Investigation of the strength and trend of seismic anisotropy beneath the Zagros collision zone

Abstract

The Zagros collision zone is known as an active tectonic zone that represents the tectonic boundary between the Eurasian and Arabian plates. A popular strategy for gaining insight into the upper mantle processes is to examine the splitting of seismic shear waves and interpret them in terms of upper mantle anisotropy and deformation. Core phases SK(K)S from over 278 earthquakes (M_W≥ 6.0) occurred between years 2010 and 2017 at epicentral distances between 90° and 145° are examined, which were recorded by 27 broadband stations located in the Zagros collision zone. In compressional tectonic regimes such as the Zagros collision zone, a dominant pure shear deformation in the mantle is expected that could develop lattice preferred orientation (thus anisotropic fabrics) subparallel to the strike of the mountain belt. The findings show that the majority of the fast axes of seismic anisotropy are oriented in the NE-SW direction (perpendicular to the trend of the belt) with delay times (a proxy for the strength of anisotropy) varying between 1 and 1.5 seconds. If deformation in the mantle lithosphere was the main factor of the observed anisotropy, then the fast direction of anisotropy would be parallel to the belt. Therefore, the main source of anisotropy is thought to be residing in the sub-lithosphere mantle. Crack-induced anisotropy in the upper crust that can be perpendicular to the trend of the belt (parallel to the maximum compressional stress direction) may also have some contribution to the observed splitting of shear-waves.