Geological Summary for New Zealand Area, January 2008

Earthquake activity continued at an increased level. White Island, Mt Ngauruhoe, and Mt Ruapehu remained at Alert Level 1.

Geonet, the U.S. Geological Survey (USGS) and GNS Science reported 50 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during January 2008.

The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (3), M4 to 4.9 (10) M3 to 3.9 (30).
An additional 7 events in the magnitude 2 range were deemed worthy of mention.

Mainland earthquakes were reported from Paihia to Fiordland and in the Kermadec Islands during January, while the southern ocean remained largely quiet. The month started with activity confined to the Bay of Plenty, and the 6th was a busy day for seismologists with 8 events reported near Matata, Waipawa, Gisborne and Pahiatua.

The Kermadec Islands were relatively quiet with only 4 events reported for the area. The quakes were between magnitudes 4.6 and 5.4 and ranged in location from north of Raoul Island to west of L’Esperance Rock.

Paihia in Northland experienced an infrequent earthquake when a magnitude 2.8 event struck on the afternoon of the 22nd. The quake was located within a few kilometres of the town at a depth of 5 km.

The Matata earthquake swarm continued with 17 earthquakes with magnitudes between 2.8 and 4.3 being felt. Five of these struck on January 6th. A magnitude 4.3 quake which struck on the morning of January 25th was swiftly followed by a magnitude 3.4 event 50 seconds later.

Gisborne residents, who experienced a magnitude 6.8 quake on December 20th, felt shallow magnitude 4.1 and 4.2 quakes on the 6th and 17th. The absence of strong aftershocks was considered normal for the type of quake, and the community quickly returned to normal.

Turangi experienced three very shallow quakes during the middle of the month. A magnitude 2.6 quake on the 15th was followed by a pair of quakes with magnitudes 3.1 and 3.7 two minutes apart on the 18th. The quakes were between 4 and 7 km deep.

A pair of quakes struck near Seddon in Marlborough on the 7th. The magnitude 4.2 and 3.5 quakes were at depths of 12 and 15 km and located 30-40 km south of the town.

The major seismic event of January 2008 was a magnitude 5.3 quake which struck 80 km west of Milford Sound on the morning on the 21st. The 20 km-deep quake was felt in Fiordland, Southland, Otago and Westland, and was described as damaging at Te Anau and strong at Riverton, attracting 178 felt reports. Unlike the Gisborne event a month earlier, aftershock activity was more typical and five quakes with magnitudes between 3.5 and 4.8 were recorded in the area over the next 2 hours or so.

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 January 2008. It referred to increased volcanic earthquake activity at Mt. Ngauruhoe, where between 5 and 30 quakes had been recorded each day since May 2006. On January 6th, the activity began increasing and reached a peak rate of 80 quakes per day by 9-10 January before decreasing slightly. The magnitudes of the larger events at depths of about 1 km on the volcano’s northern flank was magnitude 1.2 to 1.5.

Measurements of volcanic gas concentrations, temperatures and soil gas flux made at the volcano indicated no change since the unrest commenced in May 2006 despite the increased number of earthquakes. It was considered that an eruption was not imminent, and Ngauruhoe’s Alert Level remained at 1.

At the end of January 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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