Foreshocks of Koyna, India, earthquakes of magnitude ≧ 5.0 are investigated using data from the Poona WWSSN Seismic Station situated at a distance of 120 km from Koyna. Schemes for estimating earthquake magnitude on the SPZ component seismogram at Poona have been developed. These schemes are applicable to earthquakes of magnitude ≧ 2.0 occurring in the Koyna region.
Earthquakes of magnitude ≧ 5 on 13 September 1967, 10 December 1967, and 17 October 1973 in the Koyna region were each preceded by two earthquakes of magnitude ≧ 4.0 occurring within the preceding 15 days. In comparison, the activity of September 1980 was different. An earthquake of magnitude ≧ 5.0 occurred on 2 September and two more ≧ 5.0 earthquakes occurred on 20 September. These earthquakes were not preceded by pairs of ≧ 4.0 events. All events of magnitude ≧ 4.0 during 1974 through 1976 were singular (no two such events occurred within 15 days of one another). There appears to be a 50 per cent probability of occurrence of a magnitude ≧ 5.0 earthquake if two earthquakes of magnitude ≧ 4.0 are closely spaced in time (within 15 days) in the Koyna region.