Main Himalayan Thrust
{{Short description|Geological feature}}
{{Infobox fault
|name=Main Himalayan Thrust
|location=Himalayas
|type=Thrust fault
|length=>2,000km
|earthquakes=
|status=Active
|strike=northwest-southeast
}}
File:2 2 himal tecto units.png
File:Compressional decollement.svg
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{{cite journal |last1=Sieh |first1=Kerry |title=The Sunda megathrust - Past, present and future |journal=Journal of Earthquake and Tsunami |date=2007 |volume=1 |page=1-19 |url=http://tecto.caltech.edu/sumatra/downloads/papers/Snu.pdf |publisher=California Institute of Technology |doi=10.1142/S179343110700002X |issn=1793-4311}} fault in the world.{{Cite journal |last1=Liu |first1=Y. |last2=Hubbard |first2=J. |last3=Almeida |first3=R.V. |last4=Foster |first4=A. |last5=Liberty |first5=L. |last6=Lee |first6=Y.S. |last7=Sapkota |first7=S.N. |date=2020 |title=Constraints on the Shallow Deformation Around the Main Frontal Thrust in Central Nepal from Refraction Velocities |url=https://scholarworks.boisestate.edu/cgi/viewcontent.cgi?article=1511&context=geo_facpubs |journal=Tectonophysics |volume=777 |page=228366 |doi=10.1016/j.tecto.2020.228366 |via=Boise State University|doi-access=free |bibcode=2020Tectp.77728366L |hdl=10356/143621 |hdl-access=free }}
Overview
The MHT accommodates crustal shortening of India and Eurasia as a result of the ongoing collision between the Indian and Eurasian plates.{{Cite journal |last1=Hubbard |first1=J. |last2=Almeida |first2=R. |last3=Foster |first3=A. |last4=Sapkota |first4=S.N. |last5=Bürgi |first5=P. |last6=Tapponnier |first6=P. |date=2016 |title=Structural segmentation controlled the 2015 Mw 7.8 Gorkha earthquake rupture in Nepal |url=https://www.researchgate.net/publication/305269615 |journal=Geology |volume=44 |issue=8 |pages=639–642 |doi=10.1130/G38077.1|doi-access=free |bibcode=2016Geo....44..639H |hdl=10220/50430 |hdl-access=free }} 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 {{Nowrap|{{val|6.6e19}} 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.Ader, Thomas, et al. “Convergence Rate across the Nepal Himalaya and Interseismic Coupling on the Main Himalayan Thrust: Implications for Seismic Hazard.” Journal of Geophysical Research: Solid Earth, vol. 117, no. B4, 2012, {{doi|10.1029/2011jb009071}}. 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).{{Cite journal |last1=Le Roux-Mallouf |first1=R. |last2=Godard |first2=V. |last3=Cattin |first3=R. |last4=Ferry |first4=M. |last5=Gyeltshen |first5=J. |last6=Ritz |first6=J.–F. |last7=Drupka |first7=D. |last8=Guillou |first8=V. |last9=Arnold |first9=M. |last10=Aumaitre |first10=G. |last11=Bourlès |first11=D.L. |last12=Keddadouche|first12=K.|date=2015 |title=Evidence for a wide and gently dipping Main Himalayan Thrust in western Bhutan |url=http://www.gm.univ-montp2.fr/spip/IMG/pdf/cattingrl15.pdf |journal=Geophysical Research Letters |volume=42 |issue=9 |pages=3257–3265 |doi=10.1002/2015GL063767|bibcode=2015GeoRL..42.3257L |s2cid=30608419 }}{{Cite web |last=Hubbard |first=J. |title=Geometry and Kinematics of the Main Frontal Thrust, Himalaya |url=https://earthobservatory.sg/project/geometry-and-kinematics-main-frontal-thrust-himalaya |access-date=27 October 2020 |website=Earth Observatory of Singapore}} 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 and 1934 Nepal–India earthquake. 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.{{cite journal |last1=Michel |first1=Sylvain |last2=Jolivet |first2=Romain |last3=Rollins |first3=Chris |last4=Jara |first4=Jorge |last5=Zilio |first5=Luca Dal |title=Seismogenic Potential of the Main Himalayan Thrust Constrained by Coupling Segmentation and Earthquake Scaling |journal=Geophysical Research Letters |date=2021 |volume=48 |issue=13 |doi=10.1029/2021GL093106|pmc=9285541 }}
In April 2015, a section of the MHT produced a blind rupture earthquake, killing nearly 9,000 people.{{Cite web|last=|first=|date=|title=M 7.8 - 36km E of Khudi, Nepal|url=https://earthquake.usgs.gov/earthquakes/eventpage/us20002926/executive|url-status=dead|archive-url=https://web.archive.org/web/20200813083448/https://earthquake.usgs.gov/earthquakes/eventpage/us20002926/executive|archive-date=2020-08-13|access-date=|website=US Geological Survey}}{{Cite journal |last1=Duputel |first1=Z. |last2=Vergne |first2=J. |last3=Rivera |first3=L. |last4=Wittlinger |first4=G. |last5=Farra |first5=V. |last6=Hetényi |first6=G. |date=2016 |title=The 2015 Gorkha earthquake: A large event illuminating the Main Himalayan Thrust fault |url=https://core.ac.uk/download/pdf/77175162.pdf |journal=Geophysical Research Letters |volume=43 |issue=6 |pages=2517–2525 |doi=10.1002/2016GL068083|bibcode=2016GeoRL..43.2517D |s2cid=53463752 }} 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 {{cvt|11|km}} 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.{{cite news |last1=Koirala |first1=Keshav P. |title=Nepal earthquake "damaged Main Himalayan Thrust fault" |url=https://thehimalayantimes.com/science-and-tech/nepal-earthquake-has-ruptured-main-himalayan-thrust-fault-study |access-date=15 May 2024 |work=The Himalayan |date=12 January 2016}}
Associated seismicity
The Main Himalayan Thrust and its splay branches has been the source of numerous earthquakes, including some that are indirectly related.