula in the second layer (20-45 km). Low velocity zones appear beneath Tokyo Bay and in the western region in the first layer and to the northwest of the Izu Peninsula in the second layer. Coincidence of the high velocity zone with the inclined seismic zone is found under Sagami trough but such a correlation is not found under Suruga trough owing presumably to poor resolution.

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Shipborne and ground magnetic surveys were conducted using a proton precession magnetometer on and around Toya caldera, Hokkaido, Japan. Modeling of the noticeable magnetic anomaly indicates the presence of highly magnetized rocks filling a basement depression. Considering the low gravity anomaly on the caldera, the magnetized infill is identified as low density caldera deposits. The proposed diameter of the depression is only about half that of the present caldera. The evidence suggests that the original caldera wall has been eroded away.
Analysis of the magnetic anomalies reveals that the andesitic central cones of Toya caldera are magnetized in a normally polarized direction and with magnetizations of 3-4 A/m. Exceptionally, a magnetization of 9 A/m is es-timated for two lava domes. The origin of the strong magnetization is possibly attributed to the rich content in Fe₃O₄, on the basis of the chemical analysis of rock samples. Two topographic highs of the lake floor are identified as sunken lava domes because of the considerably high magnetic anomalies.

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The world-wide sea level change in the Holocene was calculated with non-Newtonian flow of ε ∝ σ3 in the upper mantle, where ε and σ represent strain rate and deviatoric stress. The relative sea level patterns based on this model are divided into three categories, i.e., glaciated region, forebulge region, and continental margin and oceanic island away from the glaciated region. But the relative sea levels at the oceanic islands may depend on their size. This size dependence may give an important constraint on the lower mantle viscosity. This model predicts that the hydro-isostatic adjustment in the Holocene away from the glaciated region is localized to the boundary between ocean and continent or island. The relaxation time to complete the world-wide isostatic adjustment may be several ten thousand years, so no world-wide adjustment of ocean basins owing to the load of meltwater has proceeded until the present.

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This paper describes the behavior of fractured zones in the Yamasaki fault by means of analyzing surface waves. The results are as follows: 1) Amplitudes of the strain change caused by surface waves with periods of less than 1 min at the fractured zones are larger than those at the neighboring bedrock. A similar feature was also recognized on the M₂ component of tidal strain (NAKAHORI, 1977). 2) The magnification of strain at the fractured zone is larger in surface waves than in earth tide. 3) The distribution of relative strains along the observation tunnel suggests that the degree of fracturing decreases with the distance from the fractured zones. 4) The Rayleigh wave passing across the fractured zone causes abnormal movements along the fault strike. This feature can be explained by the concept that a slip movement occurs along the fault surface.

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We found low frequency signals which lasted for a few minutes to more than 10 min on long term visible records of Okuyama (OKY) station near Mt. Fuji (Fig. 1). The low frequency feature of these signals invokes teleseismic signals. However, these signals attenuate differently from teleseismic signals, and the corresponding signals are detected at other stations in Fig. 2 only when signals observed at OKY are fairly large. Waveforms as a whole are also quite different from teleseismic ones. Since these signals were identified more clearly in the seismograms of Fuji station on the mountain side of Mt. Fuji, sources of these signals are assumed to be near Mt. Fuji.
These signals contain P and S waves (Fig. 3), and are different from signals from artificial shots. S waves are identified clearly on traces of the horizontal component, while they are not identified clearly on the vertical component. It appears that each series of the low-frequency signals consists of many micro-earth-quakes which occurred in succession. The magnitudes estimated from the total duration time of each micro-earthquake are one or smaller.

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