M 5.9 - 11 km NNE of Harnai, Pakistan

Tectonic Summary

On October 6, 2021, an M5.9 earthquake occurred in the north of the Balochistan province of Pakistan, approximately 100 km east of the city of Quetta. The earthquake occurred at shallow crustal depths (approximately 9 km) as the result of thrust faulting on either a fault dipping shallowly to the north, or a fault dipping steeply to the south. The earthquake mechanism, depth, and location are consistent with the earthquake having occurred on or near the shallowly northward dipping plate boundary between the India and Eurasia plates. At the location of the earthquake, the India plate moves north at 29 mm/yr relative to the Eurasia plate.

The October 6, 2021 earthquake occurred near the western edge of the Sulaiman Mountains, a U-shaped active fold-and-thrust belt mountain range in central Pakistan formed at the boundary between the converging Eurasia and India plates. The Sulaiman Mountains are characterized by intense faulting and folding of a thick package (approximately 10 km thick) of sedimentary rocks that are decoupled from underlying crystalline rock of the India plate by a nearly horizontal thrust fault, or decollement, that dips northward beneath the Eurasia plate.

Moderate magnitude (M6+) earthquakes are common in the vicinity of the October 6 earthquake; 21 earthquakes of M6 or larger occurred within 250 km of the October 6 earthquake in the past 100 years. Notably, an M7.1 thrust earthquake with similar faulting characteristics to the October 6 earthquake occurred 40 km to the southeast on February 27, 1997. The 1997 earthquake led to ~100 casualties and strong shaking in Quetta. West of the Sulaiman Mountains, an M7.2 earthquake occurred on August 27, 1931 75 km to the southwest of the October 6 earthquake. An M7.5 earthquake on May 30, 1935 located 205 km southwest of the October 6 earthquake caused widespread destruction in Quetta and approximately 20,000 fatalities.