Temporal variations of tS /tP ratios before and after Koyna earthquake of 10 December 1967 and calculation of its magnitude

Abstract

An analysis of the body wave travel time ratios (t_S /t_P) has been made for four seismographic stations Koyna Nagar, Satara, Mahabaleswar and Pune-from 24 October to 15 December 1967. The arrival times of body waves in different stations have been taken from published reports on Koyna Earthquakes. Origin times of the fore-shocks and aftershocks of the main Koyna earthquake of 10 December 1967 (M =6.2) have been calculated from the plots of S-P intervals and P arrival times at different stations. A plot of t_S / t_P ratios against time shows the anomalous variation of the ratio with a fall of about 10 to 14 % from the mean values for the period under consideration in Koyna Nagar, Satara and Mahabaleswar. No such t_S/t_P variations have been observed in Pune, which shows more or less steady values. The time interval, which we call anomalous period, from the date of fall of t_S/t_P from the mean value upto the time of occurrence of the main shock on 10 December 1967 is 32 days in each station. this anomalous period seems to coincide with the reported sharp and simultaneous changes indicated by some strain gauge placed in Koyna Dam, for about a month prior to the occurrence or the main Koyna earthquake. On using empirical relations based on dilatancy-fluid diffusion model, this amount of anomalous period is equivalent to that of a smaller shock of magnitude between 4 and S. Consideration of aftershock area coupled with other empirical relations also indicates similar magnitude.

These facts tend to suggest either (1) the empirical relations used here do not hold good for Koyna earthquake or, (2) a smaller shock of magnitude between 4 and 5, occurring just ahead of the main shock, gave rise to t_S /t_P anomalies.