EVIDENCE FOR A LARGE TO VERY LARGE EARTHQUAKE AT ~AD 1100 ON THE HIMALAYAN MAIN FRONTAL THRUST, NEPAL

The Himalayan orogen has produced four Mw>7.8 thrust earthquakes during the past century. Despite their great magnitude, no surface ruptures associated with these events have been documented. We present evidence from outcrop and trench exposures in the Mahra Khola region of east-central Nepal that a large to very large earthquake ruptured the Main Frontal Thrust fault (MFT) at ~A.D. 1100, with a displacement of ~17 +6/-3 m close to the surface. In contrast, no evidence exists for surface rupture in this region during the 1934 Bihar Nepal Mw8.4 earthquake. Warping of Holocene terraces to the west of the study area indicates that most of the shortening across the Himalayas is expressed in a frontal fold and that this folding probably results from strain transfer by large earthquakes. Based on paleoseismologic results in southeast Nepal (Nakata et.al., 1998), we hypothesize that the lateral extent of the ~A.D. 1100 event could have exceeded 300 km, and that its size could have reached ~Mw9.0, similar to megathrust earthquakes in subduction zones. In this scenario, the uppermost 2 km of the MFT represents a zone of velocity strengthening through which only very large earthquakes can break. Ruptures associated with Mw<8.5 events would therefore die out upward, with strain closer to the surface transferring to the frontal Himalayan fold. Although less probable, we can not discard an opposite end-member hypothesis, a Chi-Chi type earthquake, for which high local values (>10 m) of surface displacement are associated with Mw<8 earthquakes that have lateral extents of <100 km and activate only the shallow, frontal part of the decollement. Both hypotheses imply significant modifications to current seismotectonic models based on the historical seismicity and have major implications for estimating seismic hazard, modeling seismic rupture, and conducting paleoseismologic trench studies of large thrust systems.