Estimation of the two-dimensional tomography of phase and group velocities and shear wave structure for Alborz region

The delineation 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 earthquakes، interpretation of large-scale tectonics and reliable assessment of earthquake hazard. In this study، we first prepared the two-dimensional phase، group velocity images and also the shear wave velocity structure of the lithosphere and asthenosphere of the Alborz region. To achieve these goals، in the first step، we conducted a tomographic inversion of Rayleigh wave dispersion to obtain the two-dimensional (2-D) phase and group velocity tomographic images in a period range from 10 s to 100 s for the Alborz region. For this purpose، the fundamental mode of Rayleigh waves، recorded along paths by broad-band stations، has been identified by applying the frequency time analysis (FTAN) to each epicenter–station path which، at the same time، satisfies the two-station method conditions. The fundamental modes، identified by FTAN، are used to determine the interstation path average phase and group velocities at selected periods. With this procedure، group and phase velocity dispersion curves have been processed to obtain tomographic maps by applying the Yanovskaya–Ditmar formulation، for periods in the range between 10 and 100 s. Each tomographic map has been discretized with a grid of 0. 5° of latitude per 0. 5° of longitude. Our results demonstrated that the Alborz region is characterized by low crustal and uppermost-mantle group and phase velocities. Tomographic maps at high frequencies are well correlated with the upper crust structure and especially with sediment layer thicknesses. In the second step، we used fully non-linear inversion procedure، commonly known as hedgehog (Valyus et al.، 1969; Valyus، 1972; Knopoff، 1972; Panza، 1981; Panza et al.، 2007) to derive tomographic images of the elastic structure of the lithosphere and asthenosphere of the Alborz region. An estimated shear wave structure can be useful to estimate the strong ground motion as well as the realistic seismic hazard assessment. On the other hand، the derived tomographic phase، group images and the shear wave velocity structure are well correlated with major tectonic and geological features of the Alborz region. The Moho depth in this region is derived around 46 km in which the shear wave velocity varies between 3. 7 and 4. 3 km/sec when passing from the crust to the mantel. The thickness of the two resolved crustal layers are 15 and 25 km located beneath of a sediment layer with 6 km. In this region، three upper mantel layers are resolved with thicknesses of 20، 60 and 80 km having velocities of 4. 3، 4. 6 and 4. 5 km/sec، respectively.