Earthquake Mechanisms Associated with the Conjunction of the Sinking Plates beneath the Kanto District, Central Japan
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概要
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A study of the focal mechanisms was made for 49 subcrustal earthquakes which occurred in the seismically active regions of the Kanto District, by using the dada on P wave first motion. These data were collected from the observational stations of the Earthguake Research Institute (ERI) and from the seismological bulletins of the Japan Meteorological Agenay (JMA) and the International Seismological Centre (ISC). Shallower earthquakes located at a depth of about 45 km below the western end of Ibaraki Prefecture had the focal mechanism of underthrusting at a low dip angle towards the northwest. The spatial distribution of the hypocenters in this region, showing a linear alignment of the epicenters and a gentle dipping to the northwest, is consistent with the pole on the nodal plane dipping to the northwest. These earthquakes are interpreted as being caused by underthrusting towards the northwest along the upper boun- dary of the Pacific Plate. Deeper earthquakes, located in the eastern side of the 140°E line, extending from the western part of Chiba Prefecture to the southwestern part of Ibaraki Prefecture, having a focal d pth around 70 km within the downgoing Pacific Plate, show the focal mechanism of the vertical dip-slip with the western side descending. These earthquakes take place in the colliding region of the minor seismic zone sinking to the east and the Pacific Plate sinking to the west. The earthquakes in this region are interpreted as being caused by the subcrustal collision of the Philippine Sea Plate with the Pacific Plate.関東地方の地震活動域におこる地殻下地震の発震機構がしらべられた.地震研究所観測網の他,気象庁・ISCの地震報告からも初動方向の資料が収集された.茨城県南西部には2つの著しい地震活動域があるが,西端部の地震は北西-南東の線状配列を示し,東側から傾むき下がる大規模な地震帯の上面に沿って起っている(震源の深さは45km).地震メカニズムは北西への低角逆断層型を示し,震源の配列と良く一致している.従って茨城県西端部の地震は,太平洋プレートの北西方向への沈みこみに伴なう反撥運動によるものである.もう一つの顕著な活動域は,茨城県南西部から千葉県中部にかけた,東経140°線の東側に位置し震源の深さは約70kmである.この位置は東側から沈みこむ大規模な地震帯に,西側から傾むき下がる小規模な地震帯が交叉しているところにあたる.又地震メカニズムは東西圧縮型あるいは西側が落ちこむ鉛直縦づれ型が優勢であり,その主圧力軸は西側から傾むき下がっている.このような西側から傾斜している地震帯がフィリピン海プレートの北東端であるならば,この地域の地震はフィリピン海プレートの北東端が太平洋プレートに衝突していることによって起っていると考えられる.
- 1981-03-07
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