Seismotectonic model based on comparative study of Coulomb stress change and seismic parameters of Ahar-Varzaghan earthquake doublet

On 11 August 2012, an earthquake doublet (Mw 6.4 and 6.2) occurred near the city of Ahar and Varzaghan, northwest Iran. Both events were only 6 km and 11 minutes apart, producing a surface rupture of about 12 km in length. It occurred on a so far unknown structure having sparse historical and modern seismicity in the area. According to the earthquake interaction phenomenon and stress transfer during the earthquake, the Coulomb stress changes was resolved on four different binary combination of nodal planes of the first and second mainshocks to specify “optimally oriented” nodal planes for defined orientation of the principal axes of the regional stress field and an assumed friction coefficient. Those binary nodal planes were chosen having the best fit to the most positive Coulomb failure stress changes and good correlation with the spatial distribution of the aftershocks greater than 3 for 3 months after the earthquake doublet. The results led to the values of 82°, 89°, and 164° respectively for strike, dip, and rake angles of the first earthquake source fault with right-lateral strike slip mechanism and the values of 7°, 49°, and 31° respectively for strike, dip, and rake angles of the second earthquake source fault with oblique reverse mechanism. There is also an excellent correlation between the spatial variation of b-value before the Ahar-Varzaghan earthquake and the stress increased regions along the optimally oriented source faults. The b-value parameter is actually the slope of the Gutenberg-Richter law and depends on tectonics and stress distribution in the region. Based on the Coulomb stress changes calculation, the spatial changes of the b-value before the Ahar-Varzaghan earthquake, the manifestation of the surface rupture caused by the earthquake, field observations, and the dominant trend of afterschocks revealing structural dependency of two source faults, we proposed a restraining bend structure and a complex style of deformation causing the earthquake. The proposed structural model can help to estimate the seismic hazard risk of future potential failure areas.