Mapping Heterogeneities of the Short-Period S-Wave Attenuation Field in the Lithosphere of Southwest Alaska

The authors have studied the characteristics of the short-period S-wave attenuation field in the lithosphere of southwest Alaska (in the Alaska subduction zone). Records of seismic station KDAK, obtained for shallow earthquakes within the distance range of ~250–750 km, were processed. We used a method that analyzes the ratio of maximum amplitudes in Sn and Pn waves (parameter Sn/Pn). The correlation dependence of this parameter on distance for wave lines crossing rupture zones of three large and great earthquakes is plotted: the Great Alaskan earthquake of March 28, 1964 (M_W = 9.2), the Simeonov earthquake of July 22, 2020 (M_W = 7.8), and Chignik earthquake of July 29, 2021 (M_W = 8.2). It was established that this dependence goes a little higher than similar dependences obtained earlier for regions of southwest Japan and central Chile, and much higher than for the region of northeast Japan. The reasons for the substantial differences of these dependences in different regions are discussed. The authors consider heterogeneities of the attenuation field in the rupture zones of the Simeonov earthquake and its largest aftershock (M_W = 7.6), as well as of the Chignik earthquake.