CONSTRAINING UPPER-MANTLE ANELASTICITY USING SURFACE-WAVE AMPLITUDE ANOMALIES

We present an iterative method to constrain lateral variations in surf ace wave attenuation using long-period surface wave amplitude anomalie s. The method acts to isolate the anelastic signal from elastic focusi ng effects, yielding largely unbiased estimates of lateral variations in the inverse seismic quality factor, q(omega) = Q-1(omega), of the s urface wave. In the zeroth iteration, linearized ray theory motivates the construction of a reduced datum, using measurements from four cons ecutive surface wave orbits, which is insensitive to elastic heterogen eity, an operation which requires no a priori knowledge of elastic str ucture. Synthetic experiments using both ray theoretic formalisms and normal-mode calculations reveal that significant levels of elastic bia s remain in the reduced data due to deviations from linearized ray the ory. In further efforts to eliminate elastic bias, the remaining elast ic signal in each reduced datum is predicted and subtracted using accu rate forward theory and existing aspherical elastic mantle models. Thi s operation is the first iteration of a non-linear inversion in which the data at each iteration are the residuals between the observed anom alies and the anomalies predicted for a fixed phase velocity model and updated attenuation model. The zeroth and first iterations are perfor med with 1610 vertical and 790 longitudinal component seismograms from 144 events. Heterogeneity maps of Rayleigh wave attenuation deltaq(om ega, theta, phi) are retrieved for the even degrees 2, 4 and 6 of a sp herical harmonic expansion in the period range of 150-300 s (S-0(25)- S-0(60)). These surface wave attenuation maps are analogous to elastic phase velocity in their radial averaging of, and linear relationship to, intrinsic heterogeneity and are inverted for upper mantle anelasti c structure. Under the physically plausible assumption that intrinsic elastic and anelastic structure are correlated at every depth, the sur face wave attenuation maps are well explained by a source region of an elastic heterogeneity that is radially localized in the shallow mantle (100-300 km) in a region we infer to be near the solidus temperature.