Earthquake activity declined. White Island, Mt. Ngauruhoe, and Mt. Ruapehu remained at Alert Level 1.
GeoNet, the U.S. Geological Survey and GNS Science reported 32 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during May 2008.
The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (6), M4 to 4.9 (9) M3 to 3.9 (13).
An additional 4 events in the magnitude 2 range were deemed worthy of mention.
Five earthquakes with magnitudes between 5.0 and 5.8 were recorded within 330 km of L’Esperance Rock in the Kermadec Islands during May. The quakes were at depths between 40 and 210 km. Activity near Raoul Island was quieter, with two magnitude 4 events being reported.
The largest quake located near the New Zealand mainland was a magnitude 5.7 quake that struck on the 4th of May. The 170 km-deep earthquake was located off-shore 150 km north of Te Araroa and was felt in the Bay of Plenty, at various localities on the eastern side of North Island and in Wellington.
Three very shallow earthquakes were felt near Taupo on the 23rd and 25th of May. Though only of magnitude 2.4 or 2.5, the quakes were only 5 km deep, allowing them to be felt nearby.
The remaining activity was scattered across North and South islands from Gisborne to Fiordland. Taranaki and Wanganui were quiet.
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.
One Alert Bulletin was issued by GNS Science during May 2008. Continuing unrest at Mt. Ruapehu indicated that magma (molten rock) was present in the volcano conduit. The magma was thought to be responsible for keeping the crater lake hot and releasing volcanic gases that were ten times above normal.
The background level of volcanic tremor increased and short bursts of stronger volcanic tremor had been recorded. However, these changes in volcanic tremor are common at the volcano.
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, and their contributing agencies.]