MT Measurements in the San-in Region, Japan, in Cooperation with Seafloor Electromagnetic Observations in the Sea of Japan

MT measurements have been made since 2006, together with Seafloor electromagnetic observations MT measurements in the Sea of Japan, in order to investigate the resistivity structure beneath the San-in region, Japan (the Tottori and surrounding region in the northern part of Chugoku district, southwestern Japan), where epicenters of seismic activities are remarkably distributing within a line belt with a width of about 4-9 km along the coastal line of the Sea of Japan. The depths of the hypocenters are located up to about 10km depths. In the seismic belt, several large earthquakes of M6.2-7.4 took place in 1943, 1983 and 2000. Moreover, quaternary volcanoes, such as Daisen and Oginosen volcanoes are also located in the seismic belt. Wide-band magnetotelluirc (MT) observations have been made along several survey profiles of almost N-S direction in the San-in region since 1998, to investigate heterogeneity in the crustal electrical resistivity structure. Through those wide-band MT measurements we found the low resistive region beneath seismogenic zone of the high seismicity belt along each MT profile, and that the upper resistive crust corresponds to the seismogenic zone in the Tottori and northern Hyogo region. The low resistive region found along each wide-band MT profile seems to form a conductive zone extending in the almost E-W direction beneath the seismic belt extending in the almost same direction of the conductor. This result strongly suggests the existence of crustal fluid beneath the seismogenic zone in the focal area. The survey lines should be extended toward the Sea of Japan and longer period MT data on the land area should be obtained, in order to investigate deeper resistivity structure and clarify the relationship between subducting Philippine Sea plate and the deeper resistivity structure beneath the San-in region. Therefore, we have carried out not only seafloor EM measurements but also longer period MT surveys on land since 2006 along two almost N-S profiles passing through lines of longitudes, 134.3E and 133.4E. It is important to analyze both data sets to obtain high accuracy resistivity structure. We will show the outlines of the observation and the preliminary results along the two survey lines in presentation.