Geological Summary for New Zealand Area, October 2008

Earthquake activity continued at a low level, but swarm activity increased. White Island, Mt. Ngauruhoe, and Mt. Ruapehu remained at Alert Level 1.

GeoNet, the U.S. Geological Survey and GNS Science reported 31 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during October 2008.

The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (9), M4 to 4.9 (7) M3 to 3.9 (10).
An additional 4 events in the magnitude 2 range were deemed worthy of mention.

Earthquake swarms were reported near White Island, Matata, Mt. Tarawera, and Wanganui, but two clusters of quakes in the Kermadec Islands dominated the lists.

Seven earthquakes with magnitudes between 4.8 and 6.2 were reported within 500 km of Raoul Island. The activity included three very deep quakes to the north of Raoul, and four shallow events to the south. The largest quake recorded in New Zealand territory during the month was the magnitude 6.2 quake 125 km south-south-east of Raoul at a depth of 8 km on October 5th. This quake followed a 7th magnitude quake near Raoul Island on September 30th.

The southern part of the Kermadecs hosted three quakes with magnitudes between 5.0 and 5.4. The earthquakes were within 150 km of L’Esperance Rock at depths of 35 to 50 km.

The Bay of Plenty area experienced three earthquake swarms during October 2008. The Matata swarm eased back with only three quakes with magnitudes between 2.9 and 3.4 being reported by GeoNet.

Another swarm of earthquake activity near Mt. Tarawera went unreported by GeoNet which reported only one of the quakes on its website. The activity began at 7:30 p.m. on Thursday the 16th of October 2008 with a shallow magnitude 1.9 earthquake on the southern shore of Lake Tarawera. Two events, the largest of the swarm, then struck 1 minute apart at 7:42 p.m. The larger of the two was a magnitude 3.3 quake, 3 km deep located on the southern shore of Lake Tarawera 20 km north of Reporoa, 20 km south-east of Rotorua. The second quake of the pair was slightly smaller in magnitude. The swarm activity ended at 8:33 p.m., by which time a 6 km-deep magnitude 1.7 quake had been recorded at 8:05 p.m. In all there were 10 distinct events in the swarm, most minor in magnitude.

The largest quake in the swarm was felt in Rotorua and at several of Rotorua’s lakeside communities. By mid-morning Friday, 54 reports describing the quake’s effects had been lodged by the public with GeoNet. Posters on the Trademe website reported feeling the two larger events, but no damage was reported.

At the end of October a swarm of earthquakes was recorded near White Island. The quake activity began at 8:19 p.m. on the evening of Monday the 27th of October 2008 with a shallow magnitude 3.1 quake. By the time activity ceased at 6 minutes after midnight, sixteen quakes of 3rd and 4th magnitude had been recorded over a period of about 4 hours.

Quakes of magnitude 4.0 were recorded at 9:33 p.m. and 4.1 at 10:33 p.m. The swarm included three quakes of magnitude 3.8 and three of 3.9. Activity was relatively quiet until another quake of magnitude 4.0 struck at 4:13 the following morning, at which point the swarm seems to have run its course. All of the quakes were located within 30 km of White Island volcano at depths of 5 km.

A pair of shallow earthquakes with magnitudes of 3.5 and 4.0 were reported near Te Karaka on October 7th and 28th.

A swarm of four quakes with magnitudes between 3.2 and 4.2 occurred near Wanganui between the 1st and the 17th. All of the quakes were located 20 km south-west of Wanganui at depths between 12 and 30 km.

A pair of quakes with magnitudes of 3.3 and 4.7 struck near Hastings on the 9th and 25th. The magnitude 4.7 earthquake, which struck at 6:47 a.m. on Saturday the 25th of October 2008, was located 10 km west of Hastings at a depth of 30 km. The quake was widely felt in the Hawke’s Bay area, attracting 214 reports by 11 a.m. from members of the public who had felt the shaking.

The Hastings area had been quiet since the shallow magnitude 5.9 quake of the 25th of August. This quake, located 10 km south-west of Hastings, caused damage to contents and some man-made structures in Hawke’s Bay leading to 1300 claims totalling $3.6 million with disaster insurer, EQC.

A pair of quakes near Upper Hutt on the 2nd and 12th were of magnitude 3.3.

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.

GNS Science issued one Volcano Alert Bulletin during October. The bulletin, for White Island, was issued on October 23rd, prior to the nearby earthquake swarm. It noted that the crater lake was continuing to refill, changing the geothermal activity on the island.

The lake was reported to be hot at 57 °C and had changed colour to a light green. Steam, gas and mud emissions from the largest vent on the south side of the main crater floor had increased and were expected to continue at an elevated level.

At the end of October 2008, 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.]

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