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

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 m...

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Autores principales:	Magali Riesner, Laurent Bollinger, Judith Hubbard, Cyrielle Guérin, Marthe Lefèvre, Amaury Vallage, Chanda Basnet Shah, Thakur Prasad Kandel, Samuel Haines, Soma Nath Sapkota
Formato:	article
Lenguaje:	EN
Publicado:	Nature Portfolio 2021
Materias:	Medicine R Science Q
Acceso en línea:	https://doaj.org/article/abbc33bc522840088783224c9544bcec
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spelling	oai:doaj.org-article:abbc33bc522840088783224c9544bcec2021-11-08T10:50:44ZLocalized extension in megathrust hanging wall following great earthquakes in western Nepal10.1038/s41598-021-00297-42045-2322https://doaj.org/article/abbc33bc522840088783224c9544bcec2021-11-01T00:00:00Zhttps://doi.org/10.1038/s41598-021-00297-4https://doaj.org/toc/2045-2322Abstract 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.Magali RiesnerLaurent BollingerJudith HubbardCyrielle GuérinMarthe LefèvreAmaury VallageChanda Basnet ShahThakur Prasad KandelSamuel HainesSoma Nath SapkotaNature PortfolioarticleMedicineRScienceQENScientific Reports, Vol 11, Iss 1, Pp 1-18 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	Medicine R Science Q
spellingShingle	Medicine R Science Q Magali Riesner Laurent Bollinger Judith Hubbard Cyrielle Guérin Marthe Lefèvre Amaury Vallage Chanda Basnet Shah Thakur Prasad Kandel Samuel Haines Soma Nath Sapkota Localized extension in megathrust hanging wall following great earthquakes in western Nepal
description	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.
format	article
author	Magali Riesner Laurent Bollinger Judith Hubbard Cyrielle Guérin Marthe Lefèvre Amaury Vallage Chanda Basnet Shah Thakur Prasad Kandel Samuel Haines Soma Nath Sapkota
author_facet	Magali Riesner Laurent Bollinger Judith Hubbard Cyrielle Guérin Marthe Lefèvre Amaury Vallage Chanda Basnet Shah Thakur Prasad Kandel Samuel Haines Soma Nath Sapkota
author_sort	Magali Riesner
title	Localized extension in megathrust hanging wall following great earthquakes in western Nepal
title_short	Localized extension in megathrust hanging wall following great earthquakes in western Nepal
title_full	Localized extension in megathrust hanging wall following great earthquakes in western Nepal
title_fullStr	Localized extension in megathrust hanging wall following great earthquakes in western Nepal
title_full_unstemmed	Localized extension in megathrust hanging wall following great earthquakes in western Nepal
title_sort	localized extension in megathrust hanging wall following great earthquakes in western nepal
publisher	Nature Portfolio
publishDate	2021
url	https://doaj.org/article/abbc33bc522840088783224c9544bcec
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Localized extension in megathrust hanging wall following great earthquakes in western Nepal

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