Ball-and-socket tectonic rotation during the 2013 Mw7.7 Balochistan earthquake

- The Balochistan earthquake ruptured >200 km of a reverse fault with pure strike slip motion.
- Little variation between strike- and dip-slip suggest large scale ball-and-socket rotation of the SE Makran.
- The earthquake highlights source of reduced seismic risk to Karachi and structural complexity of the Chaman fault system.
- Anderson-Byerlee mechanics are perhaps inappropriate for strong ground motion modeling.

The September 2013 Mw7.7 Balochistan earthquake ruptured a ∼200-km-long segment of the curved Hoshab fault in southern Pakistan with 10±0.2m of peak sinistral and ∼1.7±0.8m of dip slip. This rupture is unusual because the fault dips 60±15° towards the focus of a small circle centered in northwest Pakistan, and, despite a 30° increase in obliquity along strike, the ratios of strike and dip slip remain relatively uniform. Surface displacements and geodetic and teleseismic source inversions quantify a bilateral rupture that propagated rapidly at shallow depths from a transtensional jog near the northern end of the rupture. Static friction prior to rupture was unusually weak (μ<0.05), and friction may have approached zero during dynamic rupture. Here we show that the inward-dipping Hoshab fault defines the northern rim of a structural unit in southeast Makran that rotates - akin to a 2-D ball-and-socket joint - counter-clockwise in response to India's penetration into the Eurasian plate. This rotation accounts for complexity in the Chaman fault system and, in principle, reduces seismic potential near Karachi; nonetheless, these findings highlight deficiencies in strong ground motion equations and tectonic models that invoke Anderson-Byerlee faulting predictions.

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