Earthquake activity declined abruptly. A tsunami warning was issued and sea level disturbances were recorded. White Island and Mt. Ruapehu remained at Alert Level 1.
GeoNet, the U.S. Geological Survey and GNS Science reported 24 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during September 2009.
The magnitude distributions were as follows:
M6 to 6.9 (3), M5 to 5.9 (3), M4 to 4.9 (6) M3 to 3.9 (10).
An additional 2 events in the magnitude 2 range were deemed worthy of mention.
Earthquake activity continued at a lower than normal level in the Kermadec Islands during September. A deep magnitude 6.2 quake struck 95 km west of Raoul Island on the 3rd and a shallower magnitude 5.0 event occurred 140 km east-north-east of L’Esperance Rock on the 26th.
Four earthquakes struck the area between the Kermadecs and the Bay of Plenty. A deep magnitude 6.0 quake located 60 km north of White Island just after midday on the 1st was widely felt. The 250 km-deep earthquake was felt from Warkworth to Gisborne, through the lower North Island, in the northern part of the South Island, and in Christchurch.
Another deep 6th magnitude quake struck at 10:32 p.m. on the 9th of September. This magnitude 6.5 quake was 420 km deep and centred 100 km south of L’Esperance Rock. With the epicentre 680 km north-east of White Island, it was thought that the shaking would be lightly felt in the North Island. A small number of felt reports were filed from the Bay of Plenty, Gisborne, Hawke’s Bay, Manawatu and Wellington.
On the 23rd and 30th a pair of shallow magnitude 4.8 quakes located 210 and 200 km south of L’Esperance Rock were not felt in New Zealand.
Two members of the Matata earthquake swarm were reported felt on the 23rd and 30th. The magnitude 3.6 and 2.6 events were located 10 km north-west of Matata at depths of 5 km.
A cluster of five earthquakes struck Canterbury between the 8th and 24th of September. The quakes ranged between magnitude 3.0 and 3.7 at depths between 5 and 33 km. The epicentres were scattered between 30 and 60 km north-west of Christchurch near Rangiora, Oxford, Springfield and Amberley. The earthquakes were felt in Christchurch and in towns near the earthquake epicentres.
The stronger quakes of the series, magnitude 3.7 and 3.5 events, were the deepest members of the swarm at 30 and 33 km, lessening their impact. Nevertheless, strong shaking was reported from Rangiora on those occasions.
Only one aftershock of July’s Dusky Sound earthquake was reported by GeoNet during September. The magnitude 4.9 quake struck just before 9 a.m. on the 10th at a depth of 5 km. Two quakes nearby on the 5th and 22nd were magnitude 5.0 and 5.1 events, 12 km deep. These were centred to the south of the other Dusky Sound activity and, while associated with the disturbance caused by the magnitude 7.6 event, may not have been aftershocks in the technical sense.
A tsunami warning was issued for New Zealand on the 30th of September when a great quake of magnitude 8 struck near Samoa. As a result of the earthquake, a six metre wave swept ashore at American Samoa, and caused serious damage to southern resorts in Western Samoa and some damage in Tonga. At least 149 people were killed in Samoa, 34 in American Samoa and nine were killed and four injured at Nuiatoputapu, Tonga according to the U.S. Geological Survey.
Some coastal evacuations took place in the North Island, with people moving to higher ground before the calculated wave arrival times for their location. Sea level disturbances were recorded on tsunami gauges at Raoul Island just before 11 a.m. and at East Cape about midday. The wave activity reflected a trend seen elsewhere in the Pacific away from the epicentre, with small waves arriving at intervals of four to ten minutes.
The initial arrivals were not the largest and the on-going nature of the wave activity resulted in public warnings of strong currents being made. The activity continued as the day progressed, with maximum wave heights of one metre recorded at Raoul Island and about half a metre at East Cape.
To the alarm of some, the installation at Raoul Island ceased transmitting seismic and tsunami data during the afternoon. However, the cause was not a massive wave wiping out the installation. A severe storm disrupted the telemetry and GeoNet’s datacentre was unable to access the stored data until late on the evening of September 30th.
The sea level disturbances continued into October 1st but variations of only tens of centimetres were recorded, except at Chatham Island where changes of half a metre were recorded
Regular reporting of the status of New Zealand’s volcanoes ceased at the end of June 2007, with the closure of the Hazard Watch service. GNS Science now only issues bulletins which record significant changes in volcanic behaviour.
No Alert Bulletins were issued by GNS Science during September.
At the end of September 2009, New Zealand’s active volcano status can be summarised as follows:
Raoul Island (Alert Level 0).
White Island (Alert Level 1).
Mt Ngauruhoe (Alert Level 0).
Mt Ruapehu (Alert Level 1).
[Compiled from data supplied by GNS Science, US Geological Survey, GeoNet, and their contributing agencies.]