Estimation of shallow sediment structure of Tehran Basin by using surface wave Love dispersion curves

The delineation of the elastic, or velocity, structures of the Earth has long been a goal of the world's seismologists. In the first few decades of seismological research, the investigation of velocity structures was restricted to the determination of one-dimensional models of the solid Earth and various regions within it. Seismologists are currently working on three-dimensional velocity models, and they are trying 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: It includes the accurate location of the earthquakes, it is used in the determination of the composition and origin of the outer layers of the Earth, it improves our ability to discriminate nuclear explosions from earthquakes, it helps to interpret the large-scale tectonics as well as a reliable assessment of earthquake hazards. In this study, we used highfrequency dispersion curves to estimate the elastic properties of Tehran and the suburbs. Tehran city is located in the Alborz major seismic tectonic zone. The Alborz is an arcuate chain of mountains in the Northern Iran that wraps around the Southern side of the South Caspian basin; the boundary is roughly the present shoreline of the Caspian Sea. The range is actively deforming on range-parallel thrusts and left lateral strike-slip faults. The thrusts dip inward toward the interior of the range from both its northern and southern sides, and the GPS-derived shortening across the range is 5 ± 2 mm/yr at the longitude of Tehran (Vernant et al., 2004b). Most are parallel to the range and accommodate the present-day oblique convergence across the mountain belt. Recent large earthquakes occurring in this region suggest that the seismicity is connected with major faults of the recent age that cut across the regional Quaternary Lineaments. In this study, in order to estimate the upper crust elastic structure, we conducted a tomographic inversion of the Love wave dispersion to obtain two-dimensional Love wave group velocity tomographic images in a period range from 2 s to 5 s for the city of Tehran and the suburbs. We used two databases to derive dispersion curves for different paths. In the first dataset, continuous ambient noise in ten stations located in and around the city of Tehran and installed by the Tehran Disaster Mitigation and Management Organization network was used to explore the inter-station Green’s Functions. In the second database, forty seven earthquakes recoded by the Parsian stations were prepressed and rotated to be used as single station records to estimate the Love wave group velocity. In the next step, the inter-station Green functions and single-station records were used to estimate the Love wave dispersion curves by applying a multiple filtering technique. All dispersion curves were used to estimate the two-dimensional Love wave group velocity models. For this purpose, Tehran region was divided into 0.1° × 0.1° cells. According to the ray coverage, the minimum dimension of distinct heterogeneity was 6 km. In our study, topographical features and near-surface known geological structures were two main criteria to assess the credibility of the estimated Love wave group velocity variations. There was a strong correlation between the estimated group velocities and topographical features. The prominent surface geology units at the mountain range consisted of varying structures including sand-stone, siltstone, claystone, and massive limestone.