Seismic anisotropy in the crust and upper mantle of the Central Alborz Region, Iran

For the last three decades, determination of anisotropy has been a subject of interest for seismologists and geologists. Less well-known information of seismic anisotropy is available from mantle of Alborz Mountains as a branch of Alpine–Himalayan orogenic belt. In this study, we investigated the crust and upper mantle anisotropy by shear wave splitting of P-to-S converted phases on receiver functions and SKS phases on both broadband and short period seismograms in the central Alborz region of Iran.We have selected more than one hundred teleseismic events with M ≥ 5.5 and 30° ≤ Δ ≤ 95° for shear wave splitting of Ps method and with Δ ≥ 65° for shear wave splitting of SKS method. The events were recorded by three broadband seismic stations; DAMV, CHTH and THKV (operated by International Institute of Earthquake Engineering and Seismology (IIEES)) and six short period seismic stations; FIR, DMV, AFJ, SHR, MHD and GZV (operated by Institute of Geophysics, University of Tehran, (IGUT)).The majority of receiver functions showed three different Ps phases. Two of them were seen before Moho Ps on radial component. We believe that the first one is generated in the upper crust (h < 18 km) while, the second one belongs to the middle crust (18 < h < 30 km). The third Ps phase which is generated at the Moho, provides the average shear wave velocity anisotropy in the crust. Results of this study showed that the direction of anisotropy which is derived from third Ps phase in the crust of Alborz Region is about 24 to 77° with magnitude ranging from 0.22 to 0.29 s. From analysis of shear wave splitting of SKS phase, the fastest direction of anisotropy is estimated in a range of 35 to 71° where the magnitude of anisotropy varies from 1.2 to 1.9 s. Our results strongly suggest coherent crust and upper mantle anisotropy in the area of study with a fast direction approximately perpendicular to the strike of the Alborz mountain ranges.