Two dimensional Rayleigh wave group velocity tomography in Northwestern part of Iranian Plateau

The explanation of the elastic, or velocity structure of the Earth has long been a goal of the world’s seismologists. For the first few decades of seismological research, the investigation on velocity structure was restricted to the determination of one-dimensional models of the solid Earth and of various regions within it. Seismologists are currently obtaining three dimensional velocity models and are working to resolve finer and finer features in the Earth. The knowledge of seismic velocity structure of the crust and the upper mantle is important for several reasons: these include accurate location of earthquakes, determination of the composition and origin of the outer layers of the Earth, improvement of our ability to discriminate nuclear explosions from earthquake, interpretation of large-scale tectonics and reliable assessment of earthquake hazard. The Iranian part of the Alpine-Himalayan collision zone consists of an assemblage of lithospheric blocks, features a complex tectonic setting, which results from the collision and convergence of the Arabian plate towards Eurasia, thus investigation of the structure of the lithosphere and the asthenosphere of the Iranian plateau is of great interest.
The North West of Iran is affected by important seismic activity concentrated along the North Tabriz Fault. In recent centuries, more than five successive and destructive seismic events have occurred along the North Tabriz Fault. The North West of Iran is particularly rich in geological and Most of NW Iran is located in a volcanic arc zone of Cenozoic age, including the Quaternary. Some of the main geological features in the North western of Iranian plateau include the Sahand Volcano, the Urmia Lake, salt deposits, travertine deposits, springs, limestone caves, tectonic structures and Cenozoic vertebrate fossils. Sahand and Sabalan peaks are the most prominent geological as well as topographic feature in the region.
The aim of this study is to obtain two dimensional tomographic maps of Rayleigh wave group velocity of the Northwest part of Iran plateau. To do this, the local earthquakes data during the period 2006 to 2013, recorded at 10 broadband stations of the International Institute of Earthquake Engineering and Seismology (IIEES) were used. firstly, Rayleigh wave fundamental mode dispersion curves using single-station method were estimated. In single-station method after the preliminary correction, Rayleigh wave group velocity for each source-station using time-frequency analysis (FTAN) were estimated. After estimating group velocity dispersion curves, using a 2D-linear inversion procedure, the group velocity tomographic maps for the period 2-50 s were obtained. Each tomographic map has been discretized with a grid of 0.5° of latitude per 0.5° of longitude. The results at period 5 s show a low velocity anomaly beneath the Sabalan volcano, whereas beneath the Sahand volcano a high velocity anomaly is observed. At period 10 s the results are different. Beneath the Sabalan volcano a high velocity anomaly is observed, whereas beneath the Sahand volcano and also along the Urumieh-Dokhtar Magmatic Arc a low velocity anomaly are observed. At period 20 s in most of the study area, a low velocity anomaly is observed. The results at period 40 s are different, so that a low velocity anomaly in the southern part and a high velocity anomaly in the northern part are observed.