Seismic Swarm PS20130924.1: Analysis of the 2013 Balochistan Sequence
On 24 September 2013, a significant seismic swarm was recorded 73 km north of Bela, Pakistan. The sequence began at 11:29 UTC and concluded at 17:20 UTC, encompassing eight earthquakes within a span of five hours and fifty minutes. The events occurred in a tectonically active region shaped by the ongoing collision between the Indian and Eurasian plates along the western boundary of the Indian subcontinent.
The initial event registered a magnitude of 7.7 at a depth of 15 km. Subsequent shocks included magnitudes of 5.8 at 10 km depth, 5.1 at 9 km, 4.8 at 10 km, 5.0 at 10 km, 5.6 at 14 km, 5.2 at 20 km, and a final 5.5 at 10 km. These parameters reflect typical aftershock behavior following a major strike-slip rupture, with most activity concentrated in the shallow crust.
The swarm center lies near the trace of the Chaman Fault system, a major left-lateral transform boundary that accommodates northward motion of the Indian plate relative to Eurasia. This fault zone has produced destructive earthquakes throughout recorded history, including events in 1892 and 1931 that caused extensive damage in Balochistan. The 2013 sequence aligns with this pattern, occurring along a segment characterized by oblique convergence and partitioned strike-slip and thrust faulting.
Geological mapping of the area reveals Quaternary alluvial fans and folded sedimentary rocks overlying ophiolitic basement, consistent with the Makran accretionary complex to the south. The mainshock epicenter was located approximately 27 km from the swarm centroid, confirming the sequence as part of the immediate aftershock distribution. Regional seismic hazard assessments note elevated strain accumulation along the Chaman Fault, with recurrence intervals for magnitude 7+ events estimated in the range of several hundred years.
Post-event studies have documented surface rupture extending over 200 km, accompanied by triggered slip on adjacent structures. The swarm activity remained confined to the upper 20 km, consistent with the brittle seismogenic layer in this continental transform setting. No significant change in regional strain rates has been detected since 2013, although ongoing monitoring continues to track microseismicity along the fault trace.
- USGS Earthquake Catalog
- Pakistan Meteorological Department Seismic Reports
- Geological Survey of Pakistan Tectonic Maps