Spatio temporal seismicity variation in earthquakes of Uttaranchal region

Abstract

The spatio temporal variations of seismicity preceding Uttarkashi, 1991 and Chamoli, 1999 earthquakes were studied based on the data during the period 1981 to 2000 using the catalogues of earthquakes prepared by the India Meteorological Department. Two scenarios were examined. In one case the epicentral distance from the respective impending earthquakes were worked out for all the earthquakes recorded during a ten  years period prior to the earthquake of Uttarkashi and Chamoli respectively. In the other case, the epicenter near latitude 30.2° N and longitude 80.2° E near India Nepal border (where earthquakes of 1966 and 1980 occurred) were considered to compute the epicentral distance. The second case was included because it is a seismically active region where Dharachulla earthquake of 1916 (magnitude 7.5) occurred. The  earthquakes of  1999, 1991 and 1980 in  Uttaranchal were  characterised  by six phases  of seismic  activity  namely (i) first quiescence or  gap, (ii) swarm, (iii) second quiescence or gap, (iv) foreshocks, (v) main shock and (vi) aftershocks. Some differences among these phases could however, be noticed which were explained through source mechanism, isoseismals, ‘b’ (Gutenberg Richter’s relationship), ‘h’ values (Omori’s law ) and fractal dimension. It is interesting to point out that prior to the occurrence of earthquake swarms (second phase) the seismic pattern exhibits the development of a seismic gap (first phase) after the decay of the aftershock activity associated with a  previous large earthquake of magnitude greater than or equal to M: 6.0  in this region. We infer that this second  ‘gap’ (third phase) is a characteristic of the complexity of the tectonics in the Uttaranchal. Thus, the simple Kanamori’s asperity model could be  modified to consist of six phases of seismic activity in the complex tectonic zone of Garhwal Himalaya. Detailed difference in the seismicity patterns prior to the earthquake were explained by the fractal dimensions estimated from the ‘b’values.