Three-dimensional imaging of Central Alborz’s crust by using local earthquake tomography

In this study, applying a tomography method to local earthquakes, a three-dimensional (3D) image of the crust of central Alborz is obtained. The result is not only consistent with the tectonics features in the region but also abale to interpret the tomography of Moho and crustal structure beneath the volcanic mountain of Damavand. More than 11000 local earthquakes, with a magnitude of 1.7 or higher, recorded by three-component short-period seismic stations of Tehran, Mazandaran, and Semnan networks between 1996 and 2006, bounded by 34-37N and 39.7-54E, were used to image the crust in central Alborz. These raw pieces of data, on one hand, were used as input for the 1D inversion method to obtain the variation of Vp and plot the velocity versus depth diagram whose rms was smaller than 0.15. On the other hand, they were used in a relocation process and when their locations were improved, a 3D model was generated based on them. After determining the preliminary 3D and forward models using the finite-difference method, the refracted and wide-angle reflected travel times were inverted and the horizontal and vertical sub-structures in our determined region were investigated. Based on these results, the discontinuities of the crust and Moho were mapped and analyzed. The final outputs showed that the resulted crust model is consistent with some of the recent geological studies. These outputs illustrat that the upper layer is thicker than the middle and lower ones as these two layers become thinner and even disappear below Alborz. It seems that the upper layer fills some hollows in the other ones. The depth of Moho increases below Damavand mountain; also, the area around the volcanic conduit of Damavand, between 6km and 18km depths, has a high velocity and is colder than the other areas.
    The P-velocity model resulted by using 1D tomography facility of Velest with RMS values less than 0.15, are compatible with the previous models. The resulted depth is 45 ± 2 km for the Moho and 7 km for the sediment layer. Frequency and distribution diagrams of the earthquakes show that about 75% of earthquakes have happened in depths less than 24 km and consequently the most seismogenic layer of the crust is estimated to be located at this depth. The 3D tomography, performed through the Zelt routines, has acceptable results with less than 2.5% error. Although an enough number of earthquakes overcome the problem of scarcity of the stations, the high depths of earthquakes cause a low resolution in shallow layers. However, this problem can be solved by increasing the density of stations.