Tuesday, May 5, 2015

Return Times Of Great Himalayan Earthquakes

Bollinger, L., S. N. Sapkota, P. Tapponnier,Y. Klinger, M. Rizza, J. Van der Woerd,D. R. Tiwari, R. Pandey, A. Bitri, and S. Besde Berc (2014), Estimating the return times of great Himalayan earthquakes in eastern Nepal: Evidence from the Patu and Bardibas strands of the Main Frontal Thrust, J. Geophys. Res. Solid Earth, 119, doi:10.1002/2014JB010970.

This is a very detailed study taken up along  two strands of the Main Frontal Thrust in Nepal south east of Kathmandu. Along this fault, the Neogene foreland basin Siwaliks are thrust over the Indo-Gangetic alluvium.

Crustal shortening taking place in the Himalayas as a result of convergence between India and Asia is accommodated along a sequence of south younging thrust faults; The Main Central Thrust, The Main Boundary Thrust and the southernmost Main Frontal  Thrust, which is active today. All these thrusts are interpreted to merge into a single decollment called The Main Himalayan Thrust (a "master fault" along which India subducts underneath Asia). The cross section below shows these major thrust faults flattening at depth and merging into the Main Himalayan Thrust. Earthquake clusters in red dots shows a region of the Indian slab which as it slides under Asia, often (over decadal to millenial times scales) gets locked. Rupture follows, thus releasing that accumulated slip. These ruptures propagate southwards and occasionally break the surface along the Main Frontal Thrust.

Source: Bollinger et al. 2014

So, along this fault in front of the Siwalik  hills, there is evidence in the form of fault scarps and fault traces, offset and deformed strata, uplifted river terraces and stacks of colluvial deposits (sediments eroded from a fault scarp) of past earthquakes. This study examines this record in detail going back several thousand years. Its a long paper and there were sections where the reading is a hard slog and when my eyes glazed over, but it is rewarding to understand the techniques (geomorphic measurements, geochronology and shallow seismic profiling)  that have been applied to reconstruct earthquake history.


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’stwo 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 ± 1.5mm/yr), thrust dip (25° ± 5°N), and apparent characteristic behavior imply 12–17.5m 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.

And in conclusion:

Certainly, the best path toward fully understanding whether and where great or giant earthquakes are likely to occur along the foothills of the highest mountain range on Earth will be to combine many exhaustive geomorphological and paleoseismological field investigations such as that presented here with extensive, long-term geodetic measurements, capable of narrowing uncertainties in estimates of the full seismic moment deficit.

No comments:

Post a Comment