Mapping the distribution of strain along multiple strike-slip faults in the Chaman fault system from InSAR

Abstract

<p>The Chaman plate boundary between India and Eurasia is a wide faulted region in Pakistan and Afghanistan, hosting distributed seismicity. Along the western edge of the deforming region, the Chaman fault currently accommodates less than 15 mm/yr of slip, while the differential left-lateral motion between both tectonic plates is close to 30 mm/yr. In the past century, significant earthquakes have ruptured structures east of the Chaman fault, including the 1931 Mach earthquake and 1935 Quetta earthquake with magnitudes (Mw) greater than seven. We aim to identify where strain focuses so that active structures likely to rupture in large earthquakes are outlined. We use ground velocities computed from 6 years-long InSAR time series in ascending and descending line of sights to map gradients of deformation in the Kirthar ranges. InSAR data reveals that most of the current plate boundary strain focuses east of the Chaman and Ghazaband fault in the central axis of the ranges. We model velocities along profiles across the plate boundary as the surface expression of left-lateral slip on several vertical faults: the Chaman fault, the subparallel Ghazaband fault, the Hoshab fault and one to three unknown faults to the east. We localise strain in the continuation of the Ornach Nal in the south and along the Quetta-Kalat fault which is thought to have hosted the 1935 Quetta earthquake (Mw 7.7). Three discrete portions of the Ghazaband fault slip with rates close to 10 mm/yr. Our description of partitioning matches known seismic ruptures, and makes sense in a geodynamical and geological perspective. We propose a tectonic model of the plate boundary evolution with an eastward migration of strain.</p>