Large paleoearthquake timing and displacement near Damak in eastern Nepal on the Himalayan Frontal Thrust

Abstract

An excavation across the Himalayan Frontal Thrust near Damak in eastern Nepal shows displacement on a fault plane dipping ~22° has produced vertical separation across a scarp equal to 5.5 m. Stratigraphic, structural, geometrical, and radiocarbon observations are interpreted to indicate that the displacement is the result of a single earthquake of 11.3 ± 3.5 m of dip‐slip displacement that occurred 1146–1256 A.D. Empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 to >800 km and moment magnitudes Mw of 8.6 to >9. Sufficient strain has accumulated along this portion of the Himalayan arc during the roughly 800 years since the 1146–1256 A.D. earthquake to produce another earthquake displacement of similar size.

Structural Control on Downdip Locking Extent of the Himalayan Megathrust

“…It has been suggested that several Gorkha‐type moderate earthquakes may be required to sufficiently load the upper part of the fault here, which may explain why the last two large earthquakes occurred only on the deeper, blind part of the fault in 1833 and 2015 (Bilham et al, ). By comparison, in eastern Nepal paleoseismic evidence suggests that the past two recorded events (in A.D. 1255 and 1934) ruptured all the way to the surface (Bollinger et al, ; Nakata et al, ; Sapkota et al, ; Wesnousky et al, ). Earthquake cycle models incorporating realistic fault geometry have shown that an upper ramp can potentially control the occurrence of partial ruptures (Qiu et al, ).…”

Post Earthquake Aggradation Processes to Hide Surface Ruptures in Thrust Systems: The M8.3, 1934, Bihar‐Nepal Earthquake Ruptures at Charnath Khola (Eastern Nepal)

“…Although the 1344 AD earthquake was very destructive (Pant, ), its surface rupture is usually located farther west along the MFT (Bollinger et al, ; Mugnier et al, ). Instead, surface ruptures associated with 1255 have been documented both west of the Charnath site, at ~20km at the Sir Khola (Bollinger et al, , ; Sapkota et al, ), and at ~65 km at Bagmati (Wesnousky, Kumahara, Chamlagain, Pierce, Reedy, et al, ), and ~160‐km eastward at Damak (Wesnousky, Kumahara, Chamlagain, Pierce, Karki, et al, ). Hence, the Charnath site is located within the area of potential surface rupture of the 1255 AD earthquake, and for this reason, we suggest that this earthquake is likely responsible for the abandonment of Pal‐T4.…”

“…In addition, trenches often present a limited amount of age control, making correlation with historical events difficult. Thus, occurrence and spatial extent of surface ruptures for historical earthquakes, including the Mw 8.3, 1934, Bihar‐Nepal event, remain vividly debated (Bollinger et al, , ; Hossler et al, ; Lavé et al, ; Le Roux‐Mallouf et al, ; Mugnier et al, ; Rajendran et al, ; Sapkota et al, ; Wesnousky, Kumahara, Chamlagain, Pierce, Karki, et al, ; Wesnousky, Kumahara, Chamlagain, Pierce, Reedy, et al, ; Wesnousky et al, ).…”

New Observations Disagree With Previous Interpretations of Surface Rupture Along the Himalayan Frontal Thrust During the Great 1934 Bihar‐Nepal Earthquake

“…The lack of a fault scarp and folding in young sediments above the trace of the HFT at Sir Khola (supporting information Figure S3) is conspicuous and inconsistent with all other paleoearthquake trench sites that have reported evidence of earthquakes occurring in the last century (e.g., Kumar et al, , ; Lave et al, ; Mugnier et al, ; Wesnousky, Kumahara, Chamlagain, Pierce, Karki, & Gautam, ; Wesnousky, Kumahara, Chamlagain, Pierce, Reedy, et al, ; Yule et al, ). Each is invariably associated with a fault scarp produced by displacement on the HFT commensurate with the size of an earthquake on the same order as the Mw 8.4 Bihar‐Nepal earthquake.…”

“…The length of surface rupture along the HFT for the 1934 event indicated in Sapkota et al () and Bollinger et al () is red. Additional citations in figure include Kumar et al (, ), Yule et al (), Hossler et al (), Wesnousky, Kumahara, Chamlagain, Pierce, Karki, and Gautam (); Wesnousky, Kumahara, Chamlagain, Pierce, Reedy, et al (), Lave et al (), Upreti et al (), Rajaure et al (), Ader et al (), NSC (), and Stevens and Avouac ().…”