The 1714 M 8.2 Bhutan Earthquake: new constraints on magnitude
Abstract
The on-going convergence between India and Eurasia plates poses a huge seismic hazard on the Himalayan foreland. However, the timing and magnitudes of paleo-earthquakes and their return periods are poorly constrained in this area. This research aims to reconstruct the seismic history of an exposed surface rupture recently identified along the Main Frontal Thrust (MFT) in eastern Bhutan to develop accurate hazard evaluations and determine the seismic risk in the region.
The large coseismic surface rupture was observed along the MFT striking 95°, and dipping about 24° to the north. The fault places Late Miocene Lower Siwalik siltstone on top of the Holocene river terraces. The mapping of the refreshed eastern riverbank revealed two slips events along the same surface. The throw of the younger event (event 2)is about 4.5 m, which may have been caused by a very large earthquake (M~8). We hypothesize that the younger surface rupture in eastern Bhutan was caused by one of the known large historic earthquakes in the eastern Himalaya, either the 1714 Bhutan earthquake or a medieval earthquake of ~1255 CE or ~1100 CE. Five dating techniques have been applied to accurately and precisely constrain the timing of the paleo-earthquakes. To avoid the limitation caused by one dating method, radiocarbon dating and conventional optically stimulated luminescence (OSL) are used together to determine the burial age of the youngest displaced river terrace layer yielding the maximum ages of the paleo-earthquakes. To overcome the imprecision of indirect dating method, we applied another three OSL methods to obtain the direct ages of the seismic events. a) Multi-OSL of feldspar and b) the Thermally-Assisted OSL are being tested to date fault gouge produced during the paleoseismic movement. c) High precision rock surface dating is applied to the cobbles exposed along the thrust and the layer boundaries. The indirect dating results indicates that the maximum age of the younger event is about 1690 CE, which implies that the surface rupture was caused by the 1714 Bhutan earthquake. The magnitude is constrained to 8.2 ± 0.4 applying the empirical scaling methods. This places the Bhutan earthquake among the three largest Himalayan earthquakes in the last 500 years; the other two being 1934 M8.4 Bihar/Nepal earthquake and the 1950 M8.62-8.37 Assam earthquake.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T41E0314G
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8004 Dynamics and mechanics of faulting;
- STRUCTURAL GEOLOGY;
- 8123 Dynamics: seismotectonics;
- TECTONOPHYSICS