Analysis of S-Wave Attenuation in South-Central Alaska

Abstract

Abstract The S-wave attenuation of the crust and upper mantle in south-central Alaska is determined from shear-wave spectral data of 60 earthquakes with magni-tudes 3.0–6.3 recorded by a network of 22 strong motion stations in Anchorage. The direct S-wave attenuation ( ) and coda attenuation ( ) were estimated by the1 1Q Qb c generalized inversion and coda decay methods, respectively. The scattering and in-trinsic attenuation coefficients, gs and gi, were determined by applying a multiple scattering model to the variation of integrated spectral energy with hypocentral dis-tance for three consecutive lapse time windows (0–15, 15–30, 30–45 sec), starting from the onset of the S-wave arrival. In order to obtain the dependence of attenuation coefficients on frequency, the spectral energies over an octave bandwidth for three central frequencies, 1.0, 3.0, and 6.0 Hz, were calculated. The results show the fol-lowing: (1) is higher than for frequencies (f) between 0.6 and 3 Hz and1 1Q Qb c less for f 3.0 Hz; (2) for 1.0 Hz f 6.0 Hz, gi increases and gs decreases gradually with f; (3) the seismic albedo, B0 gs/(gs gi), of the medium decreases from 0.48 to 0.15 with increasing frequency for 1.0 Hz f 6.0 Hz; (4) the total attenuation coefficient, gi gs, is nearly frequency independent; (5) at low1Le frequencies (1.0 Hz) the scattering attenuation ( k1gs, k is the wavenum-1Qs ber) is comparable to the intrinsic attenuation ( k1gi), while for high fre-1Qi quencies (f 3.0 Hz) dominates the attenuation process compared to; and1 1Q Qi s (6) at 1.0 Hz, lies in between and total attenuation, , but is closer to1 1 1Q Q Qc i t for frequencies higher than 1.0 Hz.1Qt