1998 年 51 巻 3 号 p. 253-264
Two earthquakes with magnitudes 5.9 and 5.7 occurred beneath the Onikobe geothermal area, northeastern Japan, on 11 August 1996. A detailed study of the earthquakes and their aftershocks revealed that the M5.9 earthquake occurred in the region between the Sanzugawa caldera and the Onikobe caldera perhaps along a geological fault. The bi-lateral fault slip motion was stopped at the caldera walls in both ends. The M5.7 earthquake with right-lateral strike slip type focal mechanism took place along the Onikobe caldera rim or a geological fault close to the caldera rim. On 13 August 1996, M4.9 earthquake occurred on the southwestern extension of the M5.7 fault and had a focal mechanism of strike slip type with some reverse fault components. The fault slip of the M4.9 earthquake did not extend into the Mukaimachi caldera. Recent earthquakes with magnitudes about 5 have occurred in this geothermal area almost every ten years. M4.9 earthquake occurred on the southwestern edge of the Onikobe caldera on 5 July 1976, having a focal mechanism of reverse fault-type. On 28 March 1985, a left-lateral strike slip earthquake with magnitude 5.3 took place also along the Onikobe caldera rim. The lengths of earthquake faults estimated from aftershock distributions are at most 10km and seem to be consistent with characteristic lengths of geological heterogeneity, i. e. lengths of geological faults and diameters of calderas in this area. Seismic tomography study shows that low S wave velocity areas are located inside the calderas. Relatively large earthquakes and their aftershocks occurred only within high S wave velocity areas. It seems reasonable to suppose that temperature inside the calderas is too high to generate earthquakes. Thermal structure is one of major factors that govern seismic activities in the crust.