Strong Quakes, Kermadecs

A burst of strong earthquake activity struck near Raoul Island in the Kermadec Islands this morning.

Tuesday, 24th June 2014

A series of strong undersea earthquakes struck near Raoul Island this morning. The sequence began at 7:19 a.m. when two magnitude 6 earthquakes struck in quick succession.

The first event was initially thought to be magnitude 7 but further analysis has revised the strength of the quake downward. The Pacific Tsunami Warning Centre reports the event as magnitude 7.2 at a depth of 5 km. A widespread tsunami threat was not expected, but the centre advised that a local event was possible. GeoNet’s tsunami gauge on Raoul Island indicates that a small local disturbance occurred.

The United States Geological Survey reports the event as magnitude 6.9 centred 96 km south-south-east of Raoul Island at a depth of 20 km. Geoscience Australia reported the event as magnitude 7.1 and estimated that it would have caused damage within 110 km of the epicentre and would have been felt up to 1300 km away.

[click for larger image] The first two earthquakes left strong traces on New Zealand’s seismograph network. The top line shows the trace of the two quakes (2 minutes apart) as detected at Raoul Island. The energy is then detected by seismographs onshore in New Zealand, with the trace sloping to the right further down the image as the energy reaches the instruments later because of the increasing distance from the earthquake epicentre.

The traces at sites in the Bay of Plenty (Te Kaha, Urewera, and inland at Matawai) seem to show more strongly because the slower S-wave (secondary wave) from the first earthquake is registering at about the same time as the primary wave from the second earthquake. The trace in the top right corner is due to another quake of magnitude 6 being recorded at Raoul Island nearly 50 minutes later – just before 8:10 a.m. when this snapshot was taken.

GeoNet reports the first earthquake as magnitude 6.6, centred 1035 km north-east of Whakatane at a depth of 30 km. No felt reports have been made by the public.


[click for larger image] GeoNet’s tsunami gauge shows a small disturbance at both of the tsunami gauges located at Raoul Island suggesting that a local sea disturbance of about 20cm may have occurred as a result of the earthquake activity.

A second quake struck at 7:22 a.m. The United States Geological Survey reports this event as magnitude 6.3, centred 87 km south-south-east of Raoul Island at a depth of 20 km. GeoNet has not reported this event.

At 8:06 another magnitude six earthquake struck the area. The United States Geological Survey reports this quake as magnitude 6.2, centred 85 km south-south-east of Raoul Island at a depth of 20 km. GeoNet reports this event as magnitude 6.3, centred 1080 km north-east of Whakatane at a depth of 30 km.

Aftershocks continue to occur in the area. In addition to the three magnitude 6 events, there have been five magnitude 5 events and one of magnitude 4.9.

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