Mapping lateral variations in upper mantle attenuation by stacking P and PP spectra

Abstract

We study the lateral variations in P wave attenuation in the upper mantle at frequencies between 0.16 and 0.86 Hz by analyzing the spectra from >18,000 P and >14,000 PP arrivals. We select seismograms from shallow earthquakes at epicentral distances of 40°–80° for P waves and 80°–160° for PP 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 average source and attenuation models. 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 P log spectra. The resulting source‐specific response functions include any remaining source spectrum and near‐source Q structure; the receiver stacks include the site response and near‐receiver Q structure. We correct the PP log spectra for the appropriate source‐ and receiver‐side stacks. Since attenuation in the lower mantle is small, the residual log spectrum approximates attenuation in the upper mantle near the PP bounce point and is used to estimate δ. We constrain the anomalies to the top 220 km of the mantle, as suggested by previous Q studies, and translate the δ measurements to variations in 1000/Qα. The patterns of more and less attenuating regions generally correlate with previously published shear attenuation models and surface tectonics. Continents are usually less attenuating than the global average, whereas oceanic regions tend to be more attenuating. There are interesting exceptions to this tectonic pattern, such as an attenuating region beneath southern Africa.