近畿地方東部および中部地方における浅発地震の発震機構

抄録

Focal mechanism solutions have been determined for 113 shallow earthquakes which occurred from 1968 to 1971 in the east part of Kinki district and Chubu district, Central Honshu, Japan. These mechanisms have been obtained from the first motion of P waves recorded by the stations of Japan Meteorological Agency (J. M. A.) and microearthquake observation stations. Mechanisms have been determined for the earthquakes with magnitude as small as 2.5.
Although the local variation of pressure direction becomes larger, the smaller the earthquakes become, almost of the pressure directions obtained in this study agree well with those of the major shocks analyzed by many seismologists. But there exists a remarkable contrast of the pressure directions in the southwest of Chubu district (Mikawa district and Ise Bay), where pressure lay east-west in the crust but south-north in the upper most mantle. Similar results were obtained in the Kii straight and the south part of Shikoku by SHIONO (1970) and SAWAMURA and KIMURA (1971), respectively. These facts can be interpreted by assuming the pressure which lay perpendicular to the Nankai Trough and the subsedence of the Philippine Sea plate.