Introduction — Chaman Fault
The Chaman Fault is a major active continental fault system exceeding 850 km in length that forms the primary structural boundary between the Eurasian and Indo‑Australian plates across Pakistan and Afghanistan. Its dominant movement is left‑lateral strike‑slip, with the western block shifting southward relative to the east, but the ongoing India–Eurasia collision imparts a significant compressional component. This combination of lateral shear and shortening classifies the boundary as transpressional and accounts for both horizontal displacement and mountain building adjacent to the fault trace.
Geologic and geodetic constraints indicate appreciable plate motion on the system: conservative slip‑rate estimates are on the order of ≥10 mm/yr for the northward approach of the Indo‑Australian plate relative to Eurasia, whereas paleogeologic offsets—notably displaced volcanic units dated to ~2 Ma—have been interpreted to imply a higher long‑term rate of roughly 25–35 mm/yr. Field observations document recent surface deformation along much of the trace; geomorphic offsets are widespread and sufficiently young that only the finest, youngest alluvial deposits in active dry washes remain undisturbed, attesting to ongoing activity at the surface.
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Structurally, the Chaman Fault originates at a triple junction off the Makran coast where the Arabian, Eurasian and Indo‑Australian plates converge, and its zone continues southward beneath the Arabian Sea as the Owen Fracture Zone to approximately 10°N, 57°E. On land the system trends northeast from the Makran junction across Balochistan, swings north‑northeast into Afghanistan passing just west of Kabul, and continues toward higher latitudes where it intersects and interacts with other major structures: it crosses the right‑lateral Herat Fault, merges with elements of the Pamir fault network north of ~38°N, and commonly includes the Ghazaband and Ornach‑Nal faults as subsidiary strands. The transpressional regime also drives regional tectonic geomorphology in eastern Balochistan, where a suite of east–west–aligned mountain ranges (e.g., Kirthar, Khude, Zarro, Pab, Mor) has formed east of the main fault trace in response to the component of shortening parallel to the system’s lateral motion.
The Chaman fault system has produced a spectrum of surface-rupturing earthquakes that illustrate its segmented geometry and variable slip behavior. In the early 16th century (5 or 6 July 1505) a major event generated a continuous ~60 km surface rupture on a transverse strand of the system—often referred to as the Paghman fault—characterized by several meters of vertical displacement. A historically recorded event on 20 December 1892 is noted in regional catalogues but remains poorly documented, with no reliable data on rupture length, magnitude, offsets, or casualties. The 31 May 1935 Quetta earthquake (Mw ≈ 7.7) ruptured the Ghazaband segment and produced catastrophic societal impact, with death toll estimates exceeding 35,000, exemplifying the potential for very large-magnitude ruptures on named fault strands. In contrast, the 16 March 1978 event was moderate (Mw 6.4) and confined to a short (~5 km) rupture that accommodated oblique slip: dominant left-lateral strike-slip of up to ~4 cm accompanied by a smaller normal/vertical component that down-dropped the eastern wall.
Taken together, these events demonstrate pronounced segmentation (distinct strands such as Paghman and Ghazaband), a wide range of rupture lengths (from ~5 to ~60 km), and diverse slip modes (from multi-meter vertical offsets to dominantly left-lateral strike-slip with minor normal motion). This variability implies a complex mechanical architecture that produces both long, high-displacement ruptures and shorter, oblique ruptures, with important consequences for regional seismic hazard assessment and patterns of surface deformation.