Finite fault slip models for the 11 August 2012 Varzaghan-Ahar, NW Iran earthquakes (Mw 6.4 and 6.3) from near-field GPS measurements of coseismic offsets

Abstract

We use coseismic offsets, derived from continuous GPS measurements at six nearby sites, due to the twin Varzaghan-Ahar earthquakes (northwest Iran) of 11 August 2012 (Mw 6.4 and 6.3) to constrain slip distribution on the ruptures of two earthquakes. We assume that slip during the two earthquakes occurred on conjugate faults. Majority of the slip occurred during the first earthquake which involved dextral slip on the east west trending vertical plane. The earthquake involved slip at shallow depth which reached up to ∼0.9 m at the surface, consistent with the evidence of surface rupture and the observed offsets. During the second earthquake oblique slip occurred on a north–south rupture having steep dip towards east and extending towards north from the eastern edge of the first earthquake. Maximum slip of ∼0.4 m occurred at depth of ∼6–8 km. Derived moment magnitude of the first earthquake (6.45) from our analysis is consistent with the reported moment magnitude, while that of the second earthquake (6.1) is slightly underestimated from our analysis.