عنوان مقاله [English]
Tehran, Iran’s capital with more than 10 million population is located in the southern foothills of the Alborz collision zone. The Alborz active mountain range consists of several sedimentary and volcanic layers, EW trending mountain belt 100-km wide and 600-km long, is bounded by Talesh Mountains to the West and by the Kopet Dagh Mountains to the East. 5±2 mm/yr shortening and 4±2 mm/yr left-lateral strike-slip motion in central Alborz implies a slip partitioning between strike-slip and reverse faults across Alborz. The city is bounded by active faults. Several of these faults have been mapped but their geometry at depth, their seismicity and kinematics are not precisely known. Historical earthquakes are associated with Mosha, Taleqan, Parchin and Garmsar faults, with the largest events on the Garmsar (Ms ~ 7.6) and Taleqan (Ms ~ 7.7) faults during the third andtenth centuries BC, respectively. Obtained information until now reveal that better understanding of the Alborz region needs more detailed studies in longer time intervals. Several questions about faults geometry, associated seismicity, their interactions and the mechanism of deformation in this region are remained unanswered. Considering the weak geological evidence of fault activity in some parts of Tehran, and rare calculated focal mechanisms for large earthquakes, moment tensor solution of small ones can help us with better understanding of fault behavior in this region.
Combining the data recorded by 29 local seismic and accelerograph stations, October 17, 2009 Ray Earthquake, Mw 4.3, was located in the westernmost part of the Parchin fault in the south of Bibi Shahrbanoo Mountain, 35.57° latitude, 51.51° longitude and 15 km depth in south-east edge of Tehran mega city. Using first motion data, a reverse mechanism with a small component of the strike-slip motion was determined.
Deviatoric moment tensor was inverted by using broadband data recorded by seven Iranian stations from National Seismic Network, INSN. We used ISOLA program (Sokos and Zahradnik, 2008) that is based on the multiple point-source representation and iterative deconvoloution method, similar to Kikuchi and Kanamori (1991) for teleseismic records, but here the full wavefield is considered, and Green functions are calculated by discrete wavenumber method of Bouchon (1981). Doing many tests, we selected the 0.06-0.095 Hz frequency range that resulted in the highest variance reduction. Besides, we examined the centroid-depth range between 5 and 23 km to find the best correlation. To calculate Green functions, we used the velocity model by Abbasi et. al. (2010) for the Southern flank of Alborz. Inversion with different data subsets verified the stability of the solution.
The deviatoric moment tensor inversion for this earthquake by waveform modeling shows almost a pure reverse mechanism, 97% DC component, in northwest-southeast direction along Parchin fault and a centroid depth of 11 km. It is another evidence of dominant reverse mechanism in the southern edge of the Alborz region that implies the accommodation of deformation in Alborz by the slip partitioning. The estimated seismic moment for this earthquake was 3.096e15 Newton meter resulting in a 4.3 moment magnitude using Kanamori (1977) relation.