Seismological studies of the Raoul Island eruption, 1964

Abstract

An eruption of mud and steam at Raoul Island on 1964 November 20, was preceded by a swarm of earthquakes, which started on November 10. Within four hours of the start of the swarm, the frequency of shocks recorded on the island's seismograph had risen to a maximum of over 80 an hour. Volcanic tremor soon appeared, and became continuous by November 12, masking all but the largest earthquakes. Thereafter, both the level of tremor and the frequency of earthquakes declined until by the time of the eruption only 10–15 shocks an hour were being recorded. Following the eruption, the island was evacuated for two weeks. For four days after the reoccupation on December 6, three additional seismographs were installed and during this time the earthquakes occurred to the west and south-west of the crater, at distances of 4–8 km from it, and at depths of up to 8 km. The amplitude-frequency relationship does not show such a high proportion of smaller earthquakes as has been reported for some other volcanic swarms, and following an initial preponderance of compressions, the recorded directions of first motion showed roughly equal numbers of compressions and dilatations.