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The short-period S-wave attenuation field has been mapped for the lithosphere of the Altai and adjacent areas in Mongolia and southern Siberia. A total of approximately 500 earthquake seismograms were used. These were recorded at the Makanchi and Ulan-Bator stations at distances of ~300–1900 km. It is shown that the attenuation of shear waves is much stronger in the west of the area of study compared with the east. A V-shaped band of high attenuation in the upper mantle has been identified in the west of the area where the epicenters of the magnitude 6.9 1990 Zaisan and the magnitude 7.3 2003 Chuya earthquakes were situated; a similar band extends northwestward to the west of Lake Ubsu Nur. The attenuation is comparatively low in the rupture zones of large (М ≥ 7.0) pre-1970 earthquakes. It was concluded that fluid-charged zones are formed in the lithosphere prior to large earthquakes in the Altai, as well as in other areas of Central Asia. Following large seismic events, the fluids were rising into the crust from the upper mantle during a few tens of years, thus reducing the attenuation of Sn waves. We have identified zones of high attenuation with no significant earthquakes being recorded there during historical time. It is our opinion that earthquake precursory processes may be occurring in these zones. 相似文献
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Izvestiya, Atmospheric and Oceanic Physics - We consider some characteristics of seismicity in the region of Southwest Alaska. The aftershock zones of the Simeonof (July 22, 2020,... 相似文献
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Izvestiya, Atmospheric and Oceanic Physics - We consider the characteristics of seismicity before large earthquakes and the great earthquakes in the periphery of the Pacific Ocean. It is found that... 相似文献
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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. 相似文献
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On the correlation between seismicity characteristics and S-wave attenuation in the ring structures that appear before large earthquakes 总被引：1，自引：0，他引：1
We discuss seismicity characteristics in the source zones of two great earthquakes: the December 26, 2004 Sumatra (Mw = 9.0) and the November 14, 2001 Kunlun (Mw = 7.8) events. Ring structures of low magnitude seismicity have been forming prior to these earthquakes for several decades.
We studied the short period shear-wave attenuation field in the area of these ring structures. The method we used is based
on the analysis of the rate of attenuation for the early Sn and Lg codas to detect attenuation inhomogeneities in the uppermost
mantle. We show that the ring structures have comparatively high attenuation of shear waves compared with the crustal volumes
inside the rings. The fact that there is no recent volcanism in the area of the seismicity rings shows that this effect is
due to a high content of free fluids in the uppermost mantle. Proceeding by analogy with our results, we identified a zone
in northern Tien Shan that is anomalous for these parameters; the zone may be related to the precursory process of a large
earthquake. We discuss the geodynamic mechanisms that may be responsible for fluid concentration in the seismicity rings. 相似文献
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This paper addresses inhomogeneities in the short-period S-wave attenuation field in the lithosphere beneath Altai. A technique based on the analysis of the amplitude ratios of Sn and Pn waves is used. High S-wave attenuation areas are identified in the West Altai, which are related to the source zones of recent large earthquakes, viz., the 1990 Zaisan earthquake and the 2003 Chuya earthquake. Associated with the Chuya earthquake, a large ringlike seismogenic structure had been formed since 1976. It is also found that ringlike seismogenic structures are confined to high S-wave attenuation areas unrelated to large historical earthquakes. It is supposed that processes paving the way for strong earthquakes are taking place in these areas. The magnitudes of probable earthquakes are estimated using the earlier derived correlation dependences of the sizes of ringlike seismogenic structures and the threshold values of magnitudes on the energy of principal earthquakes with prevailing focal mechanisms taken into consideration. The sources of some earthquakes are likely to occur near to the planned gas pipeline route from Western Siberia to China, which should be taken into account. The relationship of anomalies in the S-wave attenuation field and the ringlike seismogenic structures to a high content of deep-seated fluids in the lithosphere is discussed. 相似文献
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