Estimating the return times of great Himalayan earthquakes in eastern Nepal: Evidence from the Patu and Bardibas strands of the Main Frontal Thrust

Abstract

The return times of large Himalayan earthquakes are poorly constrained. Despite historical devastation of cities along the mountain range, definitive links between events and specific segments of the Main Frontal Thrust (MFT) are not established, and paleoseismological records have not documented the occurrence of several similar events at the same location. In east central Nepal, however, recently discovered primary surface ruptures of that megathrust in the A.D. 1255 and 1934 earthquakes are associated with flights of tectonically uplifted terraces. We present here a refined, longer slip history of the MFT's two overlapping strands (Patu and Bardibas Thrusts) in that region, based on updated geomorphic/neotectonic mapping of active faulting, two 1.3 km long shallow seismic profiles, and logging of two river-cut cliffs, three paleoseismological trenches, and several pits, with constraints from 74 detrital charcoals and 14 cosmogenic nuclide ages. The amount of hanging wall uplift on the Patu thrust since 3650 ± 450 years requires three more events than the two aforementioned. The uplift rate (8.5 ± 1.5 mm/yr), thrust dip (25°± 5°N), and apparent characteristic behavior imply 12-17.5 m of slip per event. On the Bardibas thrust, discrete pulses of colluvial deposition resulting from the coseismic growth of a flexural fold scarp suggest the occurrence of six or seven paleo-earthquakes in the last 4500 ± 50 years. The coeval rupture of both strands during great Himalayan earthquakes implies that in eastern Nepal, the late Holocene return times of such earthquakes probably ranged between 750 ± 140 and 870 ± 350 years.

Unprecedented Drought in North East India Compared to Western India

“…The M 7.8 earthquake was not completely unexpected in the Central Nepal region, as several studies had indicated the likelihood of earthquakes of magnitude greater than 8.0 based on the slip deficit estimation and accumulation of strain energy in the region. This has been anticipated in early 1990s and further confirmed by recent studies [2][3][4][5][6][7] . As shown in Figure 2, the major part of Nepal, including Kathmandu, lies in zone A on the seismic zoning map of Nepal 8,9 , whereas the districts of north Bihar adjoining the Nepal border lie in zones IV and V on the Indian seismic zone map 10 .…”

Geotechnical Effects of the 2015 Magnitude 7.8 Gorkha, Nepal, Earthquake and Aftershocks

“…Paleoseismic data suggest that earthquakes in 1255 and 1934 ruptured to the surface along the MFT, south of Kathmandu (Bollinger et al, 2014). Field observations after the 2015 rupture, which nucleated on MHT (Avouac et al, 2015), suggest the rupture only extended to the base of the MFT.…”

Investigation of Coulomb stress changes in south Tibet (central Himalayas) due to the 25th April 2015 M W 7.8 Nepal earthquake using a Coulomb stress transfer model

“…The MCT is a series of thrusts separating the High Himalaya from the Lesser Himalaya (ibid.). Both the MBT and the MCT dip northward are no longer very active (Takada and Matsu'ura 2007); only the youngest, southernmost thrust (MFT) appears to be active in central Nepal (Bollinger et al 2014). Some studies have also suggested out-ofsequence thrusting, with possible thrust fault reactivation in the MCT zone (e.g., Hodges et al 2004;Seeber and Gornitz 1983).…”