2004年留萌支庁南部地震震源域の重力測定

概要

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An earthquake (MJMA 6.1), occurred in December 14, 2004, at southern area of Rumoisub-prefecture, northern Hokkaido, Japan. The earthquake intensity was assumed to 6 inObira town. In Hokkaido, this is the first inland earthquake larger than M6 sinceTeshikaga earthquake (MJMA 6.5, 1967). Detailed aftershock distribution shows that theaftershocks occur on an eastward dipping plane of 10 km square, depth of which 5 km. Themainshock assumed to have occurred near the southern end of this plane. Despite thesparseness of existing gravity stations over the focal region, it was found that the aftershocksoccur just above the gravitational rise of about 20 mGals. So the gravity survey wasoperated in Nov. 2005 to Aug. 2006, to make a detailed Bouguer anomaly map and toexplicate the subsurface structure of the focal region. The measurement was done withCG-3 type gravimeter. Measurement positions were carefully decided by operatingdifferential GPS observation. The accuracy of altitude decision is within 1 meter in all of themeasurement points. Besides, data provided by Japan Petroleum Exploration Co. Ltd. wasvery helpful. The appropriate assumed density was detected to 2300 kg/m3, using densitylog of deep borehole near the focal region, and the correlations of gravity correction terms.The Bouguer anomaly map by current dataset also shows good coincidence with aftershockdistribution.