The status of central seismic gap: a perspective based on the spatial and temporal aspects of the large Himalayan earthquakes

The central Himalaya, considered as a prominent 'seismic gap', is generally believed to be the most vulnerable segment, due for a great plate boundary earthquake (M>8). Two significant historical earthquakes are known to have occurred in this region, in AD 1505 and 1803. Interpreted by some workers as great earthquakes, occurrences of these events question the validity of central Himalaya being a seismic gap. Here, we analyze the macroseismal data from the Central Himalaya and the Gangetic Plains, and suggest that neither of these earthquakes can be qualified as a plate-boundary-type event, and the size of at least the 1803 does not exceed Mw 7.7. Our studies further indicate that the central Himalayan frontal thrusts may be undergoing a quiescence of >1000 years in terms of generation of great earthquakes. Assuming a steady elastic strain accumulation due to plate convergence, and its simple translation as periodic great earthquakes, such long quiescence may appear intriguing. We suggest that the long-term quiescence and the spatial and temporal clustering that characterize the distribution of large earthquakes in the Himalaya, is a manifestation of the tectonic deformation associated with a highly evolved fold and thrust belt. Our analyses also indicate that the large/moderate earthquakes are mostly concentrated on higher-level thrusts. These structures probably accommodate most of the seismogenic slip in the Himalaya, and the rest of the strain could possibly be expended on passive folding. Consequently, great earthquakes (M>8) are relatively infrequent, as in the case of central Himalaya, apparently leading to long periods of quiescence.