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Kopnichev Y.F.,Russian Academy of Sciences | Sokolova I.N.,Institute of Geophysical Research
Journal of Volcanology and Seismology | Year: 2010

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. © 2010 Pleiades Publishing, Ltd. Source


Kopnichev Yu.F.,RAS Institute for Nuclear Research | Sokolova I.N.,Institute of Geophysical Research
Doklady Earth Sciences | Year: 2012

The shear wave attenuation field in the lithosphere of Eastern Tien Shan has been mapped. The method based on analysis of the ratio between amplitudes of Sn and Pn waves was used. On aggregate, about 120 seismograms made at the Makanchi station (MKAR), mainly in the period of 2003-2009, at epicentral distances of about 350-1200 km were analyzed. It was found that shear wave attenuation in the lithosphere of Eastern Tien Shan is weaker than that in the region of Central Tien Shan. This agrees with the fact that the rate of deformation of the Earth's crust in Eastern Tien Shan is lower (based on GPS data), as is the seismicity level, in comparison to Central Tien Shan. The zones of high attenuation, where strong earthquakes with M > 7.0 have not occurred for the last 200 years, have been identified: first of all, these are the area west of Urumqi and that of the Lop Nur test site. It is suggested that in the first zone, where an annular seismicity structure has formed over the last 30 years, a strong earthquake may be being prepared. The second zone is most probably related to the uplift of mantle fluids resulting from a long-term intensive technogenic effect, analogous to what has occurred in areas of other nuclear test sites (Nevada and Semipalatinsk). © Pleiades Publishing, Ltd., 2012. Source


Kopnichev Y.F.,Russian Academy of Sciences | Sokolova I.N.,Institute of Geophysical Research
Journal of Volcanology and Seismology | Year: 2014

It is shown that episodes of comparative seismic quiescence that lasted about 20–25 years in the areas of study alternated with intervals of sharply increased seismicity as series of large (M ≥ 6.9) earthquakes occurred during two to three decades. Since no M ≥ 6.6 earthquake has occurred in the area for as long as 21 years after the 1992 Susamyr event, middle-term prediction would require identification of zones of imminent large earthquakes. More reliable identification of such zones rests on data relating to inhomogeneities in the field of S-wave attenuation in the lithosphere, as well as on the characteristics of ring structures of seismicity. Such structures are formed as zones of seismic quiescence that are bounded by M ≳ Mth earthquake epicenters, where Mth is the threshold magnitude value. Correlative relationships were previously derived, lgL(Mw) and Mth(Mw), for events with different focal mechanisms (L is the length of the longer axis of a seismicity ring and Mw is the magnitude of the associated large earthquake). These relationships were used to estimate the Mw of large events that can occur in these ring structures. The greatest earthquake with Mw ≳ 7.5 is probably about to occur in southern Tien Shan, east of the 1949 Khait earthquake rupture. A smaller event (Mw ∼ 7.0) can occur in the Kyrgyz Range area. Still smaller earthquakes probably have their precursory areas north and east of Lake Issyk-Kul, as well as in Dzungaria. © 2014, Pleiades Publishing, Ltd. Source


Kopnichev Y.F.,Russian Academy of Sciences | Sokolova I.N.,Institute of Geophysical Research | Sokolov K.N.,Saint Petersburg State University
Izvestiya, Physics of the Solid Earth | Year: 2013

The characteristics of the attenuation field of short-period shear waves in the region of Nevada nuclear test site (NNTS) are studied. The seismograms of underground nuclear explosions (UNEs) and earthquakes recorded by three seismic stations in 1975-2012 at the epicentral distances of up to 1000 km are processed by the methods based on the analysis of the amplitude ratios of Sn to Pn and Lg to Pg waves, as well as the S-coda envelopes for close events. It is shown that the structure of the attenuation field in the Earth's crust and upper mantle in the NNTS region experienced significant temporal variations during the interval of nuclear operations. The strongest variations were associated with UNEs conducted in the Pahute Mesa area, which held about two-thirds of the most intense explosions. Our data indicate that temporal variations in the structure of the attenuation field are related to the migration of deep fluids. A comparison of the general characteristics of the attenuation field in the regions of the three large nuclear test sites is presented. © 2013 Pleiades Publishing, Ltd. Source


Kopnichev Y.F.,Russian Academy of Sciences | Sokolova I.N.,Institute of Geophysical Research
Doklady Earth Sciences | Year: 2011

The characteristics of seismicity prior to the series of eight very strong earthquakes (M w = 7.0-9.0) in Northeast Japan are discussed. Ring seismicity structures that appeared prior to all eight events in two depth ranges of 0-33 and 34-70 km are identified. The epicenters of the main shocks were located near areas of crossing or touching of shallow and deep rings. It was shown that the sizes of shallow rings and threshold magnitudes corresponding to seismicity rings grow with the energy of the main shocks. It was noted that the prognosis with respect to the place and magnitude of the catastrophic earthquake on March 11, 2011, had been made before it based on the data obtained prior to July 1, 2009. Use of the new data obtained prior to March 10, 2011, enabled us to specify this prognosis significantly. We obtained correlation dependences of threshold magnitudes on the energy of the main shocks (with a high correlation coefficients). It was shown that the duration of the period for seismicity rings to emerge in the considered region nearly did not depend on magnitude. The nature of annular structures and the possibility of application of their parameters for prognosis of strong earthquakes were discussed. © 2011 Pleiades Publishing, Ltd. Source

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