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The detailed investigations of the deep seismic zone beneath the island arc have revealed the existence of the double-planed structure of the intermediate-depth seismic zone. The characteristic features of the double-planed structure have been interpreted in terms of phase change, unbending, thermal stress or sagging of the descending plate beneath the island arc. The purpose of the present study is to simulate the double-planed structure experimentally based on our thermal stress model and to clarify the relationship between the thermal stress distribution and the foci of acoustic emissions (A. E.) in a sample. The sample used in this experiment is pine resin with the diameter of 10cm and the thickness of 2.5cm. The sample is, first, gradually cooled to the uniform temperature of -30C. Then, one of the sample surfaces is uniformly heated up to 20C with the three heating rates of 2.0, 5.5 and 14.5C/mmn and a non-uniform temperature distribution along the direction of the sample thickness is generated. Acoustic emissions during the heating process are detected and located using the four piezo-electric transducers which are set up on the both surfaces. The thermal stresses in the sample are estimated based on the observed temperature distribution. The regions near the upper and lower surfaces are characterized by the compressional stress with the maximum value of 25 to 70 bars depending on the heating rate, whereas the region near the middle part is characterized by the tensional stress with the maximum value of 15 to 50 bars. The focal depth distributions of A. E. are separated into three groups by the two planes with the null thermal stress. The two groups are located in the regions of the compressional thermal stress and the other is in the region of the tensional stress. The triple-planed A. E. activity induced by the thermal stresses does not correspond exactly with the double-planed seismic activity in the descending plate. This discrepancy, however, is mainly attributed to the high temperature state near the lower rim of the descending plate. The present model experiment confirms that the occurrence of the double-planed seismic activity beneath some island arcs can be explained in term of the thermal stresses induced in the descending plate.

A Model Experiment on the Generating Stresses for the Double-Planed Structure of the Intermediate-Depth Seismic Activity