反射法測線を用いた2003年宮城県北部地震の余震の精密震源決定

Abstract

A small aftershock of the 2003 Northern Miyagi earthquake was observed during the seismic reflection survey in 2005 which was carried out near the source fault of the 2003 event. The hypocenter of the aftershock determined by the Japan Meteorological Agency (JMA) and the Ministry of Education, Culture, Sports, Science and Technology (MEXT) is close to the survey lines and lies in a plane distribution of other aftershocks. Using the observed arrival times in the reflection survey stations in 2005 and the seismic structure determined by using the previous survey in 2003, we determined the hypocenter location of the aftershock. Our hypocenter located in an about 1.6km northwest and about 8km shallower position compared with the hypocenter determined by the JMA and MEXT. The relocated hypocenter is close to the deeper extension of the Asahiyama flexure. We installed one 3-component seismograph nearby during the survey. The observed S-P time of the event at the seismograph can be explained by this hypocenter. The hypocenter determined by the JMA and MEXT is too far from the seismograph to explain this S-P time. Previous reflection surveys show a 2∼3km thick sedimentary layer in the west of the Sue Hills, which area contains this survey territory. A previous gravity survey also suggests this heterogeneous structure extends to a wider area. The heterogeneous structure may cause eastward and deeper bias of the hypocenters estimated by using laterally homogeneous velocity models which have a higher velocity than those estimated from the reflection surveys at shallow depth. This supposed mislocation is consistent with our results.