Anelastic structure of the upper mantle beneath the northern Philippine Sea

We investigated the upper mantle anelastic structure beneath the northern Philippine Sea region, including the Izu-Bonin subduction zone and the Shikoku Basin. We used regional waveform data from 69 events in the Pacific and the Philippine Sea slabs, recorded on F-net and J-array network broadband stations in western Japan. Using the S-P phase pair method, we obtained differential attenuation factors, deltat*, which represent the relative whole path Q. We conducted a tomographic inversion using 978 deltat* values to invert for a fine-scale (50-100 km) three-dimensional anelastic structure. The results shows two high-Q regions (Q(P) > 1000) which are consistent with the locations of the Pacific and the Philippine Sea slabs. Also there is a low-Q (Q(P) similar to 110) area extending to the deeper parts (350-400 km) of the model just beneath the old spreading center and the Kinan Seamount Chain in the Shikoku Basin. A small depth dependence of the laterally averaged Q(P) was found, with values of 266 (0-250 km), 301 (250-400 km), and 413 (400-500 km). (C) 2003 Elsevier B.V. All rights reserved.