HR: 08:15h
AN: S21A-02 INVITED     [PDF]
TI: Compressional-Wave Studies on the Frequency Dependence of and Lateral Variations in Mantle Attenuation
AU: * Warren, L M
EM: warren@igpp.ucsd.edu
AF: IGPP, SIO, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Shearer, P M
EM: pshearer@ucsd.edu
AF: IGPP, SIO, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AB: We study the frequency dependence of and lateral variations in $P$ wave attenuation in the mantle by analyzing the spectra from $>$18,000 $P$ and $>$14,000 $P\!P$ arrivals. We select seismograms from large, shallow earthquakes at epicentral distances of 40{{$^\circ$}}--80{{$^\circ$\ }}for $P$ waves and 80{{$^\circ$}}--160{{$^\circ$\ }}for $P\!P$ waves and compute the spectrum for a 12.8-s-long window around each arrival. Each spectrum is the product of source, receiver, and propagation response functions as well as local source- and receiver-side effects, and we use a stacking procedure to isolate the propagation effects. Using separate absorption bands in the upper and lower mantles, we model the average depth and frequency dependence of mantle $Q$ by combining measurements of the amplitude decay of the propagation log spectra between 0.16~and 0.86~Hz with long-period $Q_{\beta}$ values of other workers. We find that the upper mantle is more attenuating than the lower mantle and that this contrast is greater at higher frequencies. At 1~Hz, the top 220~km of the mantle is $\sim$6 times more attenuating than the lower mantle. In addition, our results indicate that the upper corner frequency of the absorption band is higher for the upper mantle than at greater depths; the lower layer is about twice as attenuating at 0.1~Hz than at 1~Hz, whereas upper mantle attenuation is relatively constant across this band. Since lower-mantle attenuation is small, we interpret deviations in spectral decay as lateral variations in upper mantle attenuation. The resulting map of more and less attenuating regions generally correlates with previously-published attenuation models and surface tectonics. Continents are usually less attenuating than the global average, whereas oceanic regions tend to be more attenuating.
DE: 5144 Wave attenuation
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
SC: Seismology [S]
MN: 2003 Fall Meeting