Determination of 3D seismic wave velocity in Zagros collision zone

The Zagros orogenic belt was formed approximately 12 million years ago due to the convergence between the Arabian and Eurasian plates upon the closing of the Neo-Tethys Ocean. The Zagros is categorized as one of the youngest such settings on Earth, at an early stage of this collision. Many geophysical multiscale studies have been performed in the Zagros region based on different seismic and non-seismic data. Based on these studies, it can be concluded that the Zagros thrust belt has a crustal thickness of 45 ± 3 km, whereas beneath the Sanandaj-Sirjan zone, the Moho depth significantly increases up to 65 3 ± km. Among the many geophysical studies of Zagros and surrounding areas, local earthquake tomography (LET), which uses travel time data of both stations and earthquakes located in the study area, has never been performed for the entire Zagros. In this research, a 3D velocity model of body waves has been extracted using the information of the arrival time of 7783 earthquakes in the period of 2006 to 2018, recorded in the National Seismological Center and the broadband seismic network of Iran. The dataset used for tomography consists of 123,575 P- and 11,520 S-picks from 7783 events with magnitude greater than 2.5. We used the LOTOS code (Koulakov, 2009a) developed for simultaneous inversion for the 3D distributions of the P and S wave velocity anomalies and source locations. In the first step, LOTOS determines initial source locations using tabulated values of travel times previously calculated in a 1-D velocity model. The iterative algorithm of tomographic inversion includes the following steps: (1) Source relocations in the updated 3-D velocity structure based on the ray tracing bending method, (2) calculation of the first derivative matrix and (3) simultaneous inversion for P and S wave velocity anomalies, earthquake source parameters (4 parameters for each source), and station corrections. The inversion uses the LSQR method39. The distribution of estimated 3D velocity models correlates well with tectonic and geological conditions. The Vp and Vs anomalies, which are obtained independently, appear to be almost identical in the crust (depths smaller than 45 km). According to the results, the low velocity anomaly observed in the obtained models in the upper crust can be interpreted due to the presence of Cambrian-Miocene sediments with a thickness of at least 10 km that are spread throughout the Zagros. According to the obtained velocity models in the vertical sections, the Moho depth in the Sanandaj-Sirjan area increases significantly compared to the Zagros region. This increase in Moho depth is related to the subduction of the Arabic plate below the micro-continent of Central Iran, which increases the thickness of the crust (double crust) in the Sanandaj-Sirjan region. Using LOTOS code, the optimal one-dimensional velocity model for the whole Zagros collision zone is also presented. In this model, we can distinguish a ∼10 km thick sedimentary (V_p ∼4.90 km s^-1), the upper crust down to ∼30 km (V_p ∼ 5.54 km s^-1) and the lower crust down to ∼45 km (V_p ∼6.30 km s^-1).