Frequency of foreshocks

Abstract

IN February 1975, the Chinese Seismological Bureau successfully predicted a magnitude 7.3 earthquake near Haicheng, Liaoning Province. From their reports, it is obvious that many methods such as radon count, abnormal animal behaviour, seismic velocity, and tilt and telluric current anomalies were considered in making the long term prediction1. The short term prediction, however, was based primarily on foreshock activity2,3. That foreshocks were important in the prediction raises the question of how common they are. We here analyse the frequency of foreshocks before shallow earthquakes with magnitudes greater than 7. Our findings show that 44% of all large shallow events in the world from 1950 to 1973 had fore-shocks large enough to be teleseismidally recorded, and that 21% of the large earthquakes that occurred in China between 1900 and 1949 had foreshocks large enough to be noted in local records.

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