The Rudbār Mw 7.3 earthquake of 1990 June 20; seismotectonics, coseismic and geomorphic displacements, and historic earthquakes of the western ‘High-Alborz’, Iran

Abstract

The Rudbār earthquake of 1990 June 20, the first large-magnitude earthquake with 80 km left-lateral strike-slip motion in the western ‘High-Alborz’ fold-thrust mountain belt, was one of the largest, and most destructive, earthquakes to have occurred in Iran during the instrumental period. We bring together new and existing data on macroseismic effects, the rupture characteristics of the mainshock, field data, and the distribution of aftershocks, to provide a better description of the earthquake source, its surface ruptures, and active tectonic characteristics of the western ‘High-Alborz’. The Rudbār earthquake is one of three large magnitude events to have occurred in this part of the Alborz during recorded history. The damage distribution of the 1485 August 15 Upper Polrud earthquake suggests the east–west Kelishom left-lateral fault, which is situated east of the Rudbār earthquake fault, as a possible source. The 1608 April 20 Alamutrud earthquake may have occurred on the Alamutrud fault farther east. Analysis of satellite imagery suggests that total left-lateral displacements on the Rudbār fault are a maximum of ∼1 km. Apparent left-lateral river displacements of ∼200 m on the Kashachāl fault and up to ∼1.5 km of the Kelishom fault, which are situated at the eastern end of the Rudbār earthquake fault, also appear to indicate rather small cumulative displacements. Given the relatively small displacements, the presently active left-lateral strike-slip faults of the western High-Alborz fold-thrust belt, may be younger than onset of deformation within the Alborz Mountains as a whole.