DEFORMATION PATTERNS AND ITS EFFECTS ALONG MAKRAN SUBDUCTION ZONE AND ADJACENT AREAS OF BALOCHISTAN, PAKISTAN

Abstract

Pakistan lies in a region where three tectonic plates collide with each other, along with a triple junction near the west of Karachi city. At Makran subduction zone (MSZ), the Arabian plate subducts beneath the Eurasian plate, producing the world's largest arc-trench gap (500-600 km) and splay faults wedge system having ~7 km thick sedimentary strata. The Makran area has experienced more than eight major earthquakes (M > 7) in last 500 years, including the 1945 M8.1 tsunami, making this region the largest source of earthquake and tsunami hazards. The research aims to investigate the crustal deformation caused by interseismic coupling along the plate interface and its ongoing seismogenic potential for future earthquakes and tsunamis using ground- and space-based geophysical and geodetic data. To do so, various techniques are employed to analyze the deformation patterns, slab geometry, and seismic potential of MSZ, utilizing detailed seismic and velocity data analyses. Mechanical modeling of Makran megathrust and splay faults has been performed to predict slip components and its impact on tsunami generation during major earthquakes. Furthermore, the study utilizes satellite-based (GNSS, InSAR) data and computational simulations to analyze and model crustal deformation in the coastal region due to interseismic coupling. For geodetic data modeling, all the parameters related to the plate interface are fixed except the slip deficit rate and the width with lower and upper convergence rate limits. The InSAR-derived velocity field aids in monitoring deformation processes, including pre-, and post-seismic studies, but coupling analysis remains challenging due to persistent post-seismic signals even a decade after the 2013 M7.7 Balochistan earthquake. The structural complexity of Makran megathrust requires precise knowledge of subducting plate interface geometry for understanding earthquake hazards. The seismic structural interpretation reveals the dominance of the imbricate thrust fault system across the subduction zone, with megathrust geometry being interpreted in both offshore and onshore areas. Seismic attributes show fractured and porous subducting strata, while rock physics analysis indicates high pore-fluid pressure in the coastal subsurface regions reducing friction properties. The frontal interface region exhibits low friction values, possibly due to the high pore fluid pressure, fracturing, or dynamic weakening. The results found that the effective friction ~ 0.04–0.12 is required to reproduce the observed fault structures. If rupture occurs at the frontal plate ix interface and its splay faults, a range of horizontal and vertical slips could occur due to slip deficit along each fault patch. Mechanical modeling runs a number of simulations to measure the frontal horizontal and vertical slip of wedge geometry. Assuming the convergence rate of 3–4 cm/yr, the calculated slip deficit in the last 77, 158, and 257 years, after 1945, 1864 and 1765 earthquakes along offshore-ruptured patches of eastern MSZ is 2.31–3.08 m, 4.74–6.32 m, and 7.71–10.28 m, respectively. The study shows that the splay faults will observe high uplifts as compared to the plate interface amplifying thick water column and potentially causing a large tsunami in the coastal areas. The geodetic GNSS coupling model further tested different fault patches along the strike direction and identified the three fault patches as the most suitable case for this study area. The estimated slip deficit rates and width for all three fault patches along the megathrust reveals that the whole subduction interface is coupled with variations along the strike direction. The modeling indicates slip deficit rates of 2.3 cm/yrs., 0.76 cm/yrs., and 3.6 cm/yrs. for eastern, central, and western fault patches respectively with a fixed width of ~57 km. The coupling model has been further investigated through two high-resolution InSAR data sets for varying time spans, i.e., 2003-2010 (Envisat) and 2021-2024 (Sentinel-1A). To independently verify the coupling model, forward modelling is performed for both the Sentinel-1A and EnviSAT data. The study analyzed SAR data sets and predicted models along the coastal region of eastern Makran for the same data points over different time spans. Forward modelling results suggest that the similarity level for Sentinel-1A is less than Envisat model, maybe due to post seismic signal effects. The interseismic coupling model indicates that the eastern segment of the megathrust coupled north of the coast is likely to be consistent with InSAR observations i.e., subsidence observed near the coast. The coupling model for eastern fault patch reveals varying slip deficit rates from east to west of 3.5 cm/yrs., 1.0 cm/yrs., and 2.0 cm/yrs. The estimated strain rate accumulation and partitioning along these faults will assist us to forecast the impending major earthquakes and early tsunami warning system.