Fault plane determination of the 2012 August 11 Ahar-Varzaghan earthquakes based on H-C method

The study area locates between an active fault system of North Anatolian in Turkey and active Alborz and Zagros mountain ranges in Iran. The total shortening concluded from Arabia-Eurasia collision with a rate of 22 mm/year across the NE Persia is partitioned into two fault systems, right-lateral strike slip movements in the Turkish-Iranian Plateau like North Tabriz fault system and thrusting in the Caucasus. In other word the study area is transferred a part of this Arabia-Eurasia northward relative motion to the Anatolia. The region of this study is one of the most active and young tectonic areas in the Middle East which has experienced devastating earthquake during past few years. One of the most important basic parameters in earthquake seismology to identify earthquake source, understands the mechanism of earthquake fault. In this study, the focal mechanism of double earthquakes with Mw 6.4 and 6.2 on 11 August 2012 (21 Mordad 1391) and aftershocks with magnitudes greater than 5 which occurred in north-west of Iran, were determined using ISOLA software and then compared with solutions reported by Harvard CMT and other agencies. In this method focal mechanism of the earthquakes are determined using full waveform modeling and centroid moment tensor inversion. In this area, earthquakes are mostly concentrated around the Tabriz fault. However, the region of this study has no significant seismic activity. Recent activity of south Ahar fault is of great importance since it has generated destructive earthquakes. In this study, a geometrical method, called H-C is used to identify fault plane. H-C method is a simple method, applicable when a reliable earthquake location and its Centroid Moment Tensor solution (CMT) are available. The CMT solution also gives two planes passing through C (plane I and plane II) defined by the strike and dip angles of the moment tensor solution. Then, assuming a planar fault, the fault plane can be identified as that one among planes I and II that encompasses the hypocenter (H-C method). The data from broad-band stations of International Institute of Earthquake Engineering and Seismology (IIEES), Azerbaijan National Seismic Network (ANSN) and Iranian Seismological Center (IRSC) were used in this study. To analyze these earthquakes, the Hypocenter location is obtained from these agencies and gathering all available data and using a proper velocity model, by using Hypocenter program. The region in this study enclosed between 45º to 48º east longitudes, and 37.5º to 39º north latitudes. We are able to achieve a higher accuracy in this method in comparison with other methods which use teleseismic data, since the local and regional seismogram data and so higher frequencies are used. The mechanism for the first shock was obtained as Strike/Dip/Rake = 85º, 89º, 165º and for the second shock was obtained as Strike/Dip/Rake = 252º, 64º, 125º. The main mechanism of the South Ahar fault is strike-slip and strike-slip with a reverse component, according to the calculated focal mechanisms. The obtained focal mechanisms show that two separate activated fault segment in this earthquake has a right lateral mechanism, first of them with a dip toward the south and second of them with a dip toward the north.