Mw=7.2

The earthquake released twice as much energy as the Jan. 2010 Haiti earthquake, and one quarter of the energy of the Mw7.6 Pakistan Kashmir earthquake of 2005. This Mw=7.2 is the largest known earthquake to have occurred historically within 250 km of the epicenter.

Because the earthquake occurred at a depth of 80 km surface accelerations were attenuated and shaking in the epicenter was not violent. It was, however, felt in Kabul, Delhi, Karachi, Teheran, Mumbai and Saudi Arabia at distances of more than 1000 km.

The region is sparsely inhabited consisting of many small villages on the trade route between Iran and Pakistan. Single floor adobe mud brick structures fared poorly in the earthquake and several villagers were injured by the collapse of roofs and walls.

Geodesy

A triangulation survey along the Chagai mountains (the Kalat Longitudinal series) was started in 1904 and completed in 1908. The Dalbandin series was measured southwards at 65 deg W between 1931 and 32 by Guy Bomford to link with the Makran Longitudinal series. It has a mean interstation distance of 64 km quite unsuited to deformation measurements. One line is distinguished as being the longest line-of-sight ever measured by the Survey of India (128 km). The closest precision GPS site to the earthquake is to be found at Panjgur, 200 km south of the epicenter.

 

 

 

 

 

 

 

 

Dalbandin

Tuesday, January 18, 2011 at 20:23:26 UTC, 28.838 deg. N, 63.947 deg E

The earthquake occurred at 1:30 in the morning local time in the descending Arabian plate south of the volcanic arc at a depth of ~80 km. It was followed by a Mw 4.6 aftershock in the over-riding Asian plate (≈60 km depth). The volcanoes to the north, which demarcate the border between Afghanistan and Baluchistan, are dormant but microearthquakes are common in their vicinity. At the latitude of southern Baluchistan the northward velocity pf the Arabian plate beneath the EuroAsian Plate is roughly 30 mm/yr. In the section below GPS data are from Triggnet: well-located depth solutions from Engdhal et al. (1998). Click on section for large view.

Mechanism

The earthquake ruptured a normal fault, that most likely fractured through the upper part of the descending Arabian plate where it is flexed downward by the over-riding Asian plate. It occurred 300 km north of the great 1945 Mw=8.0 earthquake, and about the same distance west of the Quetta 1935 Mw=7.6 earthquake. Since the 1945 earthquake the Arabian plate has shifted roughly 2 m northward beneath the Asian plate. USGS 19 Jan focal mechanism is reproduced below. Centroid coordinates below are less precise than the hypocenter calculation although that to is subject to some uncertainty.

References

Breitzman L.L., R. W Nirnie and G. D. Johnston, Fission track ages of the Chagai Intrusive Baluchistan, Pakistan,GSA Bulletin; February 1983; 94(2); p. 253-258

Engdahl, E.R., and A. Villasenor, Global Seismicity: 1900-1999, in W.H.K. Lee, H. Kanamori, P.C. Jennings, and C. Kisslinger (editors), International Handbook of Earthquake and Engineering Seismology, Part A, Chapter 41, pp. 665-690, Academic Press, 2002.

Engdahl, E.R., R. van der Hilst, and R. Buland, Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Seism. Soc. Am. 88, 722-743, 1998

Khan, M. A. R. Bendick, I.M. Bhat, R. Bilham, D.M. Kakar. S. F. Khan, S. H. Lodi, M. S. Qazi, B. Singh, W. Szeliga amd A. Wahab, Preliminary geodetic constraints on plate boundary deformation on the western edge of the Indian Plate from TriGGNnet, J. Himalayan Earth Sciences, 41 (2008), 71-87). Published 2010

Khan, M.A., Siddiqui, R.H. and Jan, M.Q., 2010, Temporal evolution of Cretaceous to Pleistocene magmatism in the Chagai Arc, Balochistan, Pakistan, in Leech, M.L., and others, eds., Proceedings for the 25th Himalaya-Karakoram-Tibet Workshop: U.S. Geological Survey, Open-File Report 2010-1099, 2 p. [http://pubs.usgs.gov/of/2010/1099/khan/].