Double-planed structure of intermediatedepth seismic zone and thermal stress in the descending plate.

Abstract

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 Vc, 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°, Vc=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 Vc≥3cm/yr and decreases with increasing dip angle.