Main Himalayan Thrust

The Main Himalayan Thrust (MHT) is a décollement under the Himalaya Range. This thrust fault follows a northwest-southeast strike, reminiscent of an arc, and gently dips about 10 degrees towards the north, beneath the region. It is the largest active continental megathrust^[1] fault in the world.^[2]

Main Himalayan Thrust
Location	Himalayas
Characteristics
Length	>2,000km
Strike	northwest-southeast
Tectonics
Status	Active
Type	Thrust fault

A geological map of the Himalaya region. The Main Himalayan Thrust underlies the rock units.

Diagram showing a décollement

Overview

The MHT accommodates crustal shortening of India and Eurasia as a result of the ongoing collision between the Indian and Eurasian plates.^[3] The MHT absorbs around 20mm/yr of slip, nearly half of the total convergence rate. This slip can be released from small-scale earthquakes and some plastic deformation, but the MHT still accumulates a deficit of moment of 6.6×10¹⁹ Nm/yr. The MHT also remains locked with the overlying Eurasian plate from its surface expression to the front of the higher Himalayas, nearly 100 kilometres away. This locking mechanism combined with the rapid accumulation of deficit of moment are concerning, as some professionals estimate that earthquakes up to the size of 8.9 on the Richter scale could be in order for regions such as western Nepal. Earthquakes of this magnitude are estimated to have a return period of over 1000 years in this region.^[4] Deformation of the crust is also accommodated along splay structures including the Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT), and possibly the South Tibetan Detachment. The MHT is the root detachment of these splays. Currently, the MFT and MHT accounts for almost the entire rate of convergence (15–21 mm/yr).^[5][6] This fault defines where the Indian subcontinent is underthrust beneath the Himalayan orogenic wedge.

Seismic hazard

The MHT is a known hazard and potential source for large earthquakes of magnitude 8.0 or greater. The MHT is also associated with other large 20th-century earthquakes in [[1950 Assam–Tibet earthquake|1950 (M_w  8.7)]] and [[1934 Nepal–India earthquake|1934 (M_w  8.0)]]. Within the last thousand years, multiple earthquakes have occurred with magnitudes of at least M_w  8.0, as deduced by paleoseismology. Michel et al. (2021) suggested the maximum magnitude possible on the MHT to be M_w  8.7 with a recurrence interval of 200 years.^[7]

In April 2015, a section of the MHT produced a blind rupture earthquake, killing nearly 9,000 people.^[8][9] Researchers who published their findings in Nature Geoscience revealed that the M_w  7.8 earthquake failed to rupture towards the surface, with the possibility of future large earthquakes. They said that since the rupture ceased 11 km (6.8 mi) beneath the Kathmandu region, a shallow section of the MHT, south of Kathmandu, remains unruptured. The shallow section remains locked and could produce an earthquake of comparable size. The research lead, J. R. Elliott, says such an earthquake could be more devastating because of its shallowness.^[10]

Associated seismicity

The Main Himalayan Thrust and its splay branches has been the source of numerous earthquakes, including some that are indirectly related.

Date	Country	Magnitude	Depth (km)	MMI	Deaths	Comments	Source
1255-06-07	Nepal	8.0+	-			Rupture length uncertain but possibly in the hundreds of kilometers. Killed one-third of Nepal's population.	[11][12]
1344-09-14	Nepal		-				[13]
1408	Nepal		-				[13]
1505-06-06	Nepal, India and China	8.2–8.8[14][15]	-			Killed 30% of the Nepalese population.	[16]
1680	Nepal	<7.5	-				[13]
1714-05-4	Bhutan	7.6–8.6	-	IX	"Many"	Ruptured the whole Bhutan section of the Main Frontal Thrust.	[17]
1803-09-01	India	7.8–7.9	-	IX	300	Damage as far as New Delhi.
1833-08-26	Nepal	7.5–7.9	-	IX	500	Severely damaged Kathmandu and was felt as far as Calcutta.	[18]
1905-04-04	India	7.9	-	X	20,000+		[19]
1934-01-05	Nepal and India	8.1	15.0	XI	12,000	Ruptured to the surface via the Main Frontal Thrust.	[20]
1947-07-29	China	7.3	20.0	V
1950-08-15	India, China and Myanmar	8.6	15.0	XI	4,800	Ranks among the largest Strike-slip earthquake ever instrumentally recorded.	[21]
1966-06-27	Nepal and India	6.1	37.0		80		[22]
1980-07-29	Nepal and India	6.5	17.5	VIII	200		[23]
1988-08-21	Nepal	6.9	57.4	VIII	700-1400
1991-10-20	India	6.8	10.3	IX	2000	Main Central Thrust.
1999-03-29	India	6.8	21.0	VII	103
2005-10-08	Pakistan	7.6	26.0	XI	87,400		[24]
2009-09-21	Bhutan	6.1	14.0	VI	11		[25]
2011-09-18	India	6.9	50.0	VII	111	Intraplate strike-slip.
2013-05-01	Pakistan and India	5.7	15.0	VII	1	Additional 59 injured.	[26]
2015-04-25	Nepal	7.8	8.2	VIII	8,964		[8]
2015-05-12	Nepal	7.3	18.5	VIII	218	Aftershock of the April 2015 earthquake.	[3]
2015-07-24	Pakistan	5.1	17.0	V	3		[27]
2019-09-24	Pakistan	6.0	10.0	VII	40		[28]

See also

• Geology of the Himalaya