Earthquake activity continued at a low level. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.
The Matata earthquake swarm continued during May 2007, and items were shaken from shelves in both the North and South Islands by 6 earthquakes.
Geonet, the USGS (NEIC) and IGNS reported 57 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during May 2007. The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (5), M4 to 4.9 (13) M3 to 3.9 (23).
An additional 15 events in the magnitude 2 range were deemed worthy of mention.
A pair of quakes struck near New Plymouth on the afternoon and evening of May 1st. The earthquakes, with magnitudes of 3.3 and 3.1 were located within 10 km of New Plymouth at depths of 9 and 6 km. A larger magnitude 4.2 quake at a depth of 10 km located 10 km south-west of New Plymouth on the evening of May 13th, shook items from shelves at several locations.
Eighteen earthquakes were reported in the swarm occurring near Matata during May. The quakes, all within 10 km of the Bay of Plenty town, ranged in magnitude between 2.4 and 4.2 at depths between 2 and 8 km. The strongest member of the swarm to strike during the month was a magnitude 4.2 quake late on the morning of May 3rd. The 2 km deep quake shook items from shelves in the area and was strongly felt in the Bay of Plenty, according to Hazardwatch. Three days later, a 2 km deep magnitude 3.7 quake 2km south of Matata again caused items to falls from shelves.
Geonet published a brief analysis of the latest swarm on May 11th, noting that almost 700 earthquakes had occurred near Matata since December 2006, including 50 events over magnitude 3 and 5 over magnitude 4. The latest swarm is occurring at a slightly different location to a swarm of over 1000 earthquakes in 2005. Geonet expected the swarm activity to gradually die away over several months, and this appears to have been the case as May progressed.
Minor damage was caused at Aototara by a 20 km-deep magnitude 4.4 quake, 10 km south of Pongaroa on May 12th.
On May 14th, a magnitude 5.4 quake struck at a depth of 90 km near Nelson district’s Rai Valley. The 1:25 a.m. quake threw items from shelves in both the North and South Islands, but no serious damage was reported. The quake was felt throughout central New Zealand, and attracted 887 felt reports.
A magnitude 5.0 quake at a depth of 90 km struck off-shore, 60 km west of Foxton, at 1:18 a.m. on May 17th. The quake attracted 378 felt reports and items were shaken from shelves in Whitby and Paremata.
A cluster of three earthquakes with magnitudes between 4.5 and 5.1 struck 150 to 175 km south of L’Esperance Rock in the Kermadec Islands on May 12th.
An interesting pair of quakes struck simultaneously near Kaikoura on the same day. The quakes, both magnitude 4.4 were located within 10 km of the town at depths of 30 and 60 km.
A foreshock-mainshock pair of quakes struck 30 km north-west of Murchison on May 14th. The quakes with magnitudes of 3.6 and 4.1 were located at depths of 8 and 5 km respectively.
Another foreshock-mainshock pair struck 20 km north-east of Wairoa on May 27th. The quakes with magnitudes 3.5 and 4.1 struck 24 seconds apart at depths of 35 km and 30 km at 6:14 a.m.
Vulcanologists reported the nation’s volcanoes to be quiet during May 2007.
White Island continued losing its hot crater lake during May 2007. Geonet issued an alert bulletin on May 3rd reporting the rapid evaporation of the lake during March and April, the lake having dropped 19 metres to a new depth of about 10 metres. The lake’s decline exposed several steam vents and fumaroles, both allowing the lake to cool by 10 ºC to 64 ºC, and causing steam plumes as high as 3 km under suitable conditions.
As the month progressed, the lake stabilised at about 28 metres below overflow, steam plumes were seen above the volcano on several days, and seismic activity remained low.
Mt. Ngauruhoe was surveyed as part of an annual sampling of steam and gas vents early in May. The small low frequency volcanic earthquakes occurring at a shallow depth on the volcano’s northern flank continued throughout the month.
Mt Ruapehu exhibited a low level of seismic activity throughout May. Occasional bursts of low-amplitude tremor were noted in a report on the 25th, and minor seismic activity was reported during the last week of the month.
At the end of May, 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 1).
Mt Ruapehu (Alert Level 1).
[Compiled from data supplied by GNS Science, US Geological Survey, Geonet, Hazard Watch and their contributing agencies.]