Recent seismic studies using a high-gain seismograph network have demonstrated the existence of a double-planed seismic zone in the descending plate beneath island arcs such as northeastern Japan, Kurile, and Central Aleutian. Several hypotheses in terms of plate unbending, phase changes, mechanical models have been proposed to explain the characteristic features of the double-planed structure. This paper presents a new hypothesis that thermal stress due to non-uniform temperature distribution in the descending plate is the main causative force for genesis of earthquakes in the double-planed seismic zone.
Based on the estimated temperature distribution in the plate with a dip angle θ and a convergence velocity V_c, the thermal stress is calculated analytically under several assumptions. According to the results of these calculations, the upper and lower parts of the plate are characterized by compressional stress, and the central part by tensional stress. This stress pattern is well consistent with the focal mechanism solutions of earthquakes in the two planes of seismic zone. To verify out hypothesis quantitatively, a new parameter R, defined as the ratio of deviatric stress to the mean normal stress at a depth, is introduced as an index of the possibility of earthquake occurrence. In the case of the descending plate, for example, beneath northeastern Japan (θ=30°, V_c=8 cm/yr), two regions with R≥0.04 exist at the uppermost and central parts of the plate. These regions are parallel to each other with a distance of about 30km. The upper and central regions are characterized by compressional and tensional deviatric stress, respectively. These regions terminate at a depth of about 250km The above features explain the observed seismic activity under the northeastern Japan arc. This value of R=0.04 is not too different from the data of rock fracture experiments at high temperature and pressure. The value of R at the center of the plate is the largest in the case of θ=20-30° and V_c≥3cm/yr and decreases with increasing dip angle.

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The theological structure of the earth's mantle was determined based on data from the postglacial isostatic adjustment in Laurentide. According to a linear analysis with a Newtonian rheology, the apparent viscosity derived from the observed relative sea level data is about ten times larger in the central part of the glaciated region than that in the surrounding region. Namely, the apparent viscosity has spatial dependence. The observed relation ζ∝ζ^3-4 in Laurentide and Fennoscandia, where ζ and ζ respectively represent uplift rate and estimated remaining uplift, does not reflect the non-linearity of the rheological property of the earth's mantle. Rather, the observed relation ζ∝ζ3-4 means that the lower mantle viscosity is greater than 1024 poise, regardless of Newtonian or non-Newtonian rheology.
According to the analysis for a thin channel viscosity model with a power-law creep rheology ε∝σ³, where ε and σ respectively represent strain rate and deviatoric stress, the observed relative sea level variations and free-air gravity anomalies in the glaciated region in Laurentide can be explained almost satisfactorily. The same model is also consistent with the observed relation ζ∝ζ^3-4. Our calculation therefore indicates that the viscosity of the lower mantle is so high that a thin channel viscosity model is a good approximation of the mantle flow, and that the upper mantle rheology is governed by the power-law creep law with n=3 (ε∝σ³). The average temperature, strain rate, deviatoric stress, and apparent viscosity estimated in the present work are 1, 500K to 1, 700K, 2.8/H²sec^-1, 0.22H bar, and 0.04H³ poise, respectively, where H represents the thickness of the low viscosity channel in cm. The relationship between strain rate and deviatoric stress is consistent with an extrapolation of high strain rate laboratory data.

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An analysis of surface waves including the fundamental and first higher modes, and Sa phase was made to determine the upper mantle structure beneath the Canadian Shield. Four earthquakes which occurred beneath the Canadian Shield were used for this purpose. By comparing synthetic and observed seismograms, the shear velocity structure of the upper mantle was determined. The shear velocity structure of the shallower part was determined mainly using the fundamental and first higher modes. The structure in the deeper part was mainly constrained by the Sa phase. Phase of Love type Sa wave was successfully reproduced by a uniform shear velocity structure over the entire area of the Canadian Shield. The average shear velocity of the upper mantle above the depth of 400km obtained in this study is 4.67km/sec, which is higher than velocities of any other models presented before. The low velocity layer of the upper mantle is poorly developed. Since the polarization anomaly of surface waves was not observed for the fundamental and first higher modes and Sa phase, anisotropic velocity structure does not exist in the upper mantle beneath the Canadian Shield.

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An earthquake swarm that occurred east of the Kii Peninsula near the Nankai trough was investigated using seismic data from four pop-up type OBSs, four telemetering OBSs, and one land seismic observation station. The addition of the pop-up type OBSs was most effective in the analysis of the swarm. We analysed the daily frequency of earthquakes, the b-value, S-P time distribution, and hypocenter distribution. Here we discuss the swarm activity with regard to recent seismicity around the area, to submarine active tectonic lines, to oceanic topography, and to block boundaries along the Nankai trough. The swarm activity was largely divided into two periods, and seemed to consist of six or seven sequences of foreshocks-mainshock-aftershocks or mainshock-aftershocks. The earlier activity occurred in a deeper region (at a depth of about 17km), the later one in a shallower region (about 5km). The epicenter was distributed in a region no larger than 18km×8km. The b-value seems to have decreased before the largest earthquake. The swarm seems to have occurred at a submarine active tectonic line near the Nankai trough.

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Major dacite pumice eruptions began at Usu Volcano on August 7, 1977. The relationship between the change in temperature of one of the associated hot springs and earthquake frequency of the volcano has been statistically examined using the data obtained from 1975 to the onset of the eruption. It is apparent that the coefficient of correlation between them markedly increased with time before the eruption. It may be a useful indicator to predict volcanic eruptions.

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