Earthquake activity continued at a low level. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.
Geonet, the USGS (NEIC) and GNS Science reported 51 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during April 2007. The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (5), M4 to 4.9 (12) M3 to 3.9 (24).
An additional 10 events in the magnitude 2 range were deemed worthy of mention.
There was one tsunami event.
The continuation of the earthquake swarm which commenced near Matata in the Bay of Plenty in December dominated earthquake reports during April 2007. A further 23 earthquakes with magnitudes between 2.6 and 4.0 at depths of 6 km or less continued to rattle the area.
The major event of the month occurred outside the region under study. The magnitude 8.1 quake in the Solomon Islands struck at 8:40 a.m. on Monday April 2nd 2007 (New Zealand time), and followed more than a week of quakes from the northern Bay of Plenty through the Kermadec Islands, Tonga, Fiji and Vanuatu, including a pair of magnitude 7 events near Vanuatu on March 25th. The great quake in the Solomons caused a deadly tsunami over 3 metres high for nearby islands, and many areas of the Pacific were placed on tsunami watch. 42 fatalities were reported.
Hazardwatch reported that the peak-to-trough heights of the tsunami in New Zealand coastal waters reached a metre at susceptible West Coast sites, with this height being measured at a NIWA recorder at Charleston. It also noted that a witness reported seeing 20 cm changes every half hour in Wellington Harbour at lunchtime on April 3rd.
Seven earthquakes were reported in the Kermadec Islands, 3 near L’Esperance Rock and 4 near Raoul Island. The quakes ranged between 4.5 and 5.6 in magnitude at depths between 18 km and 540 km.
A pair of shallow quakes with magnitudes of 3.6 and 3.9 struck near Porangahau on April 13th and 15th.
Another pair of shallow quakes with magnitudes of 3.3 and 4.2 struck near Tokomaru Bay on the 25th and 27th of April.
A magnitude 5.3 quake struck 90 km north of White Island on April 13th. The 160 km-deep quake was felt in the Bay of Plenty, Te Kaha and as far east as Gisborne.
Vulcanologists reported the nation’s volcanoes to be quiet during April 2007.
White Island showed occasional minor tremor at the beginning of April and steam plumes, some as high as 3 km (10,000 feet), were seen over the island volcano on many days during April as the hot lake evaporated. The lake level declined rapidly during April, exposing high temperature steam vents which allowed the lake to cool by 10 ºC to 64 ºC by month’s end, at which point the crater lake was only about 10 metres deep. Monitoring indicated that an increase in volcanic activity was unlikely, but the lowering lake could alter pressures in the geothermal system with local steam explosions resulting.
Small volcanic earthquakes continued beneath Mt. Ngauruhoe at the rate of 20 to 30 a day throughout April.
Mt. Ruapehu also exhibited minor seismic activity throughout April, but other conditions remained unchanged since the crater lake was dramatically lowered by the lahar of March 18th.
At the end of April, 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.]