Paleoseismology along the Himalayan Frontal Thrust (HFT) of Nepal after the Mw7.8 2015 Gorkha earthquake
Abstract
The Himalayan Frontal Thrust (HFT) dips northward from its trace to merge with the Main Himalayan Thrust (MHT) that extends 120 km to beneath the High Himalaya. The MHT is generally considered to slip only during recurrent large earthquakes that function to accommodate 20 mm/yr of convergence between India and Tibet. The 2015 Mw 7.8 Gorkha earthquake was confined to a down dip section of the MHT and serves as the most recent illustration of this process. Earthquakes much larger than the Gorkha event may be expected if the entire width of the MHT slips to produce surface rupture along the HFT. Excavation of trenches across the trace of the HFT at 5 sites along a 400 km stretch of the HFT and southward of the Gorkha earthquake serve to (1) support this expectation, (2) illustrate the characteristics of shallow deformation associated with the largest thrust earthquakes along the HFT, and (3) disagree with prior reports of others claiming that the M8.4 Bihar-Nepal of 1934 produced surface rupture at one of the sites. Each exposure shows the surface scarp to be a dip panel produced as total slip is increasingly accommodated up dip toward the surface by folding at the expense of fault slip taking place at depth. Structural, stratigraphic, geomorphic and radiocarbon observations at the four most eastern sites (Bagmati, Khayarmara, Sir Khola, and Damak) that extend a distance of 250 km along the HFT, show the most recent event horizons overlap in time at around 1100 AD. Vertical scarp separations at the sites range from 5.5 to 7 m. Yet larger coseismic displacements are required when fault dip is taken into account. Empirical scaling laws show a thrust earthquake averaging 7 m of coseismic slip is on average expected to be associated with a rupture length approaching 400 km and a moment magnitude of 8.3 In this context, it seems most likely that this 250 km section of the HFT ruptured simultaneously sometime around 1100 AD in an earthquake of magnitude approaching if not surpassing Mw 9. The vertical separation of the scarp at Tribeni (the westernmost site of our survey 160 km west of Bagmati) is also 7 m. Radiocarbon observations at Tribeni appear to preclude that displacement occurred prior to 1200 AD, and allow speculation of a separate rupture that extended westward, possibly corresponding to a 1255 AD earthquake that produced damage in Kathmandu.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.S44A..04W
- Keywords:
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- 4314 Mathematical and computer modeling;
- NATURAL HAZARDSDE: 7215 Earthquake source observations;
- SEISMOLOGYDE: 7221 Paleoseismology;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS