On the correlation between seismicity characteristics and S-wave attenuation in the ring structures that appear before large earthquakes |
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Authors: | Yu F Kopnichev I N Sokolova |
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Institution: | 1.Schmidt Institute of Physics of the Earth,Russian Academy of Sciences,Moscow,Russia;2.Institute of Geophysical Research,Nuclear Science Center,Alma-Ata,Republic of Kazakhstan |
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Abstract: | 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. |
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