Investigating the strength and trend of seismic anisotropy in the western part of Makran subduction zone and southeast of Iran

Abstract

The Makran subduction zone, located in southeastern Iran and southern Pakistan, extends approximately 900 km along the Eurasian-Arabian plate boundary, where the Arabian oceanic plate subducts beneath the Eurasian plate. Using broadband seismic data recorded in the western part of Makran in the southeast of Iran, seismic anisotropy was studied to better understand geodynamic processes of this subduction zone. We analyzed shear wave splitting of core-refracted shear-wave phases (SKS, SKKS and PKS) recorded by 17 broadband stations located in the western Makran and southeast of Iran from 542 earthquakes (MW ≥ 6.0) that occurred between 2004 and 2017 at epicentral distances between 90°–145°. Few stations close to the trench line exhibit a trench-oblique trend in the west to a trench-prependicular trend to the east. The magnitude of splitting delay times suggests a mantle source of anisotropy.Our findings show that the majority of the fast axes of seismic anisotropy at farthest stations in the backarc region are oriented in the NW-SE direction with delay times (created by crust and mantle) varying between 0.8 and 1.3 s. The orientations of these fast directions are almost parallel to surface structural features. For stations near the Minab strike-slip fault system (the transition from the Makran subduction zone to the Zagros collision zone), the fast directions are sub-parallel to the strike of this fault system, indicating that simple shear is likely the main source of anisotropy.