# Main Himalayan Thrust

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

## Overview

The MHT accommodates crustal shortening of India and Eurasia as a result of the ongoing collision between the Indian and Eurasian plates. 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*10^19 Nm/yr. The MHT also remains locked with the overlying Eurasian plate from its surface expression to the front of the higher Himalayas, nearly 100km. 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. 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. At this present moment, the MFT and MHT accounts for almost the entire rate of convergence (15-21 mm/yr). This fault defines where the India subcontinent is underthrust beneath the Himalayan orogenic wedge.

## Seismic hazard

The 'MHT' is a known hazard and potential source for large earthquakes of magnitude 9.0 or greater. The MHT is also associated with other large 20th century earthquakes in 1950 (M$_w$  8.7) and 1934 (M$_w$  8.4). 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.
In April 2015, a section of the MHT produced a blind rupture earthquake, killing nearly 9,000 people. 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.

## 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 |
| 1255-06-07 | Nepal | 8.0+ | - | | | Rupture length uncertain but possibly in the hundreds of kilometers. Killed one-third of Nepal's population. |
| 1344-09-14 | Nepal | | - | | | |
| 1408 | Nepal | | - | | | |
| 1505-06-06 | Nepal, India and China | 8.2–8.8 | - | | | Killed 30% of the Nepalese population. |
| 1680 | Nepal | <7.5 | - | | | |
| 1714-05-4 | Bhutan | 7.6–8.6 | - | IX | "Many" | Ruptured the whole Bhutan section of the Main Frontal Thrust. |
| 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. |
| 1905-04-04 | India | 7.9 | - | X | 20,000+ | |
| 1934-01-05 | Nepal and India | 8.1 | 15.0 | XI | 12,000 | Ruptured to the surface via the Main Frontal Thrust. |
| 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. |
| 1966-06-27 | Nepal and India | 6.1 | 37.0 | | 80 | |
| 1980-07-29 | Nepal and India | 6.5 | 17.5 | VIII | 200 | |
| 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 | |
| 2009-09-21 | Bhutan | 6.1 | 14.0 | VI | 11 | |
| 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. |
| 2015-04-25 | Nepal | 7.8 | 8.2 | VIII | 8,964 | |
| 2015-05-12 | Nepal | 7.3 | 18.5 | VIII | 218 | Aftershock of the April 2015 earthquake. |
| 2015-07-24 | Pakistan | 5.1 | 17.0 | V | 3 | |
| 2019-09-24 | Pakistan | 6.0 | 10.0 | VII | 40 | |