Microseismicity around the Focal Area of the 1978 Miyagi-Oki Earthquake by Obs Observation

In the subduction plate boundary off-Miyagi, middle part of the Japan Trench area, M 7 class interplate earthquakes occur repeatedly at intervals of about forty years. Twenty-six years has already passed since the occurrence of the most recent earthquake, the 1978 Miyagi-Oki earthquake (M 7.4), and the Japanese government evaluated that the next large earthquake may occur within 20 years from now with over 80 % possibility. We started a series of ocean bottom seismographic (OBS) observations in September 2002 in order to clarify the spatio-temporal distribution of seismicity around the area where the next large earthquake is supposed to occur. The OBS network is composed of two kinds of pop-up OBSs, long-term (one year) recording type (LTOBS) and short-term ( 3 months) recording type (STOBS). The LTOBSs are deployed at five stations and are replaced every year to maintain the continuous observation until 2005. Additional ten stations with the STOBSs were in operation during the summer of 2003 and 2004. In this paper, we describe the hypocenter distribution of the off-Miyagi area, from September 2002 to October 2003, deduced from the OBS observation. During this period, there were two large earthquakes in off-Miyagi area; one was an interplate thrust earthquake with M 6.1 on November 3 2002 and the other was an intra-slab one of M 7.0 on May 26 2003. Combining the OBS arrival time data with those of the land seismic network data, we relocated hypocenters of the earthquakes whose hypocenters has been located in and around the off-Miyagi region by Japan Meteorological Agency. Owing to the OBS data the accuracies of focal depths was greatly improved and the most of the hypocenters were relocated along a landward dipping plane, the subducting plate interface. The shape of the plate boundary defined by the relocated hypocenter distribution shows a significant change of the dip angle at about 130 km off the coast, where the depth to the plate boundary is about 20 km. Similar geometry of the plate boundary has been reported in the southern and northern part of the Japan Trench subduction zone. Besides the interplate seismicity, it becomes clear that there is a significant seismicity in the overriding land plate as a result of examining the OBS data. It seems that the intraplate seismicity halted for about two months, after the occurrence of the M 6.1 interplate earthquake. It is notable that the period of the quiescence corresponds to that of an aseismic slip along the plate boundary around the rupture area of this earthquake detected by the continuous GPS observation on land (Miura et al.,2003). This coincidence suggests that the intraplate seismicity can be variable according to the stick/slip behavior of the plate interface.