High S-Wave Attenuation Anomalies and Ringlike Seismogenic Structures in the Lithosphere beneath Altai: Possible Precursors of Large Earthquakes

摘要

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.