Determination of improved velocity model for the north west Iran region, using simultaneous inversion of local earthquake travel times

Earthquakes travel times inversion for determination of velocity structure of crust is one of the most important objectives in seismology. In the simultaneous inversion method, the parameters of earthquakes and velocity model are determined initially and are improved during inversion steps.
In this study, we have used more than 22000 calculated travel times for first arrivals of 6000 earthquakes, which occurred in north west Iran (bounded 36°N-40°N & 44°E-50°E) to estimate P wave velocities and thickness of the crust in this region. This information has been used to determine several initial models. First, we selected 181 events with magnitudes 4 or above that were well located. Then, the simultaneous inversion method was used to obtain the best initial and final improved model. The obtained model was tested again with the travel times of 2940 earthquakes with magnitudes greater than 2.5 in six independent sets. The results were in good agreement, specially for depths greater than 8 km. The results show two low velocity shallow layers with thicknesses of 3 km and 2 km and with a corresponding average velocity of 4.8 and 5.4 km.s-1 for P wave. Also, a sharp change of velocity was detected in depth 23 km, where the P wave velocity increased from 6.1 km.s-1 to 6.6 km.s-1. The depth of Moho was found to be 45 km while the P wave velocity in the upper mantle was evaluated
at 8.0 km.s-1.