Earthquake activity continued at a low level before increasing abruptly at month’s end. White Island, and Mt Ngauruhoe remained at Alert Level 1, eruptive activity at Mt Ruapehu raised its alert status to 2. Two earthquake events triggered tsunami watches.
Geonet, the USGS (NEIC) and IGNS reported 35 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during September 2007. The magnitude distributions were as follows:
M7 to 7.9 (1), M6 to 6.9 (2), M5 to 5.9 (2), M4 to 4.9 (10) M3 to 3.9 (17).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.
Earthquake activity continued at a low level for the first three weeks of September 2007. Earthquake activity in the Kermadec Islands was centred around L’Esperance Rock with a shallow magnitude 5.3 event on September 4th, a deep magnitude 6.1 quake on the 25th and a shallow magnitude 4.8 event on the 26th. A shallow earthquake near Raoul Island on the 30th was magnitude 5.1.
The Matata earthquake swarm was declining during the first 3 weeks of the month, with only four magnitude 3 events reported. Activity began again on the 24th, and five quakes struck on the 26th. The swarm contributed 15 quakes with magnitudes between 3.0 and 4.7 to the lists by the end of the month. The largest quake of the swarm, which began in December last year, was a magnitude 4.7 quake on the 30th, followed by a magnitude 4.5 event a minute later. All of the quakes have been at depths of 10 km or less.
A shallow, off-shore magnitude 4.6 quake located 90 km west of Te Anau on the 6th did not attract any reports from the public.
A magnitude 4.9 quake located 30 km south-east of Hawera on September 21st did not cause any damage, owing to its depth of 140 km and, though it attracted few felt reports from the public, was felt as far south as Nelson and Wellington.
A magnitude 4.0 quake at a depth of 10 km located 20 km south of Gore on the 26th was felt nearby, attracting 45 reports from the public. Just over an hour earlier, a shallow magnitude 4.9 quake offshore near Mahia Peninsula was felt from Gisborne to Napier.
A magnitude 7.3 earthquake and a magnitude 6.8 aftershock in the Puysegur Trench south of New Zealand on September 30th were widely felt in the lower half of the South Island, causing some items to be thrown off shelves. The quakes were located 475 and 505 km south of Invercargill at depths of about 10 km.
The quakes raised the prospects of a tsunami having been generated, and a 250 mm rise in sea level was recorded at Dog Island near Bluff about 3 hours after the quake. Smaller waves, were recorded off south-eastern Australian coasts.
Two other Pacific quakes triggered tsunami bulletins during September 2007. A magnitude 7.3 quake in the Santa Cruz Islands on the 2nd generated a tiny wave, but a magnitude 6.7 event in the Loyalty Islands on the 28th did not disturb sea levels. An intense burst of strong activity near Sumatra, which featured magnitude 8.4 and 7.9 events on September 12th and 13th, triggered tsunami warnings for the Indian Ocean but not for the Pacific.
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.
Alert bulletins were issued when a small hydrothermal eruption at Mt. Ruapehu at 8:23 p.m. on September 25th 2007 triggered small lahar flows at the top of the Whakapapa skifield and partway down the Whangaehu Valley. A 7 minute magnitude 2.9 eruption earthquake was recorded, and the summit of the volcano was covered by ash and peppered by blocks over 1 metre across from the bottom of the crater lake. A climber in the Dome Shelter was seriously injured by some of the lahar debris, and nearly lost his life.
The eruption ejected about 500,000 cubic metres of water, causing the lake to drop by 2-3 metres and the water temperature, which had dropped to 13 ºC, began rising. By September 28th, the lake had reached 19 ºC, which is still regarded as cool for the Ruapehu lake, and investigation indicated that magmatic material was not involved in the eruption.
At 11:05 p.m. on Saturday September 29th, a smaller eruption and 3 minute volcanic eruption took place, followed by two periods of weak volcanic tremor of 5-10 minutes’ duration.
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 2).
[Compiled from data supplied by GNS Science, US Geological Survey, GeoNet, and their contributing agencies.]