Space and time variation in the seismicity patterns preceding two earthquakes in the Himachal Pradesh, India

Seismicity patterns associated with two earthquakes on November 5, 1968 ( M L = 5.0) and June 14, 1978 ( M L = 5.4) in the Himachal Himalayas were studied by monitoring earthquakes through a close network of Seismic stations in the region. Hypocentre locations of 1100 earthquakes during the period 1965 to 1982 in the magnitude range 2 $ ̌ / M ⩽ 6 on the Richter Scale in the grid 32°–33° N, 76°–77° E were determined and an epicentral map for each year is presented superimposing LANDSAT imagery. The focal mechanisms of these two earthquakes were also studied from P-wave motion data. It has been observed that the focal mechanisms of both the earthquakes were similar and do not follow the regional trend in conformity with the concept of plate tectonics. The focal mechanism is, however, attributed to lineaments close to Dharamsala identified from LANDSAT imagery. It is interesting to observe that the two earthquakes show marked variations in aftershock activity. The migration pattern of the earthquakes after the occurrence of the main shocks show similar trends but differ in velocity of migration. Space-time variations in seismicity patterns associated with these two earthquakes show an increase of seismic activity followed by a period of relative quiescence in the neighbourhood of epicentres of earthquakes and expected rupture zones.