Determination of Precursory Velocity Anomalies in New Zealand from Observatory Data on Local Earthquakes

The ratio of shear to compressional travel times from a local earthquake gives a good estimate of the average velocity ratio for the wave path, providing the origin time has been moderately well determined. Thus changes of velocity ratio can be identified and mapped from standard observatory data. The method is applied to the principal New Zealand earthquakes since 1964: Gisborne, 1966, M _L= 6. 2; Seddon, 1966, M _L= 6.0; Inangahua, 1968, M _L= 7.1. The results suggest that monitoring could be achieved with a station spacing of 100 km. It appears that velocity change is essentially a rapid process and that the return to normality may not begin until the earthquake is imminent.