東海地域下の三次元地震波減衰構造

—微小地震スペクトルを用いた推定—

抄録

Long term slow slip (LTSS) and non-volcanic low frequency earthquakes (LFEs) were reported in the central part of the Tokai district, central Japan. Such LTSS and LFE events are considered to take place at a transition zone of frictional property from stick-slip to stable sliding on the top of subducting Philippine Sea plate. To clarify the spatial variation of physical properties in this region, we estimated a three dimensional seismic attenuation structure using a joint inversion method. In the shallow depths from the surface to 5km, we found a lower Q zone located along the Median Tectonic Line which divides the southwestern Japan into two parts; an old geologic belt and a new accretionary belt. In the lower crust of the land plate at the depths of 17 to 25km, a very high Q zone (about 2000) exists just above the region where large slip rates were observed during the LTSS between 2001 and 2005. Since very few earthquakes occur in this high Q zone, that portion might consist of harder rocks than surroundings. On the contrary, the region just beneath the large slip zone has lower Q values than those of surrounding area. Comparing our results with a seismic velocity structure derived from travel time tomography, we found the high Q zone approximately coincides with a zone of relatively high velocities and the lower Q zone corresponds to a zone of relatively low velocities and high V_P/V_S values. A low Q zone with low velocities and high V_P/V_S can be interpreted as the zone which involves high-pressure fluid. Probably the high Q zone above the large slip zone works as a cap rock and prevents the fluid from moving toward the shallow part, and then the fluid pressure becomes high and it affects the occurrence of slow slip in this region.