Three-dimensional Attenuation Structure beneath the Tokai Region, Central Japan Derived Using Local Earthquake Spectra
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概要
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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 <I>Q</I> 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 <I>Q</I> 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 <I>Q</I> zone, that portion might consist of harder rocks than surroundings. On the contrary, the region just beneath the large slip zone has lower <I>Q</I> values than those of surrounding area. Comparing our results with a seismic velocity structure derived from travel time tomography, we found the high <I>Q</I> zone approximately coincides with a zone of relatively high velocities and the lower <I>Q</I> zone corresponds to a zone of relatively low velocities and high <I>V</I><SUB>P</SUB>/<I>V</I><SUB>S</SUB> values. A low <I>Q</I> zone with low velocities and high <I>V</I><SUB>P</SUB>/<I>V</I><SUB>S</SUB> can be interpreted as the zone which involves high-pressure fluid. Probably the high <I>Q</I> 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.
- 公益社団法人 日本地震学会の論文