In order to study the layered structure and characteristics of the velocity distributions in the crust and upper mantle of the northern part of the Himalayas, we have made detonations in Puma Lake, Peikü Lake, and the Dinggye region. Four seismic record sections were obtained along a 475 km long profile in a nearly E-W direction from Puma Lake to Peikü Lake.
According to data processing and inversion, 6 groups, t₁, t₂, t₃, t₄, t₅, t₆ of reflected phases through the crust and upper mantle in the area are obtained. They show different kinematic and dynamic properties. The results of data analysis are as follows:
1. The crust is multilayered and there exists a low velocity layer in the crust. The thickness of the low velocity layer is a few kilometers, and with the layer velocity 5.6-5.7 km/s. This indicates that the cause of geothermal distribution and its activity in the Xizang plateau is due to the high temperature in the crustal medium and the existence of melting or partial melting matter in the crust.
2. Structure and velocity of thick crust are horizontally inhomogeneous. The crustal thickness from the north of the Himalayas is 73-77 km and its velocity, 6.2-7.3 km/s. Crustal deformation is very strong in the Tethys Himalaya region.
3. On the basis of results from ray tracing, theoretical seismogram, and phases of reflection waves, a preliminary model of crust and upper mantle in the northern part of Himalayas is put forward.
The extremely thick crust was caused by the results of the collision of the Indian plate against the Eurasian plate, and during the process of continuous pressing the horizontal shortening took place on a large scale in the crust.

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In August 1980, observations of seismic waves generated by 35 explosions, one on land and the others at sea, were conducted at about 60 sites on land and four sites at sea bottom in a profile from Hatoyama to off Izu Peninsula to obtain the crustal structure in the northern part of the Philippine Sea plate. The profile, approximately parallel to the Suruga trough, crosses the Kan'nawa fault area, Hakone volcanoes, Izu Peninsula, and the Zenisu ridge.
By the time term analysis, a velocity of 5.9 km/s was obtained for the granitic layer and a velocity of 6.8 km/s for the basaltic layer. The P_n velocity of 7.7 km/s was assumed for the time term analysis on the basis of travel time data in this experiment as well as previous results. The crustal structure thus obtained shows an interesting transition from the continental to oceanic type towards the Shikoku basin. It is possible that there are offsets in the upper boundaries of the granitic and the basaltic layers and in the Moho. Since these offsets are located in a narrow zone, this zone might be the boundary between the Philippine Sea plate and the Eurasian plate. Comparison of hypocenter distribution along the profile with the crustal structure supports the above idea since the pattern of hypocenter distribution between the two sides of the offset zone differs. This comparison also shows that earthquakes take place in the granitic layer beneath the Izu Peninsula. The distribution of observed Bouguer gravity anomaly along the profile is consistent with that expected from the crustal structure.

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Three-dimensional patterns of tectonic deformations and stresses are calculated to simulate the tectonics of Kyushu and its surrounding regions in southwestern Japan using a finite element method. The configuration of the subducted Philippine Sea plate is modeled by means of three-dimensional finite elements. Viscosities in the crust and upper mantle assigned are taken from the data of postglacial or postseismic rebounds. Several possible loads are taken into consideration: a slab pull force arising from the density contrast between the subducted slab and the surrounding asthenosphere; a ridge push force due to evolution of oceanic plates; a traction due to flows in the asthenosphere; a crustal buoyancy acting on a low-density crust; a buoyant force in the back-arc region which may simulate the spreading of a back-arc basin; a positive buoyancy acting on the aseismic ridge; compressive forces in the E-W direction exerting on the crust and subducted slab.
The results suggest that the tensile stress field observed in the crust of Kyushu may be generated by the interaction between the slab pull force, crustal buoyancy, and flows in the asthenosphere. It may also be concluded that the stress field of down-dip extension in the Wadati-Benioff zone can be explained mainly by the slab pull force. The asthenospheric flows might have a significant effect on the genesis of near-horizontal extension which yields normal faulting in the uppermost mantle, and such stresses would be more likely to appear on the oceanic side than on the continental side. In order to explain the dip angle of the subducted slab, if it is a manifestation of the flow pattern at a subduction zone, a traction due to asthenospheric flow of about 200 bar is required in the asthenosphere. The predicted values of velocities of tectonic flows and of maximum shear stresses are a few cm/yr and 500 bar at most in the crust and the subducted slab beneath Kyushu, though these are dependent on viscosity.

