The 12 November 2017 Mw 7.4 earthquake in Sarpol-e-Zahab (Iran–Iraq): a complex fault rupture in the Zagros Mountains

Abstract

On 12 November 2017, the NW–SE-trending fold-and-thrust Zagros Mountains (Iran–Iraq) border region experienced a large (Mw 7.4) and damaging earthquake. Using seismotectonic investigations coupled with InSAR and GPS data analysis, we test several scenarios of coseismic deformation associated with either a west-dipping high angle (76°) backthrust fault geometry or a gently (~ 16°) east dipping earthquake rupture. Apparently, no surface rupture was observed and with the local Iranian seismic network the mainshock is at 18 ± 2 km depth, in agreement with IPGP, GCMT, USGS GFZ and IRSC locations. Interferograms obtained from both ascending and descending Sentinel-1A–1B images show NNW-SSE-trending deformation lobes with uplift of up to 0.9 m in the WSW hanging block and subsidence of ~ 0.4 m in the ENE footwall. The thrust focal mechanisms of the mainshock being all comparable, the forward and best-fit inversion model concur with the WSW block uplift and backthrust geometry, reaching ~ 11 m coseismic slip at depth. Any forthcoming seismic hazard models for this plate boundary need to consider a segmented Zagros fold-and-thrust belt with major earthquakes on backthrust seismogenic faults.