Investigation of the Dorouneh fault system based on the focal mechanism of the earthquakes of the last two decades

Fault plane solution is one of the most important tools to determine the orientation of the stress field. The focal mechanism of earthquakes can be applied to determine the direction of rupture propagation, the structure of the fault and the stress field of the region. Dorouneh fault is the largest fault in Iran after the main Zagros fault with about 800 km length. The purpose of this study is to investigate the seismicity, the stress fields and the focal mechanism of earthquakes that have occurred across the three main segments western, middle and eastern parts of the Dorouneh fault. Therefore, the calculation of the focal mechanism is performed using the P-wave first-motion polarity. Also the stress situation of the events and the recognition of fault planes are presented in this research. Along this fault, the blocks have moved in both left and right directions, but certainly one of its last movements has had a right lateral motion. In this study, the waveforms recorded at the seismic stations of the National Seismological Center of Tehran University (IRSC), the National Center of Broadband Seismic Network of Iran belong to the International Institute of Earthquake Engineering and Seismology (IIEES) and seismic stations of Seismological Research Center at the Ferdowsi University of Mashhad were used. At first, the waveforms of each seismic event were combined with each other, and then the relocation of the events were determined based on new data set of this study (Figure 2). A number of high quality earthquake with a magnitude of more than 3 and an average depth of 14 km have been selected (their list is presented in table1) to calculate the fault plane solution.In order to calculate the mechanism of the recent earthquakes in the Khorasan region, interesting results have been obtained based on several methods (Assar Enayati, 1400). One of the common methods for estimating the mechanism of earthquakes, especially earthquakes with small magnitudes and at close distances from the epicenter, is the polarization of the first arrival of the P wave. Due to the dependence of the amplitude and polarization of the P wave on the focal mechanism, by determining the polarization of the first arrival of the seismic phase, the earthquake focal mechanism can be calculated. The results of focal mechanism solutions for significant events around Dorouneh fault show mostly left lateral strike slip motion which is consistent with the tectonic setting of the region. The difference in the focal mechanism of the events in the eastern and western parts of the fault is justified by the northward movement of the Lut block. The integration of data shows high accuracy in calculating the focal mechanism and more certainty about the results. Therefore, in the direction of the Dorouneh fault, the movements are both left-handed and right-handed from the seismic situation. The change in thefocal mechanism obtained from the seismic results, considering the stress axes changes, can represent the second and third order stress fields that balance the stress in this area today. The second-order stress can be related to continental rifting, isostasy adaptation, topography and deglaciation, and the third-order stress field can be related to the local stress source on a scale smaller than 100 km, which is influenced by the structural geometry, and interference between fault systems, topography and local density difference (Sheikh-ul-Islami et al., 2021).