関東地方の地震メカニズム
スポンサーリンク
概要
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Fault systems of earthquakes in the Kanto district are studied by the enhanced seismicity map derived from the spatial distribution of hypocenters by the sensitive seismic network. Focal mechanisms of the major subcrustal earthquakes in the seismically active regions in the Kanto district are studied by the fault-plane solutions of the P-wave first motions, and the hypocentral distributions of the relocated earthquakes by correcting the Pn residuals. Earthquakes in the southwestern end of Ibaraki Prefecture (west of the 140°E) show the significant NW-SE linear alignment of the epicenters and gentle dipping to the northwest. Earthquakes are also located along the upper boundary of the seismic zone dipping from the east, and have a type of the earthquake mechanism of thrusting with the lower angle to the northwest. This type of the earthquake mechanism is interpreted by the rebound motion of the continental side accompanying with descending of the Pacific Plate. Earthquakes in another active region, located in the eastern side of the 140°E and extending from the southwestern part of Ibaraki Prefecture to the middle part of Chiba Prefecture, occur at the deeper part (from 60 to 80 km) within the seismic zone dipping to the west. Major ones of these earthquakes have two types of the earthquake mechanisms, or the E-W horizontal compression and the vertical dip-slip with the western side downgoing. The se types of the earthquake mechanisms are consistent with the vertical alignment of the hypocenter distribution observed in this region. These earthquakes are located at the position of intersection of the major seismic zone dipping from the east with the minor seismic zone dipping from the west, and are caused by collision of the Pacific Plate with the northeastern end of the Philippine Sea Plate.
- 1981-03-30