The analysis of principal stress directions in northwestern Iran using main shocks and aftershocks in Ahar- Varzeghan region

Northwestern Iran is one of the seismically active regions with a high seismic risk in the world as confirmed by the historical background and instrumental earthquakes. The Ahar–Varzeghan double earthquakes of August 11th, 2012 with magnitudes higher than 6 are the examples of high seismic activities in this area. Our knowledge of stress state in a region is useful for a better understanding of different rupture mechanisms. The accumulation of stress is the main cause of earthquake, and its analysis is a crucial task considering the dense population of the region. The focal mechanism of an earthquake is one of the important source parameters which is practical for studying and analyzing the stress field. This parameter is affected by fault geometry and principal stress directions. One of the big advantages of the focal mechanism solutions is the ability to study the stress regime depp within the lithosphere. In this study, we determine the focal mechanisms of 15 earthquakes using the moment tensor inversion method of ISOLA program (Sokos and Zahradnik, 2008) in Ahar–Varzeghan region. This method was first proposed in order to calculate the source parameters at teleseismic distances (Kikuchi and Kanamori, 1991). It was developed later for regional and local distances by Zahradnik et al. (2005). In this method, Green’s functions are calculated by the discrete wavenumber method (Bouchon, 1981). The events used have moment magnitudes higher than 4 and encompass latitudes between 37° N and 40° N, and longitudes between 44° E and 49° E, during the period 2012–2014 for the focal mechanisms determined in this study and the period 1997–2014 for the Global Centroid Moment Tensor (GCMT) ones. We used the broadband stations of Iranian Seismological Center (IRSC), International Institute of Earthquake Engineering and Seismology (IIEES) and also stations of several other countries bordering northwestern of Iran. The focal mechanisms determined are often strike-slip and reverse which show a correspondence with the tectonic of this region. Then we analyze the stress state using these focal mechanisms and also by the focal mechanisms of other large and moderate earthquakes determined by GCMT in the region. We calculate the principal orientations of stresses by multiple inverse method that was originally proposed by Yamaji (2000). The method is a numerical technique to separate stresses from heterogeneous fault slip and focal mechanism data. Using the information of the acquired and GCMT focal mechanisms containing strike, dip and rake angles for the main shocks, we study the state of stress in the northwest of Iran. The result shows the average stress model with 1σ and 3σ equal to 141 and 50.2 degrees with a stress ratio of 0.6 in the region between Urmia Lake and Talesh in the northwest of Iran. This stress ratio shows that most of the motions are strike-slip. Using focal mechanisms of aftershocks, the same values are respectively 132.5, 42.4 degrees and 0.3 in Ahar–Varzeghan region. The value 0.3 shows that the motions are reverse in this part of the northwest of Iran. The small difference between these values in the northwest of Iran and Ahar–Varzeghan region shows that the values of the acquired principal stress directions using aftershocks are close to those of the main shocks. The faults directions of right-lateral strike-slip motion are in accordance with the stress direction determined.