A time-series InSAR study of faulting and folding in the Tajik Basin

Abstract:

The Tajik basin, or Afghan-Tajik depression, is a relatively fast-moving fold and thrust belt in Central Asia, bounded by the Tian Shan, Pamir, Hindu Kush and Afghan platform. Large historical earthquakes have ruptured the basin margins resulting in many fatalities, while seismicity in the interior is sparse and seems to be restricted to smaller magnitudes. East-west shortening across the basin is evident from GPS measurements, earthquake focal mechanisms and geological structures. A set of north-south arcuate anticlines sweep up the basin interior, curving to the east at the northern margin. Sharp GPS velocity gradients across the anticlines hint at localisation of strain on these structures and may indicate creep on inferred blind thrusts, but the sparse point measurements alone are insufficient to characterise the nature of the deformation.

A high-resolution mapping of the interseismic deformation is required to constrain how far-field tectonic stress is partitioned seismically and aseismically at the surface on several fault segments and how faults are connected at depth. A precise mapping of the strain localisation and a better quantification of the distributed deformation is crucial for assessing the seismic hazard posed to local populations, including Tajikistan’s capital city, Dushanbe, which is situated on the basin’s northern margin.

We performed a multi-temporal InSAR time series analysis of the Tajik basin using the New Small Baselines Subset (NSBAS) processing chain. This technique allows us to derive the cumulative time series of surface displacements and image small amplitude tectonic signals. We use data from ESA’s Sentinel-1 radar satellites in interferometric wide swath mode from descending track 151 and ascending track 71 between 36 and 39 degrees latitude. We process networks of ~400 interferograms across the basin and its boundaries for each track between October 2014 and August 2019, with time intervals between 12 days and 5 months.

The resulting LOS velocity maps over this ~5 year period provide a continuous view of the rate of strain accumulation across the Tajik basin and its bounding faults. The aim of these measurements is to provide insights into interseismic strain across major faults and investigate mechanisms of fold growth.