Present-day quantification of seismic coupling along the décollement level beneath the Potwar Plateau region in Pakistan western Himalaya

Abstract

The northwestern syntax of the Himalaya is a very rapidly deforming area at the edge of the India-Asia collision zone. Therefore, we quantify the current velocity field of the Potwar Plateau – Salt Range fold-and-thrust belt using GNSS horizontal surface velocities and Sentinel-1 interferometry line-of-sight velocities. From this velocity field which indicates a creep of the Potwar Plateau along the Main Himalayan Thrust, we infer a weak subhorizontal décollement level formed by a massive Precambrian salt layer. South of the Plateau, the Salt Range is uplifted along the Salt Range Thrust up to 5 mm/yr. The Kalabagh Fault, which forms the western boundary of the Salt Range and the Potwar Plateau, exhibits a creep rate along fault of 3.3 mm/yr. To characterize the slip distribution and coupling along the faults, we use numerical modelling with a set of dislocations in an elastic half-space. The preferred model shows the presence of a large asperity along the décollement level beneath the Potwar Plateau and several smaller asperities along the eastern basal thrust. These observations are consistent with the occurrence of the 2029 Mirpur earthquake of Mw 5.9 along the eastern part of the décollement level. Along the southern and superficial parts of the Salt Range Thrust, the model indicates a slip rate of 20 mm/yr which is greater than the 14 mm/yr slip rate along the Main Himalayan Thrust at depth. This observation suggests the existence of an internal southward flow of the massive salt layer along the upper part of the Salt Range Thrust. For the Kalabagh strike slip fault, an alternation of coupled and decoupled zones is observed, meaning that this fault can be characterised by creep and asperities where earthquakes and/or slow slip events can occur. Considering the lack of instrumental and historical large magnitude earthquakes in the area since the AD 25 Taxila earthquake, it can be concluded that the Main Himalayan Thrust and the Kalabagh Fault are likely to be affected by large magnitude earthquakes.