Analysis of interaction phenomenon between 11 August 2012 Ahar-Varzaghan double earthquakes and the spatial distribution of aftershocks

During the past decade, studies related to earthquake forecasting and assessment of seismic hazard have been focused on stress transfer and fault interactions. According to the earthquake interaction phenomenon, occurrence of any earthquake alters the stress state (the shear and normal stress) on its neighboring faults which can delay (decrease), or trigger (increase) subsequent events (Stein, 1999). In recent years, one of the models which have been widely used to estimate coseismic stress perturbations has been the static Coulomb stress changes. These calculations are done based on Okada’s code with assumption of a shear modulus of 3.2 × 105 bars and Poisson’s ratio of 0.25 using the program Coulomb 3.3 (Toda et al. 2005, Lin and Stein, 2004). The aim of this research is to explore the fault interaction through static stress transfer between Ahar-Varzaghan double earthquakes and the possible stress triggering relationships between these main shocks and their aftershocks. These double earthquakes occurred on August 11, 2012, near the cities of Ahar and Varzaghan in the East- Azerbaijan Province in the northwest of Iran. The first event with a magnitude of Mw 6.5 occurred at 16:53 local time, and the second one with Mw 6.3 took place about 10 minutes later. These earthquakes killed more than 306 people and a large number of people were injured. Ahar-Varzaghan double earthquakes followed by many aftershocks the largest of which occurred with a magnitude of MN 5.4. These double earthquakes occurred in places where no active faults have been identified, but there are numerous active faults in their surrounding area such as the North Tabriz Fault, Bozquosh Fault, and Ahar Fault. In order to investigate the fault interactions between Ahar-Varzaghan double earthquakes, Coulomb stress perturbations due to slip on the first source fault were calculated on a specified oriented receiver fault parallel to the second main shock. Receiver faults were planes with a specified strike, dip and rake, upon which the stress changes caused by source faults were resolved (Toda et al. 2005, Lin and Stein, 2004). Calculations of Coulomb stress changes indicated that the second earthquake occurred when the Coulomb stress was increased by the first event. Hence, the second event of Ahar-Varzaghan double earthquakes appears to have been triggered by an increase in the static Coulomb stress transferred by the first event. This means that the positive stress changes caused by the first source fault have promoted the failure on the second fault. In order to examine the triggering relationship between the aftershocks and the main shocks, the stress field due to Ahar-Varzaghan double earthquakes were calculated along two kinds of receiver faults including a specified oriented receiver fault and an optimally oriented strike - slip receiver faults (OOPs). The optimal receiver fault orientation is defined by the orientation of the principal axes of the regional stress field. The analysis shows that there is a good correlation between the spatial distribution of the aftershocks and the stress increased in the regions along the specified orientation receiver fault. Therefore, the aftershocks took place in response to the coseismic stress caused by the occurrence of Ahar-Varzaghan double main shocks. Hence, the stress-enhanced regions on this type of receiver fault can be introduced as the most likely site of the next earthquakes.