Q in the Mantle Wedge beneath the Philippine Sea

Seismic wave attenuation provides important constraints on the structure and the physical properties of the Earth. Attenuation is sensitive to the variation in water and/or melt content as well as the variation in temperature. In this study, we focus especially on Qp/Qs and the frequency dependence of Q. Qp/Qs is sensitive to the presence of melt that could cause bulk attenuation as opposed to shear attenuation. The frequency dependence α (Q~ ω ^α ) is also an important parameter: the conversion of variation in Q to the variation in viscosity depends strongly on this parameter. We investigated the upper mantle attenuation beneath the northern Philippine Sea region, including the Izu-Bonin subduction zone and the Shikoku Basin. We used regional waveform data from the events in the Pacific slab, recorded on F-net and J-array network broadband stations in western Japan. Two kinds of phase-pair method which measure the difference in spectral decay between two phases are employed to measure the differential attenuation. P-P and S-S methods measure the difference in spectral decay of two P and S waves which sample different paths. We obtain P-P and S-S spectral ratio curves with high S/N ratio over the frequency range 0.5-8.0 Hz and 0.5-2.5 Hz respectively. For S-S method, the available frequency range is limited to below 2.5 Hz because of P-coda spectra. Qp/Qs are estimated from both P-P and S-S methods which suggest some geographical variations. Regions with low Qp/Qs are found in the middle part of Philippine Sea and many data corresponding to long path-length show relatively small Qp/Qs. Values of Qp/Qs less than ∼ 2 imply significant bulk attenuation implying the presence of mechanically heterogeneous materials possibly partial melt. Frequency dependence of attenuation is estimated by fitting the P-P spectral ratio curve as a power-law function of frequency. The estimated values of power-law exponent range ∼ 0.2-0.7 with some regional variations. This corresponds to a variation of viscosity by a factor of ∼ 3-200 corresponding to the observed variation of Q (a factor of 2-3 at a same depth: Shito and Shibutani, 2003).