Lateral Variations in Upper Mantle Shear Wave Attenuation

%\def\tstar{t^*} %\def\tstarbar{\overline{\tstar}} %\def\deltstarbar{\delta\tstarbar} %\def\deg{{^o\ }} %\def\degn{{^o}} We study the lateral variations in shear wave attenuation in the upper mantle by analyzing the spectra from S and SS arrivals from selected seismograms in the IRIS FARM database between 1988 and 1999. We use seismograms from shallow earthquakes (<=50~km depth) at epicentral distances of 40\deg--80\deg~ for S waves and 80\deg--160\deg~ for SS waves. Each spectrum is the product of source, receiver, and propagation response functions as well as local source- and receiver-side effects. We correct each spectrum for the known instrument response, a source model with an ω ^-2 falloff at high frequencies, and a one-dimensional Q_β model. Since there are multiple receivers for each source and multiple sources for each receiver, we can approximate the source- and receiver-side terms by stacking the appropriate S log spectra. The resulting source-specific response functions include any remaining source spectrum and the effect of near-source attenuation in the upper mantle; the receiver stacks include the site response and near-receiver Q structure. We correct the SS log spectra for the appropriate source- and receiver-side stacks found from the S waves. Since attenuation in the lower mantle is small, the residual SS log spectrum approximates attenuation in the upper mantle near the SS bounce point, and can be used to estimate δ /line{t^*} at frequencies between 0.01~and 0.1~Hz. The resulting bounce point δ /line{t^*} measurements, which we smooth into caps of 5\deg radius, show spatially coherent patterns of more and less attenuating regions. We will compare these patterns with our previous results using the same method for higher-frequency P wave attenuation and with other studies of lateral variations in shear wave attenuation.