Permalink : http://hdl.handle.net/2115/20433

JaLCDOI : 10.14943/gbhu.70.27

An earthquake (MJMA 6.1), occurred in December 14, 2004, at southern area of Rumoi sub-prefecture, northern Hokkaido, Japan. The earthquake intensity was assumed to 6 in Obira town. In Hokkaido, this is the first inland earthquake larger than M6 since Teshikaga earthquake (MJMA 6.5, 1967). Detailed aftershock distribution shows that the aftershocks occur on an eastward dipping plane of 10 km square, depth of which 5 km. The mainshock assumed to have occurred near the southern end of this plane. Despite the sparseness of existing gravity stations over the focal region, it was found that the aftershocks occur just above the gravitational rise of about 20 mGals. So the gravity survey was operated in Nov. 2005 to Aug. 2006, to make a detailed Bouguer anomaly map and to explicate the subsurface structure of the focal region. The measurement was done with CG-3 type gravimeter. Measurement positions were carefully decided by operating differential GPS observation. The accuracy of altitude decision is within 1 meter in all of the measurement points. Besides, data provided by Japan Petroleum Exploration Co. Ltd. was very helpful. The appropriate assumed density was detected to 2300 kg/m3, using density log 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 aftershock distribution.