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[1] fault in the world.[2]
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*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.[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. At this present moment, the MFT and MHT accounts for almost the entire rate of
convergence (15-21 mm/yr).[5][6] This fault defines where the India subcontinent is underthrust beneath the Himalayan orogenic wedge.
^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.
^National Geophysical Data Center / World Data Service (NGDC/WDS): NCEI/WDS Global Significant Earthquake Database. NOAA National Centers for Environmental Information (1972).
"Comments for the 1255 Earthquake". NOAA. National Geophysical Data Center, NOAA.
doi:
10.7289/V5TD9V7K. Archived from
the original on 2017-12-16. Retrieved 2020-10-17.
^Pant, C.C.; Pathak, V.; Joshi, S. (2016). "Extant Seismicity and Regional Tectonic Interpretation: An illustration from Kumaun Himalaya, Uttarakhand, India". In Singh, S.P.; Khanal, S.C.; Joshi, M. (eds.).
Lessons From Nepal's Earthquake For The Indian Himalayas And The Gangetic Plains(PDF). Central Himalayan Environment Association. p. 31.