The passive imaging of the Philippine Sea slab underneath Tokai region in Japan inferred from cross-correlation analysis of teleseismic S coda

The seismic interferometry has recently been developed practically and theoretically for monitoring subsurface structure. The spatial correlation of wave fields, such as noise and coda, at two receivers describes a wave field that is recorded at a receiver if there was a source at the other, the Green"fs function. If such a wave field between the two receivers can be retrieved, it would be possible to detect scattered or reflected phases from subsurface structure by taking migration with the cross correlated functions. In this study we applied the this technique to teleseismic S-coda observed at Hi-net tiltmeters, deployed by NIED, to investigate crustal and uppermost mantle structure, especially the Philippine Sea slab, underneath Tokai region in Japan. We used 246 teleseismic events with magnitudes 5.5 or larger for a period from April 2003 to Dec 2007, and applied bandpass filter of 0.05-0.5 Hz to S-coda on radial component. The time-window used in this study is 20 sec before and 500 sec after the S arrival. The specific features for calculating cross correlation functions used in this study is the followings that effectively enhance the effect of S-coda portion: we subtracted the effect of microseisms, depressed the portion of source-time function by applying taper, and removed the effects of later phases, such as ScS and SS. After these processing, we stacked the cross- correlated waveforms with good S/N ratios among all teleseismic events, and made cross-sections by taking migration technique. In the Tokai region, the Philippine Sea slab subducts west-northwestwards from the Nankai trough. A resultant migrated image for Tokai region clearly shows a dipping feature corresponding to the Philippine Sea slab, and this feature is good agreement with the hypocenter distribution. The dipping feature just below the hypocenters presumably corresponds to the oceanic Moho inside the slab, implying that this technique can be used to detect body waves. However, the continental Moho seems to be unclear in northern part of the cross-section, presumably due to a lack of station density. Moreover, another migrated image also shows a clear features corresponding to the Philippine Sea slab underneath central Japan.