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The Niigata earthquake of June 16, 1964. (M⁸=7.5) is studied using a body-wave inversion technique on teleseismic P waveforms of the mainshock and a joint hypocenter determination of 27 of the largest (M_JMA≥4.0) regionally recorded aftershocks. The mainshock is interpreted as being composed of two subevents with moments of 2.1×10²⁶ dyn-cm and 2.0×10²⁷ dyn-cm. The focal mechanism indicates that faulting could have occurred on either a plane dipping toward the west or a plane dipping toward the east. The initial subevent is located at a depth of 8 to 13 km, while the centroid of the second sub-event is located 27 km southwest of the first and at a depth of 3 km. The relocated aftershocks all appear to have depths of less than 15 km which is consistent with the depths obtained for the mainshock. They appear to define a trend dipping shallowly toward the east which is consistent with the relative locations of the two subevents comprising the mainshock. We conclude, therefore, that a fault plane dipping toward the east is more consistent with the relative locations of the two subevents and the relocations of the aftershocks.

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An approximate inversion method, which has been widely used in medical science and called ART (Algebraic Reconstruction Technique), is applied to obtain a 3-D image of the uppermost mantle beneath the Hokkaido-Tohoku region for the period from 1973 to 1981. About 7, 600 P-wave arrival times recorded by seismic stations of the Japan Meteorological Agency are used. We present three types of 3-D models. 1) A model from the 7, 600 data. 2) Models for 1973-1977 and 1978-1981, respectively. 3) Eight models for 2 years moving windowed data. The first model is in good accordance with the results of previous authors who inverted P-arrival time data by damped least-squares inversion methods. The model clearly exhibits a high-low velocity contrast along the strike of the deep-seismic zone below a depth of about 60 km. The contrast is estimated to be about 4%. A subtraction operation of the second type 3-D models does not seem to show any significant differences between the models. No significant variations are apparent in and around the source region of the 1982 Urakawa-oki earthquake (M_S=7.2) in the display of the 2 years windowed results. However, the latter two results do not necessarily deny possible velocity change if we consider the sensitivity of event location process to the expected velocity change in the source area of earthquake.

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The source mechanisms of subcrustal and upper mantle earthquakes with magnitudes from 6.0 to 6.8 that occurred around the northeastern Kyushu region have been closely investigated to clarify its tectonic features in relation to the subducting Philippine Sea plate with a laterally bending configuration.
Two subcrustal earthquakes that occurred in Suonada and the Bungo channel, which are located close to the leading edge of the subducting Philippine Sea plate, show the mechanism of normal faulting type. These events may have been generated by lateral bending of the oceanic plate. The Kunisaki peninsula earthquake of August 26, 1983 (M=6.8, h=116 km), which was the largest upper-mantle earthquake in this region, shows a reverse fault type mechanism. Comparisons between the observed and synthetic seismograms suggest that the southeastward dipping nodal plane may be the fault plane, and that the rupture propagated northwest-upwards. The seismic moment was estimated to be M_O=-1.13×10²⁶ dyn·cm. The focal mechanism of three upper mantle earthquakes inland of Kyushu, including the Kunisaki peninsula earthquake, suggests that they may have been caused under the stress regime of down-dip extension. The tensional stress working southwest-downwards at intermediate depths in the Suonada and inland Kyushu regions may be explained by the gravitational pull acting on the Philippine Sea plate subducting deeper southwestwards.

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