RADIAL UPPER-MANTLE ATTENUATION STRUCTURE OF INACTIVE BACK-ARC BASINSFROM DIFFERENTIAL SHEAR-WAVE MEASUREMENTS

We measure differential attenuation between sS-S and sScS-ScS phase pa irs to characterize the variation of attenuation with depth in the upp er mantle of five inactive back arc basins: the Kuril Basin, Sea of Ja pan, Banda Sea, the Celebes and Sulu Seas, and the Shikoku Basin. A sp ectral ratio technique is used to measure the differential attenuation operator of the transversely polarized waveforms over a frequency ban d of 10 to 83 mHz. Two algorithms are employed to compute the vertical ly averaged attenuation structure: a spectral stacking procedure and a least squares inversion. In the spectral stacking method, the individ ual spectra are corrected for the elastic structure at the sS or sScS bounce point, and the differential attenuation operator is computed by spectral division. The attenuation operators are then normalized and stacked by source depth to obtain more stable spectra, and an average deltat for sources within a restricted depth range is obtained from t he slope of the log-amplitude spectrum. A model for the depth dependen t Q structure is then calculated from the deltat measurements assumin g Q is frequency independent. Alternatively, deltat measurements for individual phase pairs are made using a similar technique and analyzed by ray tracing and a least squares inversion to obtain the Q-1 estima tes. The Q results obtained from the stacking and inversion methods ar e generally in good agreement. The Q structures for the various back a rc regions are similar to each other within the uncertainties of the d erived Q models. In addition to computing Q models for each individual region, we partitioned the data into three tectonic provinces based u pon bounce point locations. The resulting average radial Q(beta) struc ture for an inactive basin shows a Q of 54 in the uppermost mantle, 11 5 at intermediate depths, and 173 in the transition zone; we also find a low Q zone of 36 beneath active island arcs. The Q values for inact ive back arc basins are lower than the global averages derived from no rmal modes but are generally consistent with previous body and surface wave studies of other young oceanic regions. The most striking featur e of this study is the observation of very strong attenuation concentr ated at shallow depths (<160 km) in the upper mantle beneath these bas ins.