[J08-3] Imaging and interpreting lithospheric structures using seismic and geodetic approaches III

[J08-3-05] Three dimensional attenuation structure in and around the source region of low frequency earthquakes beneath the Kii Peninsula, southwest Japan, revealed by dense seismic array observation

^○Noriko Tsumura¹, Hiroki Nakasako¹, Eri Umeyama¹, Naoki Mizuno¹, Eiji Kurashimo², Aitaro Kato², Shinichi Sakai², Takashi Iidaka², Takaya Iwasaki² (1.Chiba University, Chiba, Japan, 2.ERI, Univrtsity of Tokyo, Tokyo, Japan)

Non-volcanic deep low frequency earthquakes (DLFE) have occurred along the isodepth contour of 30 to 40km of the subducting Philippine Sea plate (PSP) beneath the Kii Peninsula, southwest Japan. The DLFE's activity is not uniform and some clusters align from the WWS to the EEN of Kii Peninsula. Further seismicity of local earthquakes which occur around the PSP's Moho in the eastern part of the Kii Peninsula is lower than that of western part. To clarify the physical property which controls the seismicity of DLFEs and local earthquakes, we applied a combined inversion method to P wave spectra and obtained an attenuation (Q) structure there. Adding to routine seismic network's data, we used seismic waveforms recorded in three dense seismic array observation which were settled in perpendicular or parallel direction to DLFE clusters' alignment. Since length of the arrays reach approximately 90km (or 60km) with 1km station interval, it made us possible to estimate a detailed Q structure along the arrays.
Derived Qp images show that patch-like high Qp zones were distributed in the region just around the depth from 30 to 40km at the western part of the Kii Peninsula. These high Qp regions coincide with the regions where the DLFE clusters exist. On the contrary, we can find a low Qp zone at the same depth range in which high activity of the DLFEs is seen in the eastern part of the Kii Peninsula. This low Qp zone corresponds to the lower Vp and high Vp/Vs region derived from travel time tomography. Low Vp and high Vp/Vs generally suggest the existence of fluid and similar low Vp and high Vp/Vs region were found at the DLFE cluster of the western side. Then these results suggest the occurrence of the DLFEs may be associated with fluids dehydrated from the PSP, but its activity might be affected by different physical properties in both sides of the Kii Peninsula.