Interseismic strain accumulation on the Himalayan crustal ramp (Nepal)

Abstract

Abstract. The Departement of Mines and Geology has been monitoring the seismicity of the Central Himalayas of Nepal since 1985. Intense microseismicity and frequent medium-size earthquakes (mL<4) tend to cluster beneath the topographic front of the Higher Himalaya. This 10-20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data. Both microseismic and geodetic data indicate strain accumulation on a mid-crustal ramp that had been previously inferred from geological and geophysical evidence.This ramp connects a flat decollement under the Lesser and SubHimalaya with a deeper decollement under the Higher Himalaya, and probably acts as a geometric asperity where strain and stress build up during the interseismic period. The large Himalayan earthquakes could nucleate there and probably activate the whole flat-and-ramp system up to the blind thrusts of the Sub-Himalaya.

Constraints on Source Parameters of the 25 April 2015, M_w = 7.8 Gorkha, Nepal Earthquake from Synthetic Aperture Radar Interferometry

“…As a consequence, the locked portion of the MHT along the mid-crustal ramp could have yielded the Gorkha earthquake that propagated east-southeast of the hypocentre 22 . The high conductivity zone characterized by intense micro-seismicity deduced from magnetotelluric and seismological data also supports the role of midcrustal ramp, which must have acted as a barrier for stress build-up in central Nepal Himalaya 13,42 . Nevertheless, we suggest that the combination of unreleased background store of energy and the strain energy accumulated since the release of high tectonic stresses associated with the 1934 Nepal-Bihar earthquake would have triggered the 25 April 2015 Gorkha, Nepal earthquake.…”

“…The elastic strain energy thus stored during the interseismic period is released periodically due to earthquakes causing rupture along the interseismically locked, brittle upper part of the MHT system beneath the outer and lesser Himalaya, which is characterized by a southern frontal ramp (MFT) 6,7,19,20 . Whereas, aseismic slip induces stress accumulation at zones beneath the higher Himalaya, which triggers intense micro-seismic activity and elastic strain in the upper crust 13 . Hence, large-magnitude earthquakes are mostly confined to the basal thrust of MHT, which is close to the locked zone 21 .…”

Estimation of the source process of the 2015 Gorkha, Nepal, earthquake and simulation of long-period ground motions in the Kathmandu basin using a one-dimensional basin structure model

“…In the Himalayan continental collision region, a belt of microseismicity has been observed (e.g., Pandey et al 1995Pandey et al , 1999, which corresponds to the creeping-locked transition (e.g., Ader et al 2012). From the microseismicity belt to the surface, the Main Himalayan thrust fault is considered to be fully coupled and it has been predicted that large interplate earthquakes will occur (e.g., Bilham et al 1997;Ader et al 2012).…”

Earthquake loss estimation for the Kathmandu Valley

“…The main cause of earthquakes in Nepal is due to the subduction of the Indian plate underneath the Eurasian plate. The major damaging earthquakes in Nepal took place in the years of 1255, 1408, 1681, 1803, 1810, 1833, 1934and 1988(Bilham et al, 1995Pandey et al, 1995). As presented in figure1, Nepal and adjoining Himalayan arc has experienced some great historical earthquakes including the 1897 Shillong earthquake, 1905 Kangara earthquake, 1934 Bihar-Nepal earthquake, and 1950 Assam earthquake.…”