The 2012 August 11 Ahar earthquakes: consequences for tectonics and earthquake hazard in the Turkish–Iranian Plateau

Alex Copley,James Hollingsworth,Mohammad Faridi,Manoucher Ghorashi,Behnam Oveisi,Hamid Nazari,Morteza Talebian,James Jackson

doi:10.1093/gji/ggt379

Abstract

We have examined the faulting in the 2012 August 11 Mw 6.4 and 6.3 Ahar (NW Iran) earthquakes using a combination of field mapping, remote-sensing observations of tectonic geomorphology, the cross-correlation of optical satellite images and the inversion of seismic waveforms. The first event was close to pure strike-slip, and the second was an oblique combination of thrust and strike-slip motion. Mapped surface ruptures indicate at least one of these events accommodated mostly right-lateral strike-slip motion on an ∼E–W striking plane. The occurrence of these earthquakes highlights the spatially distributed deformation in NW Iran, which has implications for both hazard assessment (the Ahar events killed over 300 people and injured over 3000), and also tectonic models of the region. Furthermore, these earthquakes demonstrate that the tectonics of the Ahar area is characterized by strike-slip faulting and a component of shortening, and not the previously suggested extension.

Highlights

On 2012 August 11 two destructive earthquakes occurred 11 min apart near the town of Ahar in NW Iran (Mw 6.4 at 12:23 UTC and Mw 6.3 at 12:34 UTC; Figs 1 and 2), resulting in over 300 deaths and 3000 injuries
The wider tectonics of the Arabia–Eurasia collision zone at the longitude of NW Iran and E Turkey is characterized by right-lateral strike-slip faulting on WNW–ESE planes in the Turkish–Iranian Plateau, and thrust faulting with the same strike in the Greater Caucasus further north (Fig. 1a)
Field observations show that one of the 2012 August 11 twin Ahar earthquakes ruptured to the surface on an ∼E–W striking fault plane, with mostly right-lateral strike-slip motion, close to the crest of a broad topographic ridge. [A ridge top strike-slip fault was responsible for the Mw 7.3 1990 Rudbar earthquake in the western Alborz mountains of Iran, marked ‘R’ on Fig. 1 (Berberian et al 1992; Berberian & Walker 2010).] With the available information, we are not able to state definitively which of the two events caused the observed surface ruptures

Summary

Introduction

On 2012 August 11 two destructive earthquakes occurred 11 min apart near the town of Ahar in NW Iran (Mw 6.4 at 12:23 UTC and Mw 6.3 at 12:34 UTC; Figs 1 and 2), resulting in over 300 deaths and 3000 injuries. The wider tectonics of the Arabia–Eurasia collision zone at the longitude of NW Iran and E Turkey is characterized by right-lateral strike-slip faulting on WNW–ESE planes in the Turkish–Iranian Plateau, and thrust faulting with the same strike in the Greater Caucasus further north (Fig. 1a). This arrangement of faulting accommodates the oblique northwestward convergence of Arabia with Eurasia by the spatial separation (partitioning) of the faults accommodating the strike-slip and shortening components of motion (Jackson 1992; McClusky et al 2000; Copley & Jackson 2006). The Ahar earthquakes are within the zone of right-lateral strike-slip faulting in the Turkish–Iranian

Methods

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Discussion

Conclusion