Dependence of seismic mean residuals on distance, azimuth and time

Abstract

MOST studies on the changes of sesimic velocity which precede earthquakes (see, for example, refs 1–5) are based on data from highly sensitive, but local, seismic networks. Large earthquakes very rarely occur in the neighbourhoods of these networks, so to study velocity changes associated with earthquakes of large magnitude, it is better to use a worldwide network of stations such as those reporting to the International Seismological Centre (ISC) in Edinburgh. Studies of this kind, on the teleseismic residuals over an eight-year period at Matsushiro, have been reported by Wyss and Holcomb⁶. Their results show a clear jump in the mean P-wave residuals in 1963, 2.5 yr before the beginning of a large earthquake swarm in the vicinity of the seismic station. Since the variation is very close to the level of statistical significance, I have made a study of similar anomalies at other stations close to the epicentres of strong earthquakes. The period covered by the data, which have been taken directly from the ISC Bulletins, is 1964–70.