Major earthquakes are often followed by a number of smaller magnitude events called, aftershocks that originate from the same source region of the main shock or principal shock, Aftershock sequences may continue for several days, weeks, months and even years. Generally, the frequency of occurrence of aftershocks (which in some case may be of the order of a hundred or more per day) decreases exponentially with time; and the magnitudes of these aftershocks also show a decreasing trend with time. Sometimes, the main shock is preceded by a few earthquakes, called foreshocks, which are usually smaller quakes occurring shortly, several days or weeks, before and near the source region of the main shock. Aftershocks are more common than the foreshocks. It may be emphasized here that significant deviations from this standard pattern of energy release i.e. foreshocks – main shock – aftershocks are quite common. To cite an example, the recent Bhuj earthquake of 26th January, 2001 was followed by a sequence of aftershocks, which are still continuing although at a very low frequency. So far more than eleven hundred aftershocks of magnitude 3.0 and above have been recorded till February, 2003.