The 2010–2011 South Rigan (Baluchestan) earthquake sequence and its implications for distributed deformation and earthquake hazard in southeast Iran

Abstract

We investigate the source processes and tectonic significance of two earthquakes that occurred on 2010 December 20 (Mw 6.5) and 2011 January 27 (Mw 6.2) within a desert region south of the town of Rigan, SE Iran. The two earthquakes, which we refer to as the South Rigan events, occurred close to one another at the northern margin of the Shahsavaran mountains: a mainly volcanic chain in which the potential for active faulting has not previously been considered in detail. Surface displacements mapped using SAR interferometry, multiple-event relocation analysis of epicentres, body-waveform modelling and field measurements of surface rupture together reveal that the 2010 December 20 earthquake involved an average of ∼1.3 m right-lateral slip on a vertical fault trending ∼210° whereas the 2011 January 27 resulted from ∼0.6 m of slip on a conjugate left-lateral fault striking ∼310°, parallel to the trend of the Shahsavaran mountains and confined within a zone of increased Coulomb stress from the earlier main shock. The main slip for the 2010 and 2011 main shocks failed to reach the surface though minor cracks and en-echelon fissures were mapped following both events. Some of the surface cracks may have been enhanced during a period of minor afterslip in the days following the 2010 main shock. Using the insights gained from our investigation of the two South Rigan earthquakes we perform a regional reconnaissance of the active faulting using SPOT5 (2.5 m) satellite imagery. We show that distributed ∼N–S right-lateral faulting is widely distributed north of the Shahsavaran mountains. We also show evidence for left-lateral strike-slip faulting parallel to the Shahsavaran mountains, with a component of extension in the east and shortening in the west, which is likely to accommodate regional N–S right-lateral shearing by clockwise rotation about a vertical axis. The distributed strike-slip faulting is closely associated with the distribution of towns and villages and constitutes a continuing hazard to local populations.