Precursory seismic observations in Himachal Pradesh S.K. 1976 and Shillong plateau

Abstract

Precursory changes in P-wave residuals were noticed before the earthquakes on 1 June 1969 (M=5.0) and on 15 May 1974 (M=4.8) near Shillong and another near Nurpur (Himachal Pradesh) which occurred on 23 January 1969 (M=4). For these earthquakes, decrease in P-wave velocity was observed one to two months prior to their occurrence.

Six monthly b values in Gutenberg Richter's frequency magnitude relationship showed a decrease in the value about 15 days and 3½ months before two other earthquakes of October 1969 and January 1970 in Himachal Pradesh. The decrease in b value for foreshocks as compared to the aftershocks was also noted for another earthquake on 5 November 1968.A well marked seismicity gap has been demarcated in Himachal Pradesh on the basis of epicentral distribution obtained with a close river valley project network of observatories. This could be Indicative of a possible damaging earthquake in the region.

The foreshock occurrence in the Himalayan region was investigated on the basis of data upto 1975. Although no relationship between the time of first foreshock and the main shock could be established, the results show that there is a time lag of about 10 hours between the observed foreshock and the main shock on several occasions. However, foreshocks could be detected much earlier in Himachal Pradesh where a close network of seismological observatories is being maintained.