The oceanic Moho extends to the 34.5oN at least. The corresponding depth is about 25 km beneath the Pacific seashore and 45 km beneath the central part of the study region. We found that the oceanic Moho continues down to northwest and shows a complicated configuration with ridges and valleys. By interpolating the results, we separately draw the depth contour of the continental and the oceanic Moho beneath Chugoku-Shikoku region under the assumptions: (1) the Moho of the Pacific side and the Japan sea side are the oceanic and the continental Moho, respectively and (2) each type of the Moho is continuous. We read the depth of these clear discontinuities from the inverted velocity models and map the Moho depth at the conversion point. However, at stations in the northern and southern part of the study region, we find only one major velocity discontinuity. The continental Moho of the Eurasian plate lies above the low velocity oceanic crust. The LVL corresponds to the subducting oceanic crust of the PHS and the oceanic Moho is the bottom of the oceanic crust. The Philippine Sea plate (PHS) is subducting toward the northwest from the Nankai Trough beneath the Chugoku-Shikoku region where both the continental and the oceanic Moho exist.

The depth of the upper boundary of the LVL is 45 to 50 km.

In the central part, a low velocity layer (LVL) with 10 km thickness exists under the Moho. The depth of the discontinuity is about 30 km beneath northern (the Japan Sea) and southern (the Pacific) coastlines and more than 40 km beneath central part of the study region. We detect a clear velocity discontinuity corresponding to the Moho across which the S wave velocity changes to 4.5 km/s from 3.7 km/s. This improved method adopts a weighting function to determine the shallow structure well and estimate both S and P wave velocity, simultaneously. Integrating estimated receiver functions into six groups according to the back azimuth of each station, we estimate the seismic velocity structure for every group of the receiver functions by using the improved linearized time-domain waveform inversion method. We apply the receiver function analysis to teleseismic waveforms from more than 250 earthquakes with magnitude 5.5 or larger recorded by the High Sensitivity Seismograph Network (Hi-net). We determine the depth variation of the Moho discontinuity beneath Chugoku-Shikoku region, southwest Japan.

Moho pro 12 obj mtl