2. Seismic/Geodetic Imaging Technologies -Invited-
High-resolution Receiver Function Imaging of Velocity Discontinuities in the Crust and the Upper Mantle Beneath the Japan Islands
Kazuro Hirahara(1), Takashi Tonegawa(2) and Takuo Shibutani(3)
(1) Graduate School of Science, Kyoto University, Japan. (2) Graduate School of Environmental Studies, Nagoya University, Japan. (3) Disaster Prevention Research Institute, Kyoto University, Japan.
Abstract
Over the Japan Islands, there have recently been deployed short-period and broadband seismic networks with one of highest density of the world. We have executed receiver function (RF) analyses for P-wave coda portions observed at these stations from teleseismic events to investigate velocity discontinuities in the crust and the upper mantle beneath the Japan Islands. RFs are radial waveforms deconvolved by vertical ones and are produced through conventional spectral division with water level and low-pass Gaussian filter. Assuming all phases following the direct P-wave are Ps phases converted at discontinuities beneath the station, time-domain RFs are transformed to depth-domain ones based on reference velocity models. Singular value decomposition filtering is applied to enhance the coherent phases included in RFs. Then CCP (Common Conversion Point) stacking is applied to produce clear RF profiles in several cross-sections. For crust and uppermost mantle structures, we mainly use RFs observed at short-period stations. We detect undulation of continental Moho and the 3-D configuration of the top and the oceanic Moho in the subducting Philippine Sea plate (PHS) in southwest Japan (Yamauchi et al., 2003). And we detect also the aseismic slab of PHS down to 100 km, where there is the seismic activity only down to 60 km depth. In northeast Japan, we detect the top and the oceanic Moho of the Pacific slab (PAC) down to 120 km. The lower boundary of the PAC can also be traced down to 200 km depth or more. It is parallel to the slab surface and the oceanic Moho, and the thickness between the slab surface and the lower boundary is ~80 km (Tonegawa et al., 2006). For upper mantle discontinuities, we mainly use data observed at broadband stations (Tonegawa et al., 2005). The 410 km discontinuity is locally elevated by 30 km in the region where the PAC penetrates the 410 km one. The 660 km one is gradually downwarped by 50 km caused by the stagnant slab. Frequency dependence implies that the thickness of the 410 km discontinuity varies in a relatively local scale, while the transition of the 660 km one is sharp.
Last modified: Mon May 22 10:10:14 2006