A fall in P-wave velocity before the Gisborne, New Zealand, earthquake of 1966

A fall in P-wave velocity before the Gisborne earthquake of March 4, 1966 is indicated by arrival-time residuals of P waves from distant earthquakes recorded at the Gisborne seismograph station. Residuals were averaged over 6-month intervals from 1964 to 1968 and showed an increase of about 0.5 sec, implying later arrival times. The change began about 480 days before the earthquake. This precursory time interval is about that expected for an earthquake of this magnitude (ML = 6.2), but unlike most other reported instances, there was no obvious delay between the return of the velocity to normal and the occurrence of the earthquake. Similar analyses were carried out over the same period for two other New Zealand seismograph stations; at Karapiro there was no significant variation in mean residuals, and at Wellington the scatter was too large for the results to be meaningful. The Gisborne earthquake has a focus in the lower crust, about 25 km deep and was deeper than other events for which such precursory drops in P-wave velocity have been reported.