Inhomogeneities in the Attenuation Field of Short Period Shear Waves in the Kuriles and Kamchatka, and Their Relation to Large and Great Earthquakes

Areas where large shallow earthquakes can occur in the future are identified by considering characteristics of the attenuation field of short period shear waves in the lithosphere. A total of over 360 records of earthquakes at depths of 0–33 km have been processed; these earthquakes were recorded at the PET station from two regions enclosed in the coordinates 45.0°–50.5° N and 54.0°–56.5° N (for the sake of brevity these regions will be referred to in what follows as the southern and the northern region, respectively). In addition, for comparison purposes we used records of earthquakes made at the KGB station from the area between 52° and 54° N. We used a method based on the ratio of peak amplitudes of Sn and Pn waves. It was found that the overall attenuation in the lithosphere of the northern region is much greater than that for the southern. At the same time we note that the attenuation in both of these areas is below that in northeastern Japan. The relatively lower attenuation occurs in the rupture zones of the great earthquakes occurring in 1952 (M_w = 9.0) and in 1963 (M_w = 8.6), that is, at least 60 years ago in the southern region, while the higher attenuation occurs in the rupture zones of recent events, 1997 (M_w = 7.8), 2006 (M_w = 8.3), and 2018 (M_w = 7.3). These data are consistent with the previous inferences, namely, that typical large earthquakes in subduction zones occur in earth volumes with higher concentrations of fluids in the uppermost mantle. Following large and great earthquakes, deep fluids are rising during a few decades, producing a decrease in the attenuation of the uppermost mantle. We have identified zones of high attenuation where large (M_w ≥ 7.7) earthquakes occurred long ago. We hypothesize that active precursory processes are occurring in these zones (primarily in the Avacha Bay and east of it) before large earthquakes.