Abstract

The Rudbar-Tarom earthquake of 20 June 1990 in NW Persia (MS 7.7, mb 6.4, MW 7.3) killed approximately 40,000 people, left more than 500,000 homeless, destroyed three cities (Rudbar, Manjil, and Lowshan) and 700 villages, and slightly damaged another 300 villages, in the densely populated area of the western Alborz mountains, southwest of the Caspian Sea.

Both faulting and folding associated with the earthquake were observed in the epicentral area. Co-seismic surface faulting was associated with at least three main discontinuous, complex fault segments with a total length of more than 80 km. These three main fault segments are arranged in a right-stepping en-echelon pattern and are separated by gaps in the observed surface ruptures. Each segment has a strike of 95° to 120°, with oblique left-lateral and reverse motion on faults that are subvertical or have steep dips to the S or SSW. Maximum surface displacements were 60 cm horizontal (left lateral) and 95 cm vertical (south side up). Bedding-plane slip in a reverse or thrust sense was also observed at the surface following the earthquake and may have occurred during co-seismic folding. Seismic body waves of the mainshock confirm the left-lateral nature of the faulting and show that slip occurred in three main subevents that may correspond to the fault segments seen at the surface. The focal mechanism obtained from broadband body waves is almost pure strike slip, with little evidence of the vertical component of motion seen at the surface. There were numerous aftershocks, one of which had a thrust faulting mechanism, different from that of the mainshock.

The complex system of surface faults on which the Rudbar-Tarom earthquake occurred had not previously been identified as active. The lack of obvious Quaternary scarps on the faults that moved in 1990 suggests that return periods for earthquakes of this type are relatively long; but this is difficult to quantify, as erosion rates are high. Although the Persian seismic network has insufficient coverage to locate small earthquakes with enough precision to identify active faults, there is much modern and historical seismicity in the area, and abundant geomorphological evidence of rapid active tectonic processes. Previous studies of focal mechanisms and active faulting had emphasized the reverse faulting and folding that are undoubtedly the main processes that build the Alborz mountain belt. The 1990 earthquake provided the first evidence that the belt also undergoes left-lateral shear. On the southern edge of the Alborz mountains the slip direction in earthquakes is approximately NE and occurs on oblique faults. We suspect that in the high Alborz, where the amount of shortening and the topography are greater, this oblique convergence between Persia and the southern Caspian Sea is partitioned into essentially pure reverse motion and left-lateral strike-slip.

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