Faulting structure above the Main Himalayan Thrust as shown by relocated aftershocks of the 2015 M_w7.8 Gorkha, Nepal, earthquake

Abstract

The 25 April 2015, Mw7.8 Gorkha, Nepal, earthquake ruptured a shallow section of the Indian‐Eurasian plate boundary by reverse faulting with NNE‐SSW compression, consistent with the direction of current Indian‐Eurasian continental collision. The Gorkha main shock and aftershocks were recorded by permanent global and regional arrays and by a temporary local broadband array near the China‐Nepal border deployed prior to the Gorkha main shock. We relocate 272 earthquakes with Mw>3.5 by applying a multiscale double‐difference earthquake relocation technique to arrival times of direct and depth phases recorded globally and locally. We determine a well‐constrained depth of 18.5 km for the main shock hypocenter which places it on the Main Himalayan Thrust (MHT). Many of the aftershocks at shallower depths illuminate faulting structure in the hanging wall with dip angles that are steeper than the MHT. This system of thrust faults of the Lesser Himalaya may accommodate most of the elastic strain of the Himalayan orogeny.

Significant lateral dip changes may have limited the scale of the 2015 Mw 7.8 Gorkha earthquake

“…For the Gorkha earthquake, however, aftershocks do not suggest a clear geometric shape of the fault. Instead, most events are above the MHT [ Bai et al , ], making them difficult to use for rebuilding the fault geometry. Fortunately, high‐quality geodetic data are available, offering us the opportunity to investigate the fault geometry of the earthquake.…”

The 2015 Gorkha earthquake: A large event illuminating the Main Himalayan Thrust fault

“…The bootstrap results indicate centroid depths shallower than 15 km (except for the 25 April aftershock associated with large uncertainty). These estimates are globally consistent with aftershock locations from Adhikari et al [] shown in Figure d and relocations from Bai et al [] showing that more than 65% of aftershocks occurred at depth between 10 and 15 km. Such bootstrap analysis does not fully capture the uncertainty due to inaccuracies in the Earth model used to compute the Green's functions.…”

Geometry of the Main Himalayan Thrust and Moho beneath Satluj valley, northwest Himalaya: Constraints from receiver function analysis

“…The MHT is mainly inferred from source model studies of large and great earthquakes [e.g., Seeber and Armbruster , ; Ni and Barazangi , ; Avouac et al , ; Bai et al , ; Duputel et al , ;] and using balanced geological cross sections [e.g., Schelling and Arita , ; Powers et al , ; Srivastava and Mitra , ]. Some geophysical studies including active [e.g., Prasad et al , ; Gao et al , ] as well as passive source seismic experiments [ Nábělek et al , ; Schulte‐Pelkum et al , ; Acton et al , ; Caldwell et al , ] and magnetotelluric (MT) studies [ Lemonnier et al , ; Unsworth et al , ; Rawat et al , ] were conducted to investigate the geometry of the MHT in different parts of the Himalaya.…”