Crustal Velocity Structure Model has a significant role in truly understanding of seismicity and also in relocating earthquakes. On the other hand, it can be used for recognizing major and potential seismic sources which is very critical for seismicity and earthquake hazard assessment studies. Seismic parameters simultaneous inversion modeling is one of the most prevalent methods in the study of seismic velocity structure. This approach optimizes the coordinate parameters, time of the events and the velocity structures simultaneously by processing the initially assumed values. The resulted velocity model can be used for relocating the seismic events, registered on the local seismic network, and locating future seismic events as well as establishing the future seismic tomography studies.
In this study, the simultaneous inversion modeling and VELEST software were used in order to find an optimum crustal velocity model. located in east of Caspian Sea, north east of Iran and south of Touran plate tectonics, the Kope Dagh region lies within a broad zone of deformation and forms part of Alpine-Himalayan orogenic belt which is actually the conjunction zone of Touran and Iran plates. This region is separated from Touran plate by Main Kope Dagh Fault from the north, and its southern boundary is assumed Sabzevar Reverse Fault and Mayamey Reverse Fault. Our study area covers Sabzevar, Mashhad, Shirvan and Quchan cities. Quchan is located in the central Kope Dagh region and has experienced four destructive earthquakes in the past centuries (1851, 1871, 1893 and 1895). These earthquakes caused widespread devastation and heavy human loss in Quchan and many surrounding villages.
To estimate the layer configuration, velocities and thicknesses; we used first arrival travel times. More than 14000 first arrival data related to 2200 seismic events, registered by the local seismic network, were considered. They are all registered in the stations located less than 400 meters of epicenters. First arrival times were plotted versus their distances. The chart suggests three major layers; therefore we decided to fit three lines on three sections of the chart which are 50-100 meters, 120-180 meters and 190-350 meters, using least square method. By inversing the slopes of these three lines, we calculated the mean velocities for the three layers which are 6.01, 6.36 and 8.10 km/sec related to 0-20 km, 20-46 km and more than 46 km respectively. The second anomaly is corresponding to Moho Depth so it shows thickness of the crust in the study area. We used this resulted information as an initial velocity model to prepare numerous models needed for VELEST software runs.
In the next step of our work, in order to improve resulted velocity model, we used VELEST for seven run groups, each containing 20 independent runs using 20 initial models. These initial models are created by a FORTRAN program in a way that 20 initial models have all same thickness but different velocities which are restricted in defined intervals. We considered the convergence of the resulted models in each group to select one as the best run and then to determine a proper velocity model and Moho Depth as well. Hence the third group was selected as the best run group and therefore the related Moho Depth is 46. It is exactly the same as Moho Depth resulted from the first arrival travel times.
In the final step, we used the resulted velocity models in the previous step and calculated their mean values as the mean velocity model. This model was used as another initial model for the final run of VELEST program, but in this run we added several layers to initial model so that their velocities increase regularly. The calculated model, by the VELEST, is very similar to the mean resulted model of the second step. We determined this three layers model as an optimum velocity model for the study area in which the thicknesses of layers are 5, 10 and 31 and velocities of P-wave in these layers are 5.95, 6.1 and 7.97 km/sec respectively. Quchan station is determined as our origin station, therefore its time correction was assumed zero. The most time correction resulted by the final VELEST run is related to Moghan Station.