Analysis of S-Wave Attenuation in South-Central Alaska

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 magnitudes 3.0–6.3 recorded by a network of 22 strong motion stations in Anchorage. The direct S -wave attenuation ( \(Q_{{\beta}}^{-1}\) ) and coda attenuation ( \(Q_{\mathrm{c}}^{-1}\) ) were estimated by the generalized inversion and coda decay methods, respectively. The scattering and intrinsic attenuation coefficients, η s and η i , were determined by applying a multiple scattering model to the variation of integrated spectral energy with hypocentral distance 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 following: (1) \(Q_{{\beta}}^{-1}\) is higher than \(Q_{\mathrm{c}}^{-1}\) for frequencies ( f ) between 0.6 and 3 Hz and less for f > 3.0 Hz; (2) for 1.0 Hz ≤ f ≤ 6.0 Hz, η i increases and η s decreases gradually with f; (3) the seismic albedo, B 0 = η s /(η s + η i ), 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, \(L_{\mathrm{e}}^{-1}={\eta}_{\mathrm{i}}+{\eta}_{\mathrm{s}}\) , is nearly frequency independent; (5) at low frequencies (∼1.0 Hz) the scattering attenuation ( \(Q_{\mathrm{s}}^{-1}=k^{-1}{\eta}_{\mathrm{s}}\) , k is the wavenumber) is comparable to the intrinsic attenuation ( \(Q_{\mathrm{i}}^{-1}=k^{-1}{\eta}_{\mathrm{i}}\) ), while for high frequencies ( f ≥ 3.0 Hz) \(Q_{\mathrm{i}}^{-1}\) dominates the attenuation process compared to \(Q_{\mathrm{s}}^{-1}\) ; and (6) at 1.0 Hz, \(Q_{\mathrm{c}}^{-1}\) lies in between \(Q_{\mathrm{i}}^{-1}\) and total attenuation, \(Q_{\mathrm{t}}^{-1}\) , but is closer to \(Q_{\mathrm{t}}^{-1}\) for frequencies higher than 1.0 Hz.