Increased Tectonic Activity, New Zealand

New Zealand has experienced a short period of increased tectonic activity, with an abrupt eruption at Mt. Ruapehu and earthquakes in the Southern Kermadec Islands, Bay of Plenty, Hawke’s Bay and Southland.

The early part of September 2007 was a relatively quiet period for earthquake activity, with only 17 earthquakes of note occurring in mainland New Zealand. The quakes ranged between magnitude 2.2 and 4.9 and were distributed across both the North and South Islands. Bursts of activity somewhere between the Bay of Plenty and the southern Kermadecs were noted on the 12th and 19th, but data on the events has not appeared in the earthquake catalogues.

Following the major earthquake activity near Sumatra when a magnitude 8 and two magnitude 7 quakes struck on September 12th and 13th, earthquake activity spread along the western boundary of the Pacific Plate up through Asia to the Aleutian Islands, and into some Pacific Islands as the plate boundaries reacted to changed stresses. Earthquakes of magnitude 6 were also reported to the south of New Zealand as the plate interfaces between the Antarctic, Australian and Pacific tectonic plates adjusted.

Apart from the 17 reported on-shore events, New Zealand seems to have been relatively quiet with a moderate amount of deep activity such as the magnitude 4.5 quake at a depth of 156 km near Lake Taupo on September 18th going unnoticed.

The latest sequence of activity started with a magnitude 6.1 earthquake in the southern Kermadec Islands at 5:16 p.m. on September 25th. This 400 km-deep earthquake was located deep within the subducting slab of the Pacific Plate 130 km west-north-west of L’Esperance Rock, about 800 km north-east of Auckland.

Just over 3 hours later, Mt. Ruapehu in the central North Island erupted, spilling some of the crater lake’s contents onto part of the Whakapapa skifield and down the Whangaehu Valley in small lahars. GeoNet has posted some images of the volcano following the ten minute hydrothermal eruption which was not large, but deposited ash, mud and small rocks up to 2 km from the crater. Shaking during the event reached magnitude 2.9 but was not sufficiently strong to trigger automated evacuation systems.

A climber injured by the lahar debris was sleeping near the door of a hut at Dome Shelter on the volcano. The debris smashed through the door of the hut and crushed his lower legs. The climber was lucky to survive the traumatic injuries which were complicated by being thoroughly soaked in the chill environment resulting in hypothermia. His lower right leg was amputated below the knee in Waikato Hospital yesterday, and reportedly he is making steady progress which will allow further examination to determine whether he has internal injuries.

Whilst the Kermadec quake and the hydrothermal eruption are not directly linked, they are part of the same tectonic process which causes volcanoes to form above subduction zones. It may be that the altered stress resulting from the earthquake triggered a small change in the hydrothermal pressure within Ruapehu, which regularly erupts, causing part of the cool crater lake to spill from the vent.

Skifield operators have reacted well to the precautionary closure of the Turoa and Whakapapa skifields yesterday. The fields re-opened today, and will quickly recoup lost school holiday business, benefitting from heightened interest in Tuesday night’s eruption. The volcano has been quiet since the eruption, but the top of the volcano near the crater lake remains off-limits, in case another eruptive event occurs. Previous history indicates that this is possible, and the next few days will provide some further clues on how likely this will be.

Just over three hours after the eruption at Mt. Ruapehu, earthquake activity near the Bay of Plenty coastal town of Matata increased. Matata has been experiencing an earthquake swarm since December last year. Activity reached a peak in April and May of this year and had been declining as expected. A similar swarm of over 1000 earthquakes in the same area during 2005 showed a similar pattern.

Overnight Tuesday and Wednesday, Bay of Plenty towns reported experiencing six earthquakes with magnitudes between 3.0 and 4.3, before activity again declined. All of these quakes were located at depths of 10 km or less, within 20 km of Matata, which is close to an earthquake fault zone marking the western boundary of the Rangitaiki Plains. The Plains mark the northern landward end of the Taupo Volcanic Zone which runs from Mt. Ruapehu to White Island. The zone is being stretched by tectonic forces, and the Earth’s crust is only 10 km thick in this area.

The Taupo Volcanic Zone is an active area containing volcanoes, geothermal springs and earthquake faults. Residents often experience earthquake swarms such as the recent events near Taupo, Rotorua, Awakeri, Kawerau and, of course, Matata. Most seem to be due to changing stresses along the earthquake faults as the area is being stretched, while some of the localised events result from changes in pressures in small hydrothermal systems.

Once again, the connection between the eruption at Mt. Ruapehu and the burst of earthquake activity near Matata is not direct, instead being different manifestations of adjusting pressures and tensions in an active tectonic area.

The two latest events in the latest sequence of activity were the earthquakes near Gore and off the coast of the North Island yesterday. The magnitude 4.0 earthquake at a depth of 10 km twenty km south of Gore at 1231 Wednesday is even more tenuously associated with current activity. Despite containing several earthquake faults, the Southland area is generally tectonically quiet, and the earthquakes felt by residents usually originate from the active area near Fiordland and the subduction zone in the Puysergar Trench to the south-west, such as the magnitude 8.1 quake of December 24th 2004.

Last night’s magnitude 4.9 quake in the Hikurangi Trench 110 km south-east of Gisborne marks a further adjustment along the interface between the Australian Plate and the Pacific Plate. The 20 km-deep quake, which struck at 10:54 on Wednesday night, occurred in an area where the Pacific Plate is sliding under the Australian Plate, and is the latest event in the burst of increased tectonic activity near New Zealand.

If a brief period of equilibrium has been reached, it merely marks another paragraph in the latest chapter of tectonic activity in our exciting part of the world.

[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST; and the US Geological Survey and its contributing agencies.]

3 Responses to “Increased Tectonic Activity, New Zealand”

  1. Dorothy says:

    Thank you for this very helpful report. It has been of great benefit for people thinking of going to the mountain for the school holiday.

  2. Darren Mcmanaway says:

    A very well written overview…..

    Well done!


  3. Ken says:

    Thanks for the feedback folks. Much appreciated.

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