Three-dimensional attenuation structure beneath southwest Japan estimated from spectra of microearthquakes

Seismic attenuation (Q -factor) can be estimated by extracting the amplitude-frequency information contained in seismic waveforms and it provides an important insight into the nature of heterogeneities in the Earth's structure and composition. Compared to the numerous velocity structure studies, only a few studies on attenuation structure have been carried out in Japan. Most of these studies, however, have been concentrating on northeast Japan. Almost no Q -structure studies have been carried out in southwest Japan. Thus, little is known about seismic attenuation of that region. In this study, we have investigated the 3D P wave attenuation (Q_p, hereafter we call it Q) structure of the crust and upper mantle in southwest Japan using P-wave spectra of microearthquakes recorded by the dense High-Sensitivity Seismographic Network (Hi-net). Spectra of a 2-s, cosine-tapered time window after the first P arrival were calculated. A total of 4105 high-quality waveforms from 90 local earthquakes have been analyzed. These spectra were then inverted by using an inversion scheme to estimate the 3D Q structure. Low- Q zones are detected under volcanic areas, while high- Q zones in the uppermost mantle under Shikoku and neighboring areas show the image of the subducting Philippine Sea slab. Waveforms recorded at northern Chugoku along the Japan Sea coast were found depleted from higher frequencies compared to those recorded in eastern Kyushu, Shikoku and Kii Peninsula. This reflects large differences in seismic attenuation between the mantle wedge and the subducting Philippine Sea slab. The spatial distribution of low-Q zones detected, in this study, is generally consistent with the low-velocity (low-V) zones revealed by travel time tomography.