Investigation of one dimensional upper crust velocity structure in northeast Khorasan by the travel time inversion of P waves



North-East Khorasan is one of the most active regions in the world because of is setting on the Alpine-Himalayan belt. Historical and geological backgrounds suggest that this region has experienced many destructive earthquakes throughout history. Compared with the historical background, the seismicity of the region, in the present century, is better known both from the macroseismic and instrumental point of view. The instrumentally located earthquakes suggest that seismic activity in the present century has increased remarkably. A better understanding of crustal velocity model could help to improve the location of earthquakes and to find out the active faults and the tectonic evolution in the region.
In this study we used the travel times of local earthquakes recorded by the seismic networks of Quchan and Mashad operated by the Institute of Geophysics, University of Tehran, to investigate the crustal velocity structure in the Khorasan region. In this study, among all recorded data during 1997-2006, we selected and used the records of 103 earthquakes that were recorded by at least four seismic stations. For these selected earthquakes, the azimuth coverage was less than 270 degrees; RMS less than 1 second and the location error was less than 5 km. The travel times obtained from these earthquakes were used to find out an appropriate velocity model. First we applied the VELEST method and used the travel time data to obtain the one dimensional velocity model. We applied random velocity variations of about ±0.5 Km/s in each crustal layer and produced fifty preliminary models. We selected those models that indicated acceptable convergence during the inversion process. Then, by inversion, we obtained a preliminary three layer model. Next, we used this model as initial value to find out the appropriate velocity model. The final result indicated a simple two layer model. This model contains a first layer having a thickness about 10 km and a velocity of 4.5 km/s over the second layer that has a velocity of 6.2 km/s. As we used local data and the earthquakes had shallow depths, we could investigate the structure down to 20 km.
This result is in good agreement with the results of other studies in this region In general, the one dimensional inversion of travel time data for crustal velocity structure is sensitive to the number of seismic stations and the distance between the successive two stations. The results of this study indicate that if a good data set is available, the one dimensional inversion of travel time data is an appropriate method for the study of crustal velocity structure.