himalayan seismicity and eath quakes

IAGA WG 1.2 on Electromagnetic Induction in the Earth Available at Proceedings of the 17 th Workshop http://www.emindia2004.org Hyderabad, India, October 18-23, 2004 MTNet: www.geophysics.dias.ie/mtnet/ Keywords: seismic gap, seismicity, neotectonics, active faults, strong earthquakes HIMALAYAN SEISMICITY AND PROBABILITY OF FUTURE EARTHQUAKE BASUDEO RAI INTRODUCTION Anomalous changes of physical parameters prior to earthquakes have been reported in a variety of geological settings from different parts of the world. Scholz et al.(1973), Whitcomb et al.(1973), and Mjachkin et al. (1972) summarize many of these observations. Anomalous changes to preceding earthquakes have been reported for apparent seismic velocity or travel times, crustal deformations, electrical resistivity, b values, magnetic field, groundwater chemistry, and water levels in wells. The Indian Planetary Society 102/404 , Gorai Herumb Building, Borivali(W), Mumbai, India email: [email protected] ------------------------------------------------------------------- SUMMARY Seismicity of the Himalayan region has been examined in terms of its relationship with known geological faults and tectonic lineaments. Geophysical data obtained from magnetic, gravity, geoelectric and seismic methods have been pooled to bring out more information on the subsurface structure of the Himalayan region. The co-incidence of the conductivity anomaly with a zone of high seismicity suggests that stresses associated with underthrusting of the Indian plate at its collision boundaries with the Eurasian plate may cause fracture in the Indian shield resulting in the up-flow of the asthenosphere material in the form of a ridge. Telluric changes together with geomorphology indicate that area is seismically very active. There are major lineaments and hot water springs in the study area which are indicators of weak zones conductive for s eismic a ctivity in the region. Seismic data of major earthquakes collected from the website of the Indian Meteorology Department (IMD) reveals that most of the epicenters are lying on these weak zones. Himalayan seismicity is related to continuing northward convergence of Indian plate against Eurasian plate (Fig.1). Northward movement of India closed the Tethys Ocean to its north and subsequently it colloidal with Asia around 55 million years ago. Since this first collision event, Indian plate continues to push northward till present at an average rate of 56 mm per year. Indian –Eurasian plate collision produced extensive deformation giving rise to the highest mountain chain, the Himalayas, and the highest plateau, Tibet, on our earth. The northward converging India induced crustal shortening that is accommodated not only in the formation of the Himalayas and Tibet, but it is also distributed throughout the Asian crust including the region to the far north of Tibetan Plateau. After the four well known great earthquakes, no such events have occurred in the Himalayan belt. This leads to a deduction of a potential spatio-temporal seismic gap associated with high probability of major earthquakes in this region in the near future. In particular, telluric changes and significant drop in Gutenberg-Richter b-value together with the geodetic data exhibiting crustal shortening are among the salient factors that lend credence to the hypothesis of a strong impending earthquake in the region. 17th IAGA WG 1.2 Workshop on Electromagnetic Induction in the Earth 1/11 Hyderabad, India, October 18-23, 2004

himalayan seismicity and eath quakes

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