Timing and return period of major palaeoseismic events in the Shillong Plateau, India

The close temporal occurrence of four great earthquakes in the past century , including the great Assam earthquake of 1897 in the Shillong Plateau, nec essitated examination of the palaeoseismicity of the region. The results fr om such investigation would definitely aid in addressing the problem of the earthquake hazard evaluation more realistically. Our recent palaeoseismolo gical study in the Shillong Plateau has led us to identify and provide geol ogical evidence for large/major earthquakes and estimate the probable recur rence period of such violent earthquakes in parts of the Shillong Plateau a nd the adjoining Brahmaputra valley. Trenching along the Krishnai River, a tributary of the River Brahmaputra, has unravelled very conspicuous and sig nificant earthquake-induced signatures in the alluvial deposits of the vall ey. The geological evidence includes: (1) palaeoliquefaction features, like sand dykes and sand blows; (2) deformational features, like tilted beds; ( 3) fractures and syndepositional deformational features, like flame structu res caused by coeval seismic events. Chronological constraints of the past large/major earthquakes are provided from upper and lower radiocarbon age b ounds in the case of the palaeoliquefaction features, and the coeval timing of the palaeoseismic events is obtained from the radiocarbon dating of the organic material associated, with the deformed horizon as well as buried t ree trunks observed wide distances apart. Our palaeoseismic measurements, w hich are the first from the area, indicate that the Shillong Plateau has be en struck by large/major earthquakes around 500 +/- 150, 1100 +/- 150 and > 1500 +/- 150 yr BP, in addition to the well-known great seismic event of 1 897, thereby the C-14 dates indicate a recurrence period of the order of 50 0 yr for large earthquakes in the Shillong Plateau. (C) 1999 Elsevier Scien ce B.V. All rights reserved.