Earthquake activity increased slightly. White Island and Mt. Ruapehu remained at Alert Level 1.
GeoNet, the U.S. Geological Survey and GNS Science reported 37 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during May 2009.
The magnitude distributions were as follows:
M6 to 6.9 (2), M5 to 5.9 (4), M4 to 4.9 (7) M3 to 3.9 (21).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.
Only two earthquakes with magnitudes of 4.8 and 5.1 were reported in the northern Kermadecs near Raoul Island during May. Most of the island chain’s activity occurred near L’Esperance Rock in the south, where 5 quakes between magnitude 5.1 and 6.7 were reported.
The magnitude 6.7 event struck just before 1 o’clock on the afternoon of Saturday May 16th. The 10 km-deep event was located 10 km south of L’Esperance Rock, 825 km north-east of Auckland and was felt in Gisborne and Wellington but only 11 reports were filed by the public.
A magnitude 5.1 quake occurred to the east of L’Esperance Rock on the 21st and all again seemed quiet. Just before 1 o’clock on the afternoon of Sunday May 24th, a magnitude 6.1 earthquake struck 130 km east-south-east of L’Esperance Rock (915 km north-east of Auckland). The quake was only 13 km deep and was not reported by GeoNet, indicating it was not felt in New Zealand. Two more quakes of magnitude 5.3 and 5.1 followed during the afternoon, and activity again ceased.
The Matata earthquake swarm was quiet during May with only one event of magnitude 2.5 being reported by GeoNet as felt.
A burst of activity further south near Kawerau involved a cluster of three earthquakes with magnitudes between 3.2 and 3.4 on May 14th. The quakes, which were very shallow at 3 to 5 km, were felt at several Rotorua lakeside communities.
Three earthquakes struck the Wellington region between the 1st and 7th of May. They were located to the south of Tawa, between Redwood and Glenside (where the main trunk railway line enters the first of two railway tunnels en route to Wellington) at a depth of 30 km. Another shallow quake of magnitude 3.6 was located 10 km east of Wellington on the 8th of May.
Two earthquakes near Picton on the 15th were not obviously related. The first was a magnitude 4.5 event, 40 km deep, 10 km south-east of the town at 11:47 a.m. The second quake, a magnitude 3.8, 70 km deep event at 10:32 p.m. was located 30 km north of Picton.
The activity reported near Twizel during March continued, with two shallow quakes of magnitude 3.6 and 3.0 on the 2nd and a magnitude 4.3 quake on the 27th. The quakes were all located within 30 km of the town.
The rest of the South Island went through a quieter period, with magnitude 3 and 4 events near Seddon, Greymouth, Ross, Haast, and Te Anau.
GeoNet upgraded its facilities at Raoul Island during May, bringing a camera with views of the volcanic crater, a seismograph and two tsunami gauges online at the end of the month.
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 May.
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 0).
Mt Ruapehu (Alert Level 1).
[Compiled from data supplied by GNS Science, US Geological Survey, GeoNet, and their contributing agencies.]