Research on Seismicity and Active Tectonics in the Northern Part of Ishikari Basin

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A borehole with 750m deep has been constructed for installation high sensitive seismometer and to reveal the geological chronology of Quaternary and Neogene formations beneath Sapporo-city by using many boring data. According to epicenter location by using this new station and seismic network operated by Sapporo-city a clear seismic belt trending from northwest to southeast is recognized. This coincides the direction of the boundary between mountain and basin located at western part of Sapporo-city. Rapid depth change of the top of Pliocene formation is also founded on this line. This will be explained as the reverse faulting with dipping of the western side. There is also steep gravity Bouguer anomaly along this trend of seismic activity with high anomaly at west side and low one at east side.