2003 年 55 巻 4 号 p. 337-350
We have studied micro-seismicity in the northern part of Hokkaido (north above 44°N) from June to November in 1998 combining eleven temporal seismic stations with seven and five routine ones operated by Institute of Seismology and Volcanology (ISV) in Hokkaido University and Sapporo Meteorological Observatory, Japan Meteorological Agency (SMO), respectively.
Firstly we determined 91 hypocenters of local earthquakes in this period using the dense network. These hypocenters are about three times of the number of those from the routine network alone. One-dimensional P-wave velocity structure assuming four-layer model (assumed thickness of 2, 8, 10km and infinite) and station corrections were estimated using a P-wave travel time inversion method with 735 P-wave arrival time records of 81 events. The velocity of each layer was determined to be 2.83km/sec for the first layer, 5.32km/sec for the second one, 6.32km/sec for the third one and 6.69km/sec for the bottom half space. From the P-wave station corrections we obtained, this region can be classified into three zones parallel in the north-south direction; the western islands region in the Sea of Japan, the western part of mainland, and the eastern part of mainland. Each zone shows the value of less than-0.5sec, +0.1-+0.4sec, and-0.1--0.5sec, respectively.
Next, the hypocenters with the inverted velocity structure and the station corrections are relocated. As the results show, some hypocenters in the anomalous delayed station correction zone, i. e. the western part of mainland, are clearly located at the depth range from 20 to 25 km. Focal mechanism solutions of these deep events show normal fault type, while shallower events less than 20 km depth show strike-slip and reverse fault types. We also relocated 381 earthquake hypocenters which were routinely determined by ISV from October 1996 to December 2000. According to the relocated hypocenter distribution, a high seismic zone is shown in the western part of mainland with about 50km wide along a north-south direction. On the other hand, the eastern part of mainland is strongly characterized as aseismic zone. The boundary between the seismic and aseismic zones corresponds to the geological boundary between Kamuikotan metamorphic belt and Hidaka belt.