Wild Land

Last night’s Fiordland earthquake was the largest to strike mainland New Zealand in 80 years according to GeoNet.

Thursday 16th July 2009

The magnitude 7.8 earthquake which struck at 9:22 Wednesday night was located beneath Dusky Sound at a depth of 12 km. The quake’s size makes it comparable to the Buller (Murchison) earthquake of 1929 and the 1931 Hawke’s Bay quake.

The main event was felt throughout most of New Zealand, as far north as Hamilton and Hunua and in Taranaki. Light damage was reported from 17 locations in Southland, Fiordland and Otago. By midday, the public had filed 1981 felt reports with the GeoNet website, reporting their experiences.

Aftershocks are continuing, with many being felt. GeoNet has reported on the two largest and the U.S. Geological Survey has reported five.

A magnitude 6.1 aftershock struck at 9:41 p.m. This event, reported as magnitude 5.8 by the USGS, was located 150 km west of Tuatapere (200 km west of Invercargill) at a depth of 5 km.

The USGS reports a magnitude 5.0 aftershock at 9:52 p.m. and a magnitude 5.2 event at 10:02 p.m.

Both agencies reported the magnitude 5.9 aftershock (5.3 according to the USGS) at 1:50 this morning. This quake was located 100 km west of Te Anau (180 km north-west of Invercargill) at a depth of 5 km. The USGS reported a magnitude 5.1 aftershock at 11:41 this morning.

The mainshock is the largest earthquake to strike mainland New Zealand since the magnitude 7.8 Buller (Murchison) earthquake of June 1929 which caused 17 deaths, and the magnitude 7.9 Hawke’s Bay earthquake of February 1931 which caused 256 deaths. There have been 23 shallow earthquakes of magnitude 7 or greater in New Zealand territory since 1840 according to Jefley Aitken (“Rocked and Ruptured,” IGNS, 1999).

Last night’s quake is the largest to strike the planet this year. According to the US Geological Survey, there have been six other seventh magnitude quakes so far this year, including the magnitude 7.6 quake near Tonga on March 20th. In a typical year there are about thirteen quakes of magnitude 7 or greater. The largest event in 2008 was the magnitude 7.9 Chengdu quake of 12th of May which killed 88,000 people in China.

Damage from last night’s Dusky Sound quake is remarkably light. Residents of Southland and Otago had plenty of time to duck into doorways or shelter under tables, with many reporting a minute or so of swaying. The lack of a sharp jolt may explain the lack of reports of rockfalls and collapsed slopes, although such damage is expected to be found in remote parts of Fiordland near the earthquake’s epicentre.

Goods were shaken from shelves and ejected from cupboards, and residents report watching suspended objects sway through lengthy arcs. Water slopped out of baths, fish tanks and pools. Light building damage is reported, including cracked walls and pile damage. Some ground slumping is reported from Otatara.

Infrastructure has remained largely intact. Power and telephone outages were reported at Gore, Glengarry, Scotts Gap, Orepuki, Otautau, Merrivale and Queenstown. No serious disruption to roads is reported, but burst water mains have been reported. Aerial surveys are planned for today to assess landslide damage in Fiordland and reports from small, remote communities may add to the schedule of damage.

Since compiling this report a further burst of activity has occurred with aftershocks of magnitude 5.3 and 5.5 reported at 12:24 and 12:44. A shallow magnitude 5.1 quake was reported 80 km west of Te Anau at 2:13 p.m.

The earthquake was sufficiently large and in the right location to have generated a tsunami, and suitable warnings were issued soon after the quake. At 10:18 p.m. a wave of 17 cm was measured in Jackson Bay and a bulletin from the Pacific Tsunami Warning Centre cancelled the warning at 10:51 p.m. when no further wave activity was detected. A DART buoy in the Tasman Sea reported a 5 cm sea anomaly at 9:53 p.m. and this is now thought to have been a wave generated by a landslide dropping debris into the sea.

GeoNet has now reported that a 1 metre tsunami wave was recorded at Jackson Bay, but the time of the event is uncertain.

[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.]

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