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

We measure differential attenuation between sS-S and sScS-ScS phase pairs to characterize the variation of attenuation with depth in the upper mantle of five inactive back arc basins: the Kuril Basin, Sea of Japan, Banda Sea, the Celebes and Sulu Seas, and the Shikoku Basin. A spectral ratio technique is used to measure the differential attenuation operator of the transversely polarized waveforms over a frequency band of 10 to 83 mHz. Two algorithms are employed to compute the vertically averaged attenuation structure: a spectral stacking procedure and a least squares inversion. In the spectral stacking method, the individual 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 the slope of the log-amplitude spectrum. A model for the depth dependent Q structure is then calculated from the deltat* measurements assuming 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 estimates. The Q results obtained from the stacking and inversion methods are generally in good agreement. The Q structures for the various back arc regions are similar to each other within the uncertainties of the derived Q models. In addition to computing Q models for each individual region, we partitioned the data into three tectonic provinces based upon bounce point locations. The resulting average radial Q(beta) structure for an inactive basin shows a Q of 54 in the uppermost mantle, 115 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 inactive back arc basins are lower than the global averages derived from normal modes but are generally consistent with previous body and surface wave studies of other young oceanic regions. The most striking feature of this study is the observation of very strong attenuation concentrated at shallow depths (<160 km) in the upper mantle beneath these basins.