In October, 1978, the crust and upper mantle structure between the northeastern Honshu and the Japan trench was studied in a profile almost parallel to the trench axis by the refraction method with four ocean bottom seismometers. Sixteen explosions of dynamite in total were carried out for four ocean bottom seismometers with a spacing of about 90km. Only the northern two of which could be retrieved. The results show that the thickness of the crust is about 25km except for water and almost the same as that beneath Honshu, that the layer with a velocity of about 6.0km/sec is fairly thin near the Pacific coast and becomes thicker beneath the present profile, and that the P_n velocity in the present profile is 8.1km/sec and agrees with that in the adjacent areas. These features of the structure are of basic importance for studying tectonics of the island arc-trench system.

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A unified seismic source model is proposed which can explain diverse earthquake rupture processes, an earthquake faulting consisting only of aseismic slip, an aseismic fault slip preceded and/or followed by seismic faulting, mainshock-aftershocks sequence, multiple-rupture event, an earthquake swarm and the like.
We analyse, as a mathematical model, quasistatic extension of longitudinal shear cracks in a viscoelastic medium. A Dugdale model is adopted as a fracture model. An inhomogeneously distributed stress drop is assumed on the crack surface. It is shown that the viscoelastic properties of the medium, the fracture strength at the crack tip and the inhomogeneity of stress drop distribution play important roles in the occurrence of the diversity of earthquake rupture processes.

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The spectral content of high frequency (1-30Hz) P and S phases observed by ocean bottom seismographs (OBS's) in the Mariana basin is investigated. It is shown that autoregressive models of 10-30th order are suitable for analyzing high frequency waves. For events with S-P times greater than 50 sec, both the predominant frequency and the wave energy of observed P and S phases are confined to a narrow range, say 6-10Hz for P waves and 5.5-9Hz for S waves. In addition, high frequency phases are also characterized by their long duration. It seems likely that some intrinsic property of the oceanic crust or upper mantle allows the efficient transmission of high frequency (5-10Hz) seismic waves. A model of the oceanic upper mantle which is consistent with the observed transmission of high frequency seismic waves is presented.

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A systematic study of multiple earthquake events to be found in the Japan Meteorological Agency catalogue, 1926-1979, has been made by restricting the data to magnitudes M≥5.0, and identifying multiple events by the three-in-one-day rule. Of the 74 events identified, 33 are classed as mainshock events, 29 as swarms, and 12 as multiplets. The locations of many mainshock events have been close to those of swarms (although no swarms have been located in western Honshu), and this may indicate the occurrence of local changes in the mechanical condition of the rock, and in the complexity of the stress field, with time. The results encourage the search for a possible precursory relationship between swarms and mainshock events in Japan such as has been proposed for New Zealand.

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The rupture processes of large shallow earthquakes are investigated based on the broad-band seismograms recorded at relatively short focal distances as well as on various field surveys near the source regions. Predominant shortperiod waves are recorded only during a short period at the initial parts of P and S waves. These waves give a prominent spectral peak at high frequencies. The results from coseismic and postseismic field surveys in some source regions clearly show that there must be a particular region characterized by a large dislocation, large acceleration and extremely low aftershock activity. This specific region seems to have a relatively small dimension compared with the length of the main fault.
A source model is proposed so as to satisfy various aspects of the observed results. In this model, the rupture processes in a source region may be divided into two stages. The first stage is the smoothing dislocation propagating over the main fault and the second one is the complex ruptures of many small faults distributed in a confined small region. The long-period waves are produced by the former stage. The latter stage radiates the predominant short-period waves, yielding a spectral peak at high frequencies. It is assumed that a large number of small faults with the same fault geometry and the same fault motion are arranged en echelon or parallel in this specific region, and that these small elements are dislocated simultaneously and successively in a short time after the onset of main rupture. We call this specific region in the earthquake source area "a ripple generating core."
The stress drop by each small fault may be of the same order as that of the main fault, if static and dynamic fault parameters are estimated on the basis of the earthquake scaling law. Extremely low activity of aftershocks within the specific region may be caused by the distribution of many small ruptures. A large amount of dislocation around this specific region appear to arise from the accumulation of these abundant dislocations.

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