The Bhagirthi and Alaknanda valleys of Garhwal Himalaya, were rocked, respectively, by two major earthquakes: the Uttarkashi earthquake of magnitude m(b) = 6.5, M-s = 7.0 on October 20, 1991 and the Chamoli earthquake Of m(b) = 6.8, M-s = 6.5 on March 29, 1999, during this decade. Both these seismic events are associated with ongoing deformation along the main central thrust of the Himalayas. Helium and radon anomalies on March 24 and March 27, 1999, respectively, were recorded at Palampur which is about 393 km from the Chamoli earthquake epicentre. A He/Rn ratio anomaly was recorded on March 20, 9 days before the Chamoli earthquake. The precursory nature of radon and helium anomalies is a strong indicator of the physical basis of earthquake prediction and a preliminary test for the proposed conceptual He/Rn ratio model. (C) 2001 Elsevier Science Ltd. All rights reserved. [References: 20]
展开▼
