Global teleseismic S wave attenuation

We have measured 140,000 teleseismic S wave spectra from 150 deep (focal depth > 200 km) earthquakes recorded at 890 broadband global and regional network stations up to 0.7 Hz. We have analyzed these data to constrain the (1) epicentral distance and (2) spatial variation of the shear wave attenuation parameter tS*. tS* increases by about 2 s between 30° and 98°. The increase in tS* is consistent (correlation coefficient of 0.9) with global QS profiles (Dziewonski and Anderson, 1981; Durek and Ekström, 1996; Lawrence and Wysession, 2006). However, there are well resolved departures in the distance dependence of tS*. Most notably, tS* is lower than PREM-predicted values between 58°-64°. This indicates a reduction in shear attenuation from the global average at ~1600 km depth beneath Central America, eastern Asia, and Alaska. These regions have previously been identified as downwelling mantle regions on the basis of seismic tomography (Grand et al., 1997), plate reconstructions (Ricard et al., 1993), and waveform analysis (Lay et al., 2004). Stations terms of tS* represent the spatial variation of attenuation in the upper mantle. Using multi-channel cross-correlation and least-squares inversion of differential spectra we resolve high attenuation in the upper mantle beneath western North America, western Europe, and eastern Africa. Attenuation is low beneath eastern North America, the Baltic regions, and central and southern Africa. This variation correlates well with global variations in heat flow (Pollack et al., 1993) and crustal age (Mooney et al., 1995).