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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 (Mw = 9.0) and in 1963 (Mw = 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 (Mw = 7.8), 2006 (Mw = 8.3), and 2018 (Mw = 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 (Mw ≥ 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. [ABSTRACT FROM AUTHOR]

The article considers the characteristics of the short-period S-wave attenuation field in the lithosphere of Southwestern Japan. Records of local shallow earthquakes, obtained by station MAJO at distances of ~250–900 km, were processed. We used a method that analyzes the ratio of the maximum amplitudes of Sn- and Pn-waves (Sn/Pn parameter). The dependence of this parameter on distance was plotted. We compared this dependence with analogous dependences, obtained earlier in Northeastern Japan and in the rupture zone of the great Maule earthquake (Central Chile, February 27, 2010, Mw = 8.8). The reasons for the substantial differences of these dependences in different regions are discussed. The study considers the correlation of the Sn/Pn parameter values, recorded near the rupture zones of large earthquakes in Southwestern Japan (Mw ≥ 7.0) with the passage of time after the corresponding events. Particular attention is paid to the apportionment of high attenuation zones in areas where large earthquakes that have occurred since 1900 are not known. By analogy with earlier data, it is suggested that preparation for a large seismic event can occur in such a zone. [ABSTRACT FROM AUTHOR]

We have mapped a short-period S-wave attenuation field of in the lithosphere of the Caucasus. We have used a method based on the analysis of the ratio of the maximum amplitudes for Sn- and Pn-waves. We have processed more than 200 earthquakes records, recorded by the Kislovodsk station (KIV) at distances of ~200–750 km. It is shown that, in general, the highest attenuation of shear waves is observed in the western part of the region. It is established that increased and intermediate attenuation takes place for paths simultaneously crossing rupture zones of the 1988 Spitak (Mw = 6.8) and 1991 Racha (Mw = 7.0) earthquakes. At the same time, the source zone of the 1970 Dagestan earthquake with Mw = 6.8 is characterized by mainly lower attenuation. These data are consistent with an earlier conclusion that deep-seated fluids have been ascending into the Earth's crust from the uppermost mantle for a few decades after large seismic events, and this process leads to a decrease in attenuation of Sn-waves. A high attenuation zone has been identified in the western part of the region, where, according to instrumental and historical data, no seismic events with M ≥ 6.0 have been recorded. It is assumed that processes of a large earthquake preparation may be taking place here. [ABSTRACT FROM AUTHOR]

Abstract: This paper is concerned with space-time variations in the attenuation of shear waves in the Hindu Kush area. We examined the ratios of maximum amplitudes in S and P waves (the S/P parameter) obtained from records of deep-focus earthquakes made at the AAK station in 1993-2016 at epicentral distances of ~700-800 km. The dependence of the amplitude on the radiation pattern for S and P waves was taken into account by averaging the S/P parameter over different time spans. Substantial space-time variations in S wave attenuation was found in different depth ranges in the zone of deep-focus seismicity. We show that the lowest attenuation in 2013-2015 prior to the great earthquake of October 26, 2015 (Mw = 7.5, h = 231 km) was observed for hypocenters above the rupture zone, at depths of 151-210 km, while the highest attenuation occurred at depths of 231-270 km. Following the earthquake, the attenuation rapidly decreased at depths of 231-270 km and increased in the depth range of 191-230 km. These effects are hypothesized to have been caused by dehydration of mantle rocks, as well as by migration of deep-seated fluids. [ABSTRACT FROM AUTHOR]

The characteristics of the short-period shear wave attenuation field in the lithosphere of the Turanian Plate, West Tien Shan, Pamir, and Hindu Kush have been studied. The method based on analysis of the logarithm of the ratio between maximal amplitudes of Sn and Pn waves ( Sn/ Pn parameter) has been applied. More than 400 records of earthquakes, obtained at distances of ∼400-1000 km from the AAK digital station, have been processed. It has been found that relatively weak attenuation is observed in the regions of the West Tien Shan and Pamir. The largest area of strong attenuation is located in the region of the Afghan-Tadjik Depression adjacent to Hindu Kush. A wide band of low Sn/ Pn parameter values, stretched northeastwards, has been distinguished. Along with the analogous band of strong attenuation, distinguished before in the regions of Central Tien Shan and Dzungaria, it is the continuation of the largest Chaman Fault, which stretches 850 km along the boundary of the Indian Plate. Source zones of strong earthquakes with M ≥ 7.0 that occurred in the first half of 20th century correspond to relatively weak attenuation. Areas of high attenuation, where strong seismic events have not occurred for the last 110 years, are outlined. Analogously to other seismoactive regions, it is supposed that these areas are related to preparation of strong earthquakes. [ABSTRACT FROM AUTHOR]

The article explores the heterogeneous factors in the absorption field of the transversal waves or short-period S waves in the lithosphere of Tien Shan and Dzungaria. It says that the absorption field has been examined with the use of a mapping method which was based on the analysis of the ratio between maximal values of the amplitudes of the waves. It also mentions that records of earthquakes from the MKAR station were processed. Moreover, it states that the upper mantle in the East Tien Shan area has a higher S wave absorption than in the West Tien Shan area. It concludes that the mapping method has exposed the features of the S code in records of local earthquakes. Meanwhile, constant monitoring of geophysical and geochemical parameters in the areas with marked anomalies is suggested.