Ocean Bottom Seismometer Study on the Seismic Activity in the Mariana Island Arc Region
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概要
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Using OBS's (ocean bottom seismometers) and island based stations, microearthquake observations were conducted in the Mariana island arc region during July and August, 1973 and again during January and February, 1976. Although microearthquake activity was very high along the Mariana island arc, no local earthquakes were detected within the Mariana basin. A few local earthquakes from the Caroline Islands region were recorded. Seismic activity was particularly high around the island of Guam towards the southwestern Mariana area, whereas very few earthquakes occurred around the island of Saipan. Hypocenters were determined for 40 events, only two of which were reported by the Earthquake Data Report (EDR). An analysis of earthquake hypocenters in the Mariana region determined from this study delineates distinct active seismic elements of this area. These are, (a) shallow earthquakes (h<70km) under the trench slope break along the trench axis, (b) intermediate earthquakes (70≦h<210km) under the Mariana ridge axis, (c) shallow earthquakes (h<70km) around the terminus of the Mariana ridge, extending from southeast to southwest off Guam Island. A close correlation exists between the hypocenter distribution and bathymetric features such as the ridge axis and the trench slope break, implying that these tectonic elements are active. Long term data from ISC regional catalogue show that they are associated with several major tectonical elements of this island arc system. High sensitivity microearthquake observations also revealed that earthquake activity occurred at several places in a wide region along the Mariana island arc, and included two swarms and intermediate earthquakes. The temporal pattern of these observations suggests that a broad region was tectonically activated as a whole almost simultaneously, a process that we have called "regional activation".マリアナ海盆上の海底地震計とGuam,Saipan及びMarcusにおける島上の臨時観測点を用いて,マリアナ地域における微小地震観測を行った.観測期間は1973年7月～8月及び1976年1月～2月の2度.観測期間中マリアナ島弧に沿って群発地震を含むきわめて活発な微小地震活動が観測された.カロリン地域の地震も観測されたが海底地震計近傍のマリアナ海盆には地震は起っていないように思われる.微小地震はGuam島付近から南マリアナにかけて特に活発であるが,同じマリアナ海嶺上にあるSaipan島近傍にはほとんど地震が起っていない.計40個の地震の震源が求められた.これらの震源は大別して(a)海溝軸に沿った「trench slope break」下の浅い(h≦70km)地震,(b)マリアナ海嶺下の稍深発(h＞70km)地震,(c)Guam島付近のマリアナ海嶺末端から南西にかけた浅い(h≦70km)地震に分けられる.地震活動と海底地形との対応は非常に明瞭であり,これらの造構要素が活動していることを示している.ISCのデータの示す長期間(1964年1月～1974年6月)の震源分布は,これらの微小地震活動がマリアナ島弧における主要な造構運動を反映したものであることを示している.高感度の微小地震観測により,マリアナ島弧に沿って,きわめて広範囲にわたっていっせいに地震活動が活発になる現象が見出された.数百kmに及ぶいくつかの地域に広がっており群発地震や稍深発地震も含んでいる.このことは広域にわたって造構運動が一時期いっせいに活発になったことを示しており,この現象は「regional activation」というべき現象と思われる.
- 1981-08-31
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