Seismic Swarm PS20010126.1: The 2001 Bhuj Earthquake Sequence

The seismic swarm designated PS20010126.1 occurred on 26 January 2001 in the Kachchh region of northwestern India. The sequence began at 03:16 UTC and concluded at 07:32 UTC, encompassing six earthquakes within a span of four hours and fifteen minutes. The events were centered approximately 24 km northwest of R?par, with the primary shock located 8 km from the swarm centroid.

The initial and largest event registered magnitude 7.7 at a depth of 16 km. Subsequent shocks included magnitudes 5.3, 5.2, 5.1, 4.6, and 5.5, all at depths of 10 km. This rapid succession of events aligns with the well-documented mainshock-aftershock pattern of the 2001 Bhuj earthquake, whose epicenter lay 17 km northwest of Bhachau.

The Kachchh peninsula lies within the intraplate setting of the Indian tectonic plate. Seismicity arises from the reactivation of pre-existing faults formed during Mesozoic rifting and later influenced by the Himalayan collision far to the north. The region features east-west trending reverse faults and a thick sedimentary cover overlying Precambrian basement. Historical large events, such as the 1819 Allah Bund earthquake of estimated magnitude 7.7–8.2, demonstrate the area’s long-term potential for destructive shaking.

The 2001 Bhuj mainshock caused extensive surface deformation, including liquefaction and ground fissures across the Rann of Kachchh. Post-event studies confirmed rupture along a blind south-dipping thrust fault at mid-crustal depths, consistent with the 16 km focal depth recorded for the initial swarm event. Aftershock activity decayed over subsequent months, yet the initial four-hour cluster captured the highest energy release phase.

Modern monitoring networks have improved resolution of such sequences, allowing precise mapping of fault segments activated during the swarm. The provided event parameters from internal classification underscore the compact temporal window and consistent shallow-to-mid crustal depths that typified the early aftershock distribution.

References

SeismoSight internal swarm classification PS20010126.1
USGS Earthquake Catalog (event details for 2001 Bhuj mainshock)
Geological Survey of India reports on Kachchh tectonics