Geophysical Journal | 2009 volume 31 1

Deep structure of the central part of the Crimean seismogenic zone and its velocity characteristics as a prognostic feature

© Gobarenko V.S.

122 earthquakes of energy classes 5-11,4 occurred in the vicinity of the Crimea coast in the zone with coordinates φ=44,45÷44,56 N, λ=34,3÷34,6 E during five months of 1984. Study of travel times of P- and S-waves from these earthquakes recorded at the Crimea stations shows that travel time residuals calculated for a reference velocity-depth section are of opposite signs at Alushta and Yalta, and their values achieve ±0,8 s from sources in the southern part of the region. Tomography inversion based on the Backus-Gilbert method extended to 3D case was applied to the travel time data. P- and S- wave velocities as well as the VP/VS ratio and the Poisson coefficient were mapped at the depths of 17, 19 and 21 km. Analysis of the maps shows that the southern part of the area is characterized by low both P- and S-wave velocities, whereas in the northern direction a high velocity body is revealed. However reduction of P-wave velocity in the southern part is much larger than that of S-wave velocity that results in anomalously low values of the VP/VS ratio (1,4-1,6) and the Poisson coefficient (σ=0,15÷0,5). The tomography images of the area under study at the depth of 19 km obtained in the other time intervals prior to (1975-1983) and after (1985-1990, 1991-2002) the above-mentioned set of earthquakes in 1984 have no anomalies of the velocities, VP/VS ratio and σ, and the velocity values at the three maps are practically identical. Thus we may conclude that the anomalies of velocities, VP/VS and σ are characteristics of the medium and tectonic processes in the region of earthquake origination and may be regarded as a prognostic phenomenon.

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