Yasuhiro Kumahara

Large Paleoearthquake Timing and Displacement near Damak in Eastern Nepal on the Himalayan Frontal Thrust

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 the displacement is the result of a single earthquake of 11.3 ± 3.5 m of dip-slip displacement that occurred 1146 – 1256 AD. Empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 - > 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 AD earthquake to produce another earthquake displacement of similar size.

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

Reinvestigation reveals observations that do not support prior claims that the great Mw 8.4 Bihar-Nepal earthquake produced surface rupture along the Himalayan Frontal Thrust of Nepal. While it may be viewed as reasonable to suggest that the Main Himalayan Frontal Thrust was the source of the 1934 Bihar-Nepal earthquake on geophysical grounds, decisive and substantiating geological evidence that it produced surface rupture along the trace of the Himalayan Frontal Thrust remains lacking. Plain Language Summary Great earthquakes on continents such as the Mw 8.4 Bihar-Nepal earthquake of 1934 are generally expected to produce ruptures along a fault trace where the causative fault intersects the ground surface. The 1934 earthquake for a long time remained enigmatic because surface ruptures were never reported until recently when investigators interpreted an outcrop along the Himalayan front to record such evidence. Our reinvestigation of the outcrop and presentation of new observations does not support their interpretation and so the enigma remains: there are no observations that clearly confirm that the 1934 earthquake produced surface rupture.