Three dimensional fine structure of seismic attenuation beneath Japan Islands derived with NIED Hi-net maximum amplitude data

The attenuation structure beneath the Japanese islands should be complicated three dimensionally because of the plate convergence area with subducting Pacific and Philippine Sea plates. We carried out tomographic inversions for the three-dimensional attenuation structure beneath the Japanese islands with the high-sensitivity seismograph network of Japan (Hi-net) data operated by the National Research Institute for Earth Science and Disaster Prevention (NIED). We use the maximum velocity amplitudes from vertical component of seismograms for earthquakes which occurred from April 2001 to May 2004. 205,501 amplitudes data from 3,410 earthquakes are used in this analysis. Location of hypocenter and focal mechanism of these earthquakes are reported by NIED Hi-net. The study area between latitude of 30-46N degree and longitudes of 130 - 145E, mainly covers the four main islands of Japan. Many grids are placed with an interval of 0.5 degree in the horizontal subsurface at depths of 10, 25, 40, 65, 90km and every 30km up to 360km. The shallowest two depths correspond to the upper and lower parts of the crust respectively, and the others are in the mantle. From the checkerboard test, the resolution is generally good for the layers shallower than 90km. There are three significant results. (1) High attenuation (low-Q) zones can be found beneath the volcanic front in the northeastern Japan clearly, and the distinct low attenuation (high-Q) zone is recovered in the east of the front. Low-Qp bodies appear only just below volcanoes in the upper and lower crust, while the low-Qs area extends continuously along the volcanic front. In the mantle wedge the low-Q zone are laid toward the west. (2) In the central Japan, a low-Qs area is found at a depth of 40 km in the Kanto region, east side of the volcanic front. In this area low-velocity materials with larger Poisson ratios was found, and considered the materials to be serpentine on the Philippine Sea slab (Matsubara et. al., 2004). Below 65km in the Chubu district, we found a distinct low-Q layer, west side of the volcanic front. (3) The high-Q area is found along the upper boundary of the Philippine Sea slab, which is determined from seismicity in the southwestern Japan. In the Shikoku region, the high-Q area does not extend beyond latitude of 34.2N, and the area looks falling down into a deeper part there. On the other hand, in the Kyushu region, the high-Q zone reaches a depth of 100km or deeper coincident with the intraslab seismicity.