The Armenian earthquake of 1988 December 7: faulting and folding, neotectonics and palaeoseismicity

Abstract

The 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 échelon system of strike-slip faults, with a maximum offset of 50 cm. The second branch is the main one and breaks for about 8 km 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 160 cm of vertical motion and 90 cm 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 200 m 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 40 km 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 17 000 years BP and the beginning of the formation of present-day soil.