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Mapping Lateral Variations in Upper Mantle Attenuation



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JOURNAL OF GEOPHYSICAL RESEARCH VOL 107 NO B12 2342 doi 10 1029 2001JB001195 2002 Mapping lateral variations in upper mantle attenuation by stacking P and PP spectra Linda M Warren and Peter M Shearer Cecil H and Ida M Green Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography University of California San Diego La Jolla California USA Received 12 September 2001 revised 10 April 2002 accepted 7 May 2002 published 12 December 2002 1 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 sourceand 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 dt We constrain the anomalies to the top 220 km of the mantle as suggested by previous Q studies and translate the dt measurements to variations in 1000 Qa 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



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