Mainshocks/aftershocks study of the August 2012 earthquake doublet on Ahar-Varzaghan complex fault system (NW Iran)

Abstract

Seismotectonic features of the Ahar-Varzaghan complex fault system are reviewed by the mainshocks/aftershocks study of the 2012 Ahar-Varzaghan earthquake doublet (Mw = 6.5 & Mw = 6.3). The early aftershocks were accurately monitored by a dense local seismological network. The mainshocks hypocenters were relocated using a new velocity model for the area. The regional displacement waveforms of the mainshocks and large aftershocks were jointly inverted for their moment tensors and centroids.Precisely located aftershocks are mostly distributed in three distinct clusters. These E-W trending clusters are situated to the north of the surface rupture over a ∼30 km long and 10 km wide zone, extending from the ground surface down to the depth of ∼15 km. Based on our results, the first mainshock (M1) with Mw = 6.5 nucleated 3 km east of the surface rupture at a depth of 10 km. The rupture evolved toward the west and shallower depths on an almost E-W striking right-lateral strike-slip vertical fault plane and produced ∼12 km surface rupture. Its main energy was released at about 5 km west of the hypocenter at an average depth of 5 km. About 11 min later, the second mainshock (M2) with Mw = 6.3 nucleated 5 km northwest of the M1 hypocenter and at a depth of 15 km. It occurred on an ENE-WSW striking, north-dipping (65° to 70°) fault plane with a dominant reverse mechanism and strike-slip component. The M2 related rupture also expanded to the west and to shallower depths and released most of its energy ∼5 km west of the respective hypocenter at an average depth of 11 km. The eastern aftershocks mainly show the right-lateral strike-slip mechanism on almost E-W trending fault, close to the location of the first main centroid (M1). The northern aftershocks mostly include reverse mechanisms with strike-slip component near the M2 centroid. A big gap of aftershock activity is observed close to the M1 centroid location, most likely associated with the area of maximum slip. We do not observe any N-S lineament of aftershocks neither near the M2 hypocenter and nor its centroid, thus making the suggestion of some previous investigators that M2 ruptured an N-S trending fault unlikely.