Existence of the dilatancy phenomena in the Assam region

Abstract

In accordance with the dilatancy model for earthquakes, P-wave velocity decreases in the source and region during the precursor   time, and consequently the mean, teleseismic P-wave travel-time residual at a station earthquakes close to the source should increase, We have analysed teleseisemic P-wave travel-time data at Shillong for the period from October 1964,through March 1974. The total number of events is 8021 including 1502 events with focal depths> 100 km, Six monthly average residuals have been calculated for the considered period. For the averaging period, the standard deviations are found to be less than 0.15 sec. This indicates that the residuals of the order 0.3 sec could be detected in the average P-wave residual histograms, the 1 June 1969 earthquake of Mb=5.0, at a distance of 20 km is well ref1ected. The outstanding feature of the six monthly average residuals is that the P-wave velocities have decreased since 1969 and it appears that the Assam region is presently experiencing a precursory dilatancy stage.