부산대학교 도서관

The Tohoku-Oki earthquake (Mw 9.0) occurred off the Pacific coast of Japan, on March 11, 2011. The seismic activity in Japan changed after the Tohoku-Oki earthquake. Especially around the border between Ibaraki and Fukushima prefectures, so many earthquakes within the crust have occurred. The largest earthquake in this region is the Fukushima Hamadori earthquake (Mw 6.6) on April 11, 2011, which caused severe damage. In this region, the seismic activity had been very low before the Tohoku-Oki earthquake, but the seismic activity became significantly active after the Tohoku-Oki earthquake. There are few previous studies about this region that investigated the crustal structure in detail because the seismic activity was low before the Tohoku-Oki earthquake. Thus, to investigate the crustal structure in this region is very important from the perspective of elucidating the mechanism of large inland earthquakes such as the Fukushima Hamadori earthquake. Earthquake Research Institute, The University of Tokyo, and other organizations deployed a temporary dense seismic array in this region after the Fukushima Hamadori earthquake. We examined the waveforms obtained from the seismic array and found two reflected S waves from the deep part of crust. We estimated the locations of the reflection points in the deep part of the crust by applying Reverse VSP (Vertical Seismic Profile) method which has been used for seismic exploration. As a result, we found two S wave reflection boundaries at depths of (1) 15～23 km and (2) 26～34 km in the crust. The reflection boundary (2) was found to be the Moho discontinuity by comparing with previous studies in this region. It can be interpreted that the reflection boundary (1) indicates the upper boundary of the layer with crustal fluids in terms of the large amplitudes of the reflected S waves. We compared the locations of the reflection boundary (1) with seismicity in this region and found that the location of the reflection boundary (1) was consistent with that of the Fukushima Hamadori earthquake and the focal area at the depth of 15～20 km which was one of the characteristics of seismic activity in the region. Thus, the crustal fluids may be related to the seismic activity in the region triggered after the Tohoku-Oki earthquake.