S U M M A R YThe Spitak earthquake of 1988 December 7 is the first well-documented event directly associated with surface breaks in the Transcaucasian regions of the USSR. The earthquake was located within the ESE-WNW oriented Pambak-Sevan thrust and fold zone corresponding to the southern front of the Lesser Caucasus. The mechanism of the earthquake is consistent with the nearly NS compressive tectonics due to the active continental collision between the Arabian block and the Russian Platform. The rupture is composed by several branches, two of which reach the surface. The first branch, oriented N140", begins near the village of Alavar, has a length of 11 km and disappears at about 4 km SE of Spitak. It consists of a right lateral en kchelon system of strike-slip faults, with a maximum offset of 50cm. The second branch is the main one and breaks for about 8 k m between Spitak and Gekhasar, with a general orientation N120", showing reverse faulting dipping to the north with a right lateral component. Surface ruptures between Gekhasar and Spitak show either a narrow band of pressure ridges in alluvial deposits and soil, or a fault scarp in the bed rocks when soil is absent. Maximum displacements, observed between Spitak and Gekhasar, attain 160cm of vertical motion and 90cm of horizontal dextral offset. The displacement also varies within this branch, showing finer segmentation. Few secondary deformations are observed: some normal faults near Gekhasar correspond to the collapse of the uplifted block. An anticline fold, oriented parallel to the fault scarp and situated along its northwestern prolongation, emphasizes the regional compressive tectonics. Landslides were activated on its flanks. A 200m long reverse fault break observed along the hinge of a secondary fold suggests the occurrence of a blind thrust at depth and that the hidden branches, which continue the main central segment towards the NW, are associated to surface folding. Therefore, the total length of the fault and fold zone is about 40km in agreement with the rupture length and seismic moment obtained on the basis of surface wave modelling. Well-developed uplifted terraces in the'northern block and subsiding valleys in the southern block indicate past Quaternary activity in the fault region. Palaeoseismological evidence of ancient earthquakes has been recognized in trenches across the Spitak fault. One old event occurred between 17000 years BP and the beginning of the formation of present-day soil.

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Abstract:In the regions of high seismic activity, investigations of fault zones are of paramount importance as such zones can generate seismicity. A top task in the regional studies is determining the rates of activity from the data obtained by geoelectrical methods, especially considering the data on the faults covered by sediments. From a practical standpoint, the results of these studies are important for seismic zoning and forecasting of natural and anthropogenic geodynamic phenomena that may potentially occur in the populated areas and zones allocated for construction of industrial and civil objects, pipelines, roads, bridges, etc. Seismic activity in Gorny Altai is regularly monitored after the destructive 2003 Chuya earthquake (M=7.3) by the non-stationary electromagnetic sounding with galvanic and inductive sources of three modifications. From the long-term measurements that started in 2007 and continue in the present, electrical resistivity and electrical anisotropy are determined. Our study aimed to estimate the variations of these electrophysical parameters in the zone influenced by the fault, consider the intensity of the variations in comparison with seismicity indicators, and attempt at determining the degree of activity of the faults. Based on the results of our research, we propose a technique for measuring and interpreting the data sets obtained by a complex of nonstationary sounding modifications. The technique ensures a more precise evaluation of the electrophysical parameters. It is concluded that the electric anisotropy coefficient can be effectively used to characterize the current seismicity, and its maximum variations, being observed in the zone influenced by the fault, are characteristic of the fault activity. The use of two electrophysical parameters enhances the informativeness of the study.