Near-surface azimuthal anisotropy using the Rayleigh wave inversion in the Tehran region, Iran.

Surface wave azimuthal anisotropy in the uppermost crust could be essential to obtain the patterns of the local stress field and shallow Earth’s interior structures. We used the micro-earthquakes (M ≤ 4) waveforms recorded by the Tehran Disaster Mitigation and Management Organization (TDMMO) and Iranian Seismological Center (IrSC) networks between 2006 and 2018. We applied the multiple filter analysis, FTAN, to measure Rayleigh wave group velocities and then inverted them to obtain isotropic and fast anisotropic direction maps at periods of 0.5 to 3.0 s. After obtaining the local dispersion curve for each geographic grid point, we applied a 1D V_S inversion procedure. We inserted the resulting model into the original grid point to obtain a quasi-3D V_S model in the Tehran region. According to these results, we divided the study area into five local fast anisotropic direction sectors. The resulting velocity maps indicate that the Tehran basin, with relatively low velocity, has been filled out by alluvial deposits with thicknesses between 0.4 km (in the north) and 1.2 km (in the south). In contrast, the fast-direction pattern in this basin changes from W-E (sector #2-west) to N-S (sector #4). A low-to-high-velocity anomaly change inside the basin and near-surface depths (up to 3 km) can illustrate the secondary faults, such as the Pardisan fault. The northern Tehran mountains appear with a high velocity in these maps with three different fast anisotropy directions (sectors #1, part of #2-west, and #5). This feature has also been observed in other stress field studies. In general, our tomographic results in the uppermost crust indicate that the azimuthal anisotropy can provide the velocity structure and illustrate the local stress field.