Localized extension in megathrust hanging wall following great earthquakes in western Nepal

Magali Riesner,Amaury Vallage,Judith Hubbard,Soma Nath Sapkota,Chanda Basnet Shah,Laurent Bollinger,Cyrielle Guérin,Samuel Haines,Marthe Lefèvre,Thakur Prasad Kandel

doi:10.1038/s41598-021-00297-4

Abstract

The largest (M8+) known earthquakes in the Himalaya have ruptured the upper locked section of the Main Himalayan Thrust zone, offsetting the ground surface along the Main Frontal Thrust at the range front. However, out-of-sequence active structures have received less attention. One of the most impressive examples of such faults is the active fault that generally follows the surface trace of the Main Boundary Thrust (MBT). This fault has generated a clear geomorphological signature of recent deformation in eastern and western Nepal, as well as further west in India. We focus on western Nepal, between the municipalities of Surkhet and Gorahi where this fault is well expressed. Although the fault system as a whole is accommodating contraction, across most of its length, this particular fault appears geomorphologically as a normal fault, indicating crustal extension in the hanging wall of the MHT. We focus this study on the reactivation of the MBT along the Surkhet-Gorahi segment of the surface trace of the newly named Reactivated Boundary Fault, which is ~ 120 km long. We first generate a high-resolution Digital Elevation Model from triplets of high-resolution Pleiades images and use this to map the fault scarp and its geomorphological lateral variation. For most of its length, normal motion slip is observed with a dip varying between 20° and 60° and a maximum cumulative vertical offset of 27 m. We then present evidence for recent normal faulting in a trench located in the village of Sukhetal. Radiocarbon dating of detrital charcoals sampled in the hanging wall of the fault, including the main colluvial wedge and overlying sedimentary layers, suggest that the last event occurred in the early sixteenth century. This period saw the devastating 1505 earthquake, which produced ~ 23 m of slip on the Main Frontal Thrust. Linked or not, the ruptures on the MFT and MBT happened within a short time period compared to the centuries of quiescence of the faults that followed. We suggest that episodic normal-sense activity of the MBT could be related to large earthquakes rupturing the MFT, given its proximity, the sense of motion, and the large distance that separates the MBT from the downdip end of the locked fault zone of the MHT fault system. We discuss these results and their implications for the frontal Himalayan thrust system.

Highlights

The largest (M8+) known earthquakes in the Himalaya have ruptured the upper locked section of the Main Himalayan Thrust zone, offsetting the ground surface along the Main Frontal Thrust at the range front
There are reactivated segments of the Main Boundary Thrust system that may be the onshore analog of landward-dipping normal faults observed in accretionary wedges slipping in association with large earthquakes
The first-order scale of the normal fault in Japan is comparable to the one we describe for the Himalayan frontal system

Summary

Introduction

The largest (M8+) known earthquakes in the Himalaya have ruptured the upper locked section of the Main Himalayan Thrust zone, offsetting the ground surface along the Main Frontal Thrust at the range front. The largest earthquakes on megathrust fault systems propagate from the brittle-ductile transition zone to the surface, rupturing the ground along the frontal fault[1,2,3] Such large earthquakes can cause significant offfault deformation during the co- and post-seismic periods due to the large stress changes induced by the main slip event, observed as slip on secondary faults within the hanging wall (e.g. following the 1905 Kangra and 2011 Tohoku-Oki e arthquakes[4,5]). In the present fault geometry, direct observations of active deformation are limited, as we see only a portion of the seismic cycle, but in general elastic deformation accumulates around the base of the fault system during the interseismic period, and is released via updip slip as large (M7) and great (M8+) earthquakes rupturing the interseismically coupled parts of the MHT (e.g.22–25, Fig. 1b) These slip events sometimes breach the surface along the most frontal fault, the M FT20–23,26,31–34, some events remain blind (e.g. Mw7.8 2015 Gorkha)

Discussion

Conclusion