Fault-based probabilistic seismic hazard assessment of the eastern Makran subduction and the Chaman transform fault, Pakistan: Emphasis on the source characterization of megathrust

Seismic source characterization (SSC) for probabilistic seismic hazard assessment (PSHA) in regions characterized by subduction megathrust involves a considerable ambiguity. Lack of detailed geologic, seismic, and geodetic data increases the uncertainties. The enigma is enhanced in regions where thin-skinned accretionary prism faults are part of active deformation. In this study, a planar SSC model for seismically active eastern Makran subduction zone, its associated accretionary prism faults and Chaman transform fault zone is proposed based on a kinematic block model accounting strain partitioning. Sensitivity tests for various parameters of the SSC model are performed by computing peak ground acceleration (PGA) maps for 475-year return period. Among alternative magnitude distribution models, the truncated exponential model gives similar to 10% higher PGA values than the composite recurrence model, which is favored by the observed subduction seismicity. Especially in gently dipping subduction zones such as Makran, estimated PGA values and their spatial distribution are highly sensitive (changing up to 0.3 g) to the megathrust interface geometry near the surface, dip amount, and defined depth limits. Gentler and deeper extending subduction interface maximizes the rupture width and results in higher PGA values towards inland, while the inclusion of shallow aseismic portion produces larger PGA values near the trench. Through eastward transition from subduction to transform motion, thrust faults within the Makran accretionary prism bend northward, forming oblique fault systems with higher slip rates and accommodate part of the compression in accordance with the lateral slip rate variations identified on the Chaman transform fault zone. Using the selected SSC model, the PGA values for short return period (475-year) are estimated to be between 0.5 g and 0.7 g within the accretionary prism down to the trench across the Makran subduction and along southern section of Chaman transform fault zone where slip rates are higher relative to the north.