HR: 1330h
AN: S22B-0451 INVITED     [PDF]
TI: Q in the Mantle Wedge beneath the Philippine Sea
AU: * Shito, A
EM: azusa.shito@yale.edu
AF: Yale University, Department of Geology and Geophysics, 210 Whitney Avenue, New Haven, CT 06520 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, Department of Geology and Geophysics, 210 Whitney Avenue, New Haven, CT 06520 United States
AU: Park, J
EM: jeffrey.park@yale.edu
AF: Yale University, Department of Geology and Geophysics, 210 Whitney Avenue, New Haven, CT 06520 United States
AB: 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 $\alpha $ ($Q\propto \omega ^{\alpha}$) 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 $\sim 2$ 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 $\sim 0.2-0.7$ with some regional variations. This corresponds to a variation of viscosity by a factor of $\sim 3-200$ corresponding to the observed variation of Q (a factor of $2-3$ at a same depth: Shito and Shibutani, 2003).
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting