Earthquake activity declined. White Island and Mt. Ruapehu remained at Alert Level 1. A swarm of quakes followed a slow-slip event near Tolaga Bay, and a slow-slip event continued beneath Poverty Bay.
GeoNet, the U.S. Geological Survey and GNS Science reported 27 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during April 2010.
The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (4), M4 to 4.9 (9) M3 to 3.9 (12).
An additional 2 events in the magnitude two range were deemed worthy of mention.
Three earthquakes were reported to the north of Raoul Island in the Kermadecs during April. Two were magnitude five events and shallow, while a magnitude 4.8 quake on the 8th was very deep.
Two magnitude five quakes near L’Esperance Rock in the southern Kermadecs were not reported felt in New Zealand. A magnitude 5.3 quake on April 3rd was located 195 km east of L’Esperance Rock at a depth of 10km, and a magnitude 5.2 event on the 16th was located 100 km east-north-east at a depth of 42 km.
A magnitude 4.0 earthquake located 20 km north-west of Matawai on the evening of the 12th was not widely felt because of its 50 km depth. Nor was a magnitude 4.1 quake on the 17th which was located 40 km east of Opotiki at a depth of 80 km.
A magnitude 3.8 offshore earthquake located 40 km south-east of Gisborne on the afternoon of the 10th was located within the zone of slow-slip detected by GeoNet during March. The slow-slip event appeared to have died away, but another pulse of activity was detected toward the middle of the month.
The Gisborne area was to experience another burst of earthquake activity over coming days as the slow-slip event off the coast near Tolaga Bay hosted a swarm of small earthquakes. The six largest events occurred on April 13th and 14th. These quakes, with magnitudes between 3.9 and 4.5 were all located 30 to 40 km south-east of Tolaga Bay, except for a magnitude 3.9 quake which struck directly east of the coastal town on the 13th of April.
A shallow magnitude 3.9 offshore earthquake located 40 km south of Porangahau (50 km north-east of Castlepoint) at a depth of 15 km on the 19th went largely unnoticed.
A magnitude 3.8 earthquake struck 20 km north-east of Rangiora at a depth of 35 km on the morning of April 4th. This quake was widely felt in Canterbury attracting more than 500 reports from the public. Reports of strong shaking were filed from Christchurch, Rangiora, The Pines Beach and Waikuku Beach.
March update: GeoNet has enhanced its monthly earthquake summaries by adding totals for the earthquakes it has analysed. Because of the time required to process the vast amount of data on large numbers of earthquakes, these totals are not available for more than a month after the reporting period. However, in time, these will provide a handy guide to earthquake trends in New Zealand. During March 2010, 18 earthquakes were felt by the public, and 1351 shallow quakes and 473 deep quakes were analysed by GeoNet.
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 April.
A gas detection flight over Mt. Ruapehu on the 29th of March showed that the overall gas levels were decreasing.
At the end of April 2010, 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.]