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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS009 Oral Presentation 2033
Patterns preceding major earthquakes in Central Himalaya
Prof. Harsh Gupta
National Geophysical Research Institute Raja Ramanna Fellow
Dodla Shashidhar, Metilda Pereira
Himalaya is seismically or one of the most active intra-continental region in the world where devastating
earthquakes have occurred. In the Global Seismic Hazard Assessment Program estimates of peak
ground acceleration in Himalayan region have been made. However it is useful if regions likely to be
affected by major earthquakes are identified on long-term basis. A successful study was made in 1986
where based on the concept of "precursory swarm and quiescence" an area of 50x40 was identified to
be the site of a future 8 magnitude earthquake to occur before the end of 1990. This medium term
forecast came true with the occurrence of an M 7 earthquake on 8 August 1988. We are now making an
effort to identify other parts of Himalayan belt where precursory swarm and quiescence precede major
earthquakes. One such region is central Himalaya where we discovered that three major earthquakes
namely 19 January 1975 Kinnaur earthquake of M 6.8, 19 October 1991 Uttarkashi earthquake of M 7.0
and 28 March 1999 Chamoli earthquake of M 6.6 were preceded by precursory swarm and quiescence.
For the Kinnaur earthquake a well-defined swarm activity with 5 shocks of M >= 5 was observed during
1963-1968 followed by a seismic quiescence during the period 1968 - 1975 before the main shock of
1975. A similar pattern was also observed for the Uttarkashi and Chamoli earthquakes. With a swarm
activity well before 20 years during 1966-1969 for Uttarkashi earthquake, followed by seismic
quiescence from 1970-1990 with only one event of M > 5 in 1979. For the Chamoli earthquake the
swarm and quiescence activity is not very clear probably due to the occurrence of Uttarkashi earthquake
in 1991 in the immediate vicinity. In any case, a seismic quiescence was observed during 1992-1999.
The purpose of this study is to extend this concept to other Himalayan regions, which might have gone
through the precursory swarm and are currently in the quiescence period preparing for a future
earthquake.
Keywords: himalaya, precursory swarm quiscence, forecast