Radial Anisotropy in the Upper Crust Beneath the Tehran Basin and Surrounding Regions

Abstract

Radial anisotropy is characterized by Rayleigh–Love wave discrepancy indicating crustal past and ongoing deformation. For this study, data from 1075 micro-earthquakes were collected based on recordings by 36 stations between 2004 and 2016, and after using data selection criteria, 375 events were used to calculate radial anisotropy in the Tehran region. More than 1908 and 1705 source-station Rayleigh and Love wave group velocity dispersion curves were, respectively, measured in the period band of 0.6–3.0 s. Furthermore, the tomographic inversion method was carried out to obtain group velocity maps for each period individually. Next, a damped least square iterative process was performed using a 3.5 × 3.5 km geographic grid size to calculate both VSV and VSH models. Horizontal and vertical spatial extents of the radial anisotropy beneath the Tehran region are revealed as maps of anisotropy as a percentage. Furthermore, the average value of the radial anisotropy as a function of depth indicates three sharp anomalies including: (1) relatively negative within a depth range from subsurface to 1.5 km, (2) relatively positive anomaly within a depth range from 2.0 to 4.0 km, and (3) an approximately isotropic half-space for a depth greater than 4.0 km. In general, the redeposition of former sediments near fault systems, geological and tectonic setting features are correlated with radial anisotropy anomalies at various depths as shown in the horizontal maps. In the radial anisotropy profiles, sedimentary thickness varies from ~ 500 to ~ 1500 m for southward transects, and it is constant for the eastward transect. These profiles clearly indicate the edges of three different tectonic settings.