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Abstract

We study characteristics of short-period shear wave attenuation field in the lithosphere to pick out areas of possible preparation for large and great shallow earthquakes. We have processed more than 360 seismograms of shallow events recorded by station PET from two areas limited by coordinates of 45.0°-50.5°N and 54.0°-56.5°N, respectively. We used method based on an analysis of Sn and Pn maximum amplitude ratio. We find that attenuation in the lithosphere of the northern area is generally much higher than in the southern one. At the same time attenuation in both areas is weaker than in the region of north-eastern Japan. Relatively lower attenuation corresponds to rupture zones of the great earthquakes of 1952(Mw=9.0) and 1963(Mw=8.6) occurred in the southern area more than 50 years ago. Higher attenuation is observed in the rupture zones of the recent events dated 1997(Mw=7.8), 2006(Mw=8.3) and 2018(Mw=7.3). The obtained data are in agreement with earlier conclusions stating that typical large subduction type earthquakes occur in the areas characterized by higher fluid content in the uppermost mantle; and large and great earthquakes are followed by deep fluids ascent during a few decades, which leads to attenuation decrease in the uppermost mantle. We also pick out the high attenuation zones where no large and great earthquakes (Mw≥7.8) have occurred for quite a long time. We suggest that active processes of preparation for large earthquakes can be observed in these zones (first of all in the area of the Avacha Bay and to the east of it).