Seismicity in far western Nepal reveals flats and ramps along the Main Himalayan Thrust

Abstract

Summary Unravelling relations between lateral variations of mid-crustal seismicity and the geometry of the Main Himalayan Thrust system at depth is a key issue in seismotectonic studies of the Himalayan range. These relations can reveal along strike changes in the behavior of the fault at depth related to fluids or the local ramp-flat geometry and more generally of the stress build-up along the fault. Some of these variations may control the rupture extension of intermediate, large or great earthquakes, the last of which dates back from 1505 CE in far western Nepal. The region is also associated to lateral spatio-temporal variations of the mid-crustal seismicity monitored by the Regional Seismic Network of Surkhet-Birendranagar. This network was supplemented between 2014 and 2016 by 15 temporary stations deployed above the main seismic clusters giving new potential to regional studies. Both absolute and relative locations together with focal mechanisms are determined to gain insight on the fault behavior at depth. We find more than 4000 earthquakes within 5 and 20 km-depth clustered in three belts parallel to the front of the Himalayan range. Finest locations reveal close relationships between seismic clusters and fault segments at depth among which mid-crustal ramps and reactivated tectonic slivers. Our results support a geometry of the Main Himalayan Thrust involving several fault patches at depth separated by ramps and tear faults. This geometry most probably affects the pattern of the coseismic ruptures breaking partially or totally the locked fault zone as well as eventual along strike variations of seismic coupling during interseismic period.

3D Shear‐Wave Velocity Structure of the Crust and Upper Mantle Beneath India, Himalaya and Tibet

“…The Moho depths from the V s models are assigned to the bottom of the steepest gradient layer within V s values of 4.1 and 4.5 km s −1 . These broadly corresponds to the Moho depth obtained from previous studies using various seismological techniques (Figure 5a) (Acton et al., 2010; Laporte et al., 2021; Priestley et al., 2019; Singh et al., 2015). To qualitatively assess the isostatic compensation of long‐wavelength structures, we compute the Moho depth from Airy's isostatic compensation using topography and density structure derived from the V s models (Supporting Information S2).…”

Seismically active structures of the Main Himalayan Thrust revealed before, during and after the 2015 Mw 7.9 Gorkha earthquake in Nepal

“…This mid-crustal seismicity exhibits significant lateral variations (Fig. 1a) including (1) a large spread of mid-crustal earthquakes in western Nepal (Pandey et al 1999 ;Hoste-Colomer et al 2018, Laporte et al 2021, (2) a narrow and straight band of seismicity which develops below the southern slopes of the high Himalayan range in central Nepal (Pandey et al 1995) and (3) a more complex zone of seismicity spread between the Moho of the India crust and mid-crustal and shallow clusters in eastern Nepal (Monsalve et al 2006).…”

Seismicity acceleration and clustering before the Mw7.9 Gorkha earthquake, Nepal

“…This body of evidence suggests that aqueous fluids -or supercritical CO 2 -are present at midcrustal depths in the vicinity of the decollement, with the potential to migrate within the fractured rocks of the shear zone as well as along subsidiary faults in its immediate hangingwall [44]. These fluids migrations could be related with hydrofracturation mechanisms and/or decrease of the friction on the basal decollement, possibly associated to transient decoupling.…”