Surface rupture and landscape response in the middle of the great Mw 8.3 1934 earthquake mesoseismal area: Khutti Khola site

Abstract

Large earthquakes breaking the frontal faults of the Himalayan thrust system produce surface ruptures, quickly altered due to the monsoon conditions. Therefore, the location and existence of the Mw8.3 1934 Bihar–Nepal surface ruptures remain vividly disputed. Even though, previous studies revealed remnants of this surface rupture at the western end of the devastated zone, ruptures extent remains undocumented in its central part. Evidence for recent earthquakes is revealed along the frontal thrust in this region. The Khutti Khola river cuts an 8 m-high fault scarp exposing Siwalik siltstone thrusted over recent alluvial deposits, with faults sealed by a colluvial wedge and undeformed alluvial sediments. Detrital charcoals radiocarbon dating reveals that the last event occurred between the seventeenth century and the post-bomb era, advocating for the 1934 earthquake as the most recent event. In the hanging wall, fluvial terraces associated with fault scarps were abandoned after a penultimate event that happened after the tenth century, a rupture we associate with the historic earthquake of 1255CE. Slips of 11–17 m and 14–22 m for the 1934 and 1255 earthquakes, respectively, compare well with the ~ 10–15 m slip deficit accumulated between the two earthquakes, suggesting that most of the deformation along the front is accommodated by surface-rupturing earthquakes.

Paleoseismological evidence for segmentation of the Main Himalayan Thrust in the Darjeeling-Sikkim Himalaya

“…Red arrows: convergence rate 1 . Paleoseismic investigation sites: (a) Sir Khola 4 , (b) Charnath Khola 17 , (c) Khutti Khola 18 , (d) Damak 7 , (e) Chenga (this study), (f) Panijhora 5 , (g) Chalsa 3 , (h) Piping and (i) Sarpang 6 , 19 . ( B ) Cross-section (AAʹ), with major structures, are the locked and creeping segments of the Indian Plate 27 , 28 .…”

“…Also, a considerable spatiotemporal gap exists in paleo-earthquake history in the DSH. However, it is enigmatic why great surface-rupturing events such as 1934 Bihar-Nepal 4 , 17 , 18 and 1714 Bhutan 6 , 19 are restricted to the periphery of the DSH bounded by the transverse structures. Towards this, we conducted a paleoseismological investigation at Chenga village in the DSH, India.…”

“…The segment between the MSR and DCFZ demarcates the probable break in the Himalayan segment and the rupture extent of the 1255 CE, 1714 CE and 1934 CE events. meisoseismal zones and possible hypocentre location (translucent blue) of great earthquakes in the past millennia 20 , 21 ; dark blue polygons are the inferred rupture area of paleoearthquakes; Locations of previous paleoseismic investigation sites in Nepal 4 , 7 , 17 , 18 , 28 , 37 , 38 , DSH 3 , 5 and Bhutan 6 , 19 , 39 region are allotted different colours that demarcate different segments. The vertical coloured bars are the age range of earthquake events inferred from numerous studies, and the bars with arrow head indicate no lower bound of the inferred earthquake, hence the event occurred post upper bounding age.…”