Microseismicity in and around Suruga Bay Revealed by an Array Observation of Ocean Bottom Seismometers

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  • 海底地震計で観測された駿河湾周辺の微小地震活動
  • カイテイ ジシンケイ デ カンソクサレタ スルガワン シュウヘン ノ ビショウ

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Abstract

By using an array of pop-up type ocean bottom seismometers (OBS), we investigated seismicity in and around Suruga Bay, the source region of the hypothetical Tokai earthquake, for about one month in 1983. We deployed 9 OBSs and successfully recovered all of them. Jointly analyzing arrival time data of the OBSs and land stations of NRCDP, we located about 50 earthquakes in and around the OBS array. The resulting hypocenter distribution shows the following characteristic features in seismicity, (1) high microseismicity in the middle to southern part of Suruga Bay, (2) quiescence in microseismicity in the north-eastern part of the Nankai Trough, (3) several earthquakes occurring along the Zenisu Ridge. For the events occurring beneath Suruga Bay, we estimated station corrections of travel times and determined their hypocenters precisely. The difference in station correction between the OBS stations and the land stations is very large, 1 to 2s for P wave and 2 to 3s for S wave, showing the importance of station corrections for precise hypocenter determination, particularly in the case of assembling data from OBSs and land stations. The depths of re-determined hypocenters by using the station corrections range from 5 to 20km for events beneath the western side of Suruga Bay. This depth range is shallower than that determined routinely by NRCDP by 5 to 10km, suggesting that the earthquakes beneath the Suruga Trough occur at the upper-most part of the subducting Philippine Sea plate.

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