Crustal velocity structure beneath Tehran based on teleseismic and mining explosion data recorded by Tehran City Seismic Network (TCSN)



Tehran, Iran’s capital with a population of more than 10 million is located in the southern foothills of the Alborz collision zone. The Alborz mountain belt of northern Iran results from the collision of a piece of the Gondwana with Eurasia in the late Triassic (Sengor et al., 1988). The Alborz active mountain range which consists of several sedimentary and volcanic layers, with east-west trending mountain belt 100-km wide and 600-km long, is bounded by the Talesh Mountains to the west and by the Kopet Dagh Mountains to the east (Stocklin, 1974). 5±2 mm/yr shortening and 4±2 mm/yr left-lateral strike-slip motion in the central Alborz imply slip partitioning between strike-slip and reverse faults across the Alborz (Vernant et al., 2004b). The crustal structure of the Alborz is rather poorly known. Determination of the accurate velocity model for the shallow and deep structure of the study area is useful for routine event locations, and for a precise study of seismic activities and tectonics around the Tehran area surrounded by many active faults.
The first crustal thickness variations computed from surface wave analysis of a few events by Asudeh (1982) suggested a crustal thickness of 45 km beneath the Alborz mountain range. Other crustal thickness estimations have been computed from Bouguer anomaly modeling by Dehghani and Makris (1984) for the whole of Iran. They showed that the Bouguer gravity along the Alborz mountain range varies between -100 and -120 mgal implying a crustal thickness of less than 35 km. Mangino and Priestley (1998), based on receiver function analysis showed that the crust in the southwestern and southeastern parts of the Caspian basin is 30-33 km thick and consists of a 10 km sedimentary section overlying a 15–20 km crystalline crust of Vp= 5.8 km s which in turn overlies a thin lower crust. Javan and Roberts (2003) applying the same technique on 20 teleseismic earthquakes recorded at seven stations of Iranian Long Period Array (ILPA) located to the southwest of the central Alborz estimated a crustal thickness of 46 ± 2 km for this area. They showed that the upper crust has a P-wave velocity between 4 and 5.8 km/s and a 14-km thickness, the middle crust has a positive P-wave velocity gradient from 6 to 6.4 km s down to ~30 km depth and a P-wave velocity gradient from 6.4 to 7.5 km s characterizes the lower crust in this area.
Ashtari et al., (2005) using microearthquakes recorded by a temporary local seismological network operating for several weeks around Tehran in addition to data from permanent Tehran Digital Seismic Network (TDSN), investigated the velocity model for the region around Tehran. They used arrival times of 36 well located earthquakes that were recorded during their two temporary experiments and estimated that the crust consists of a very thin layer, 2-km thick (Vp~5.4 km s ) over a 6 km thick (Vp~5.8 km s ) both associated with the sedimentary layer. Based on their results the crystalline crust consists of two layers 4 and 22 km thick with P-wave velocities of 6.0 and 6.3 km s respectively. They estimated a depth of 35 km for the Moho discontinuity beneath the study area. Recent studies of Rajaee et al. (2007) based on receiver function analysis on the one data set belonging to the seismic temporary profile show that the crust in the southern parts of the central Alborz has a thickness of about 54 and 52 km respectively. This study indicates a small increase of crustal thickness (about 5 km) toward the southern flank on the central Alborz, consistent with Bouguer anomalies of Dehgani and Makris (1984). P and S receiver functions analysis by Sodoudi et al. (2009) using data from 11 permanent stations of the Tehran Telemetry Seismic Network reveals ~51-54 km crustal and ~90 km lithosphere thickness beneath the central Alborz.
In this study the velocity structure beneath Tehran region was studied by 309 Teleseismic and 253 mining explosions data recorded in the Tehran City Seismic Network (TCSN), operated by Tehran Disaster Management and Mitigation Organization (TDMMO), from June 2004 to January 2007. Tehran City Seismic Network consists of 13 three component short period seismographs established in June 2004 in order to monitor the stress field and seismicity of Tehran city and its vicinity. Receiver function analysis of Teleseismic events which was recorded in the TCSN network shows Moho depth at 50 (km) southeast of Tehran. This Moho depth in this region reveals increasing Moho depth at the the southern border of the Alborz collision zone into the Central Iran block. Using 1D inversion of first P wave arrivals of mining activity (explosions) which were recorded in TCSN, we obtained two layers at very shallow depths with borders in 1 and 2 (km). Such shallow layers based on crustal velocity studies have not been seen in other Iranian regions like the Zagros. A better result could be achieved when the exact origin times and locations of explosions are reachable and if the explosions were fired bigger and recorded by a dense seismic profile.