関東地方における微小地震と大地震の震源分布

概要

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Epicenter maps and vertical sections showing the spatial distribution of microearthquakes in the Kanto district for the period of 1973-1976 were compiled. Major features of the micro-seismicity presented here are almost similar to the previous results (e.g., TSUMURA, 1973), but some fine structures of earthquake distribution are shown more clearly as followings; (1) Existence of seismicactivity in the lower crust in the vicinity of Tokyo is comfirmed. (2) The eastern group of microearthquakes of the two seismic areas in the southwestern part of Ibaraki Prefecturchas a double-plane structure dipping northwestwards with low angle. (3) The distribution of hypocenters shown in the vertical section along the zone from the Izu Peninsula to central Saitama Prefecture may suggest the northwards low angle thrusting or the Philippine Sea plate. Larger earthquakes (M≧6.0) in the Kanto district seems to have occurred in clusters in space and time. It should be noted that larger earhtquakes have not occurred so frequently in southwestern Ibaraki Prefecture and central Chiba Prefecture as expected from the very high activity of smaller earthquakes there.
1981-03-30