東北日本のサイスモテクトニクスと男鹿-牡鹿構造帯

抄録

Intraplate seismicity in the Tohoku district is high in the regions along the volcanic front and in the belt-like zone parallel to the Oga-Oshika tectonic line proposed by MOGI (1985a). Almost all large earthquakes in the Tohoku district in the past 100 years occurred in the Oga-Oshika tectonic belt and a significant linear distribution of small earthquakes is occasionally observed in the zone. Further, seismic activity after the 1983 Japan Sea earthquake migrated to the south along the tectonic belt [YOSHIDA and HOSONO (1987)]. Another noticeable feature is that earthquakes occur in the lower crust in the zone. On the other hand seismicity in the regions along the volcanic front is characterized by shallow earthquakes. Space-time distribution of earthquakes with M≥4 in the Oga-Oshika tectonic belt in the past 25 years exhibits that the seismicity in the zone became active just before and after three large earthquakes which occurred in the zone or in its extended area. The existence of a tectonic belt in almost the same site has been proposed by geologists based on geological observations and tectonic considerations. Recently TADA (1986) presented an idea that the Honjo-Matsushima tectonic zone, which had been proposed by geologists [OIDE and ONUMA (1960)] divides Northeastern Japan Arc into the northern and the southern blocks, investigating crustal strain field in the past 90 years in the Tohoku district. Submarine topographies and geomagnetic anomalies in the Japan Sea also suggest an existence of boundary of crustal structure in the zone. Neogene sediments are widely distributed in the zone as well. These all features indicate that the Oga-Oshika tectonic belt is a deep-seated weak zone, reaching to the lower crust, which corresponds to the boundary of crustal blocks. Characteristics of seismic activity in this zone such as concurrent activation and migration also suggest that the zone makes a stress guide in mechanical interactions between blocks. Its origin may be related to the difference of coupling form between the oceanic and the land plates in the northern part and the southern part of the Tohoku district.