Geological Summary for New Zealand Area, September 2008

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

Geonet, the U.S. Geological Survey and GNS Science reported 40 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during September 2008.

The magnitude distributions were as follows:
M7 to M7.9 (1), M6 to 6.9 (1), M5 to 5.9 (8), M4 to 4.9 (9) M3 to 3.9 (15).
An additional 6 events in the magnitude 2 range were deemed worthy of mention.

A magnitude 5.8 earthquake which struck south-east of Turangi on the afternoon of September 1st was the first of a series of stronger earthquakes recorded in the New Zealand area during September 2008. Damage was limited by the quake being deep at 80 km.

A few hours later, a magnitude 6.0 quake located north of Raoul Island was the first of six events of note in the Kermadec Islands. The quakes ranged between magnitude 5.2 and 7.0 at depths between 35 and 430 km.

GeoNet reported the largest of the Kermadec events at magnitude 7.3, located 340 km south-east of Raoul Island. This quake, which struck early in the morning of Tuesday September 30th was reported as magnitude 7.0, located 70 km south-south-east of Raoul by the U.S. Geological Survey. The uncertainty as to where the quake actually occurred highlights the difficulty of analysing shallow earthquakes in locations where seismic instruments are sparse.

Even though the quake was located more than 1000 km from East Cape, a small number of New Zealanders from Raglan to Christchurch reported feeling the earthquake. A tsunami was not generated.

The Matata earthquake swarm was more active during September 2008, when 7 quakes with magnitudes between 2.5 and 4.2 were reported by GeoNet on their website. An examination of GeoNet’s earthquake database showed more activity with six quakes of magnitude 2 and six of magnitude 3 in a small area off-shore from Matata in just over 24 hours to late morning on Wednesday the 3rd.

Residents of Tarawera and Okareka had a restless night on the 31st of August when a swarm of ten very shallow earthquakes rattled houses around 11 p.m. The earthquakes were clustered on the southern shore of Lake Tarawera, to the west of Lake Rotomahana. The area is known as the Waimangu Block State Forest and is close to the site of the Pink and White Terraces which were destroyed in the Tarawera eruption of 1886.
The quake activity began at 10:42 p.m., finishing just over half an hour later at 11:16. GeoNet analysed four of the events, reporting two magnitude 2.4 quakes 25 seconds apart at 10:45 p.m. followed by the largest member of the swarm, a magnitude 3.5 quake at 10:50 p.m. Another magnitude 2.4 quake was recorded at 10:56. The four quakes were at depths between 0.6 and 5 km.

Along with the Matata and Tarawera swarms, a third swarm of earthquakes occurred in the Taupo Volcanic Zone during September. More than a dozen earthquakes followed a magnitude 4.2 earthquake under Lake Taupo on the morning of Wednesday the 17th of September. The swarm was short-lived, bringing to about 50 the number of small quakes which had been recorded under the volcanic lake during a two-month period.

The Wanganui area had a quieter time compared with August. However, a magnitude 5.2 quake which struck on the evening of September 14th was widely felt in the lower North Island and upper South Island, attracting 1600 reports from the public. The quake was located off-shore, 60 km south-west of Wanganui at a depth of 90 km.

In the South Island, a magnitude 5.0 earthquake struck 20 km north-west of Hanmer Springs on the 5th of September. The quake was strongly felt locally, and reports of the tremor were filed by memebrs of the public as far north as Foxton and as far south as Christchurch.

A burst of earthquake activity off the coast near Kaikoura between the 24th and 26th included two quakes of 5th magnitude. Of the six quakes reported by GeoNet, all were shallow at depths between 20 and 30 km, and magnitudes ranged between 2.9 and 5.2. The two larger quakes caused slight damage near Kaikoura.

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 September 2008 when Mt. Ruapehu entered a heating cycle. The volcano’s crater lake, which had hovered at temperatures ranging between 34 ºC and 37 ºC during the period from October 2007 to June 2008, had fallen to 23.3 ºC on July 15th and then declined to 16 ºC in August. However, during the early part of September, the crater lake began heating again and had risen to 22 ºC when the bulletin was issued on September 18th.

It was reported that volcanic tremor was detected as the lake began heating during the early part of September, and volcanic gas levels had increased. The heating was expected to continue with the lake likely to change colour as sediments on the lake floor were disturbed by the increased gas flow.

At the end of September, 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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