article: Oct 16 2007 - Fiordland shaken again

The lower South Island once again felt the tremors from a large earthquake off the coast of Fiordland in the vicinity of George Sound. The magnitude 6.7 quake struck at 1:29 am on Tuesday morning and smaller aftershocks are expected to continue for days.

Mainshock

Tuesday, October 16 at 1:29 am, magnitude 6.7

Major aftershocks to date

Tuesday, October 16 at 1:35 am, magnitude 5.8
Tuesday, October 16 at 2:50 am, magnitude 4.6
Tuesday, October 16 at 6:34 am, magnitude 4.9
Tuesday, October 16 at 8:22 am, magnitude 4.9
Tuesday, October 16 at 10:28 am, magnitude 6.2
Tuesday, October 16 at 11:20 am, magnitude 4.1
Tuesday, October 16 at 12:04 pm, magnitude 4.5
Tuesday, October 16 at 1:26 pm, magnitude 4.9
Wednesday, October 17 at 3:34 am, magnitude 5.1
Wednesday, October 17 at 5:38 am, magnitude 5.6
Wednesday, October 17 at 6:50 am, magnitude 4.4
Wednesday, October 17 at 12:23 pm, magnitude 4.7
Thursday, October 25 at 12:09 am, magnitude 4.5

Map showing portable seismograph installation sites.

National network traces soon after the magnitude 6.7 earthquake.

Reported damage

Magnitude 6.7 earthquake at 1:29 am: We have received reports of landsliding in the Milford Sound area. One respondent in the Hollyford Valley reports their heavy VHF radio being thrown off the top of their TV, and stock in the shop falling from shelves. There have been no reports of any structural damage. There are other reports of minor contents damage such as objects falling from shelves, also from the Milford Sound and Te Anau areas.

Magnitude 6.2 earthquake at 10:28 am: Reports of goods falling from shelves at a Queenstown supermarket and in nearby Frankton. A report from Cromwell reports many things falling from shelves there.

No tsunami expected

The depth of the main shock, at 24 km, precludes a rupture of the sea floor which could cause a tsunami.

GNS Science response

GNS Science sent three staff to the area to deploy six portable seismographs at five locations. They installed the instruments on Thursday, October 18, reoccupying some of the sites used in monitoring 1993 aftershocks; the sites are shown on the accompanying map. Data recorded by these instruments enable scientists to locate aftershocks more accurately and better understand which part of the crust has ruptured, and how it might affect nearby faults. These instruments do not send back the data in real time - a further trip will be undertaken to collect the data in about a month's time. At the same time, a decision will be made as to whether to leave the recorders out for a longer period of monitoring.

The earthquake was recorded on all instruments of GeoNet's 100-strong seismograph network, and on nearly 30 strong motion recorders designed to capture data from the strongest of shakes. The image to the right shows the seismic waves as they appeared on the web site 20 minutes after the main shock.

Recent activity in Fiordland

This earthquake is the latest in a number of large shallow quakes in the region. The largest of these was a magnitude 7.0* event on August 22, 2003. In the last 25 years, the following earthquakes have occurred:

• Wednesday, May 31 1989, magnitude 6.1
  
  
• Tuesday, August 10 1993, magnitude 6.7
  
  
• Wednesday, November 1 2000, magnitude 6.2
  
  
• Friday, August 22 2003, magnitude 7.0*
  
  
• Tuesday, October 16 2007, magnitude 6.7**

*   Since revised from the initial estimate of 7.1. Note also that there were two aftershocks greater than magnitude 6 in this sequence.
** One aftershock greater than magnitude 6 in this sequence (to date).

What's going on in Fiordland?

Map showing mechanisms for Fiordland earthquakes.

The main shock and the larger aftershocks in this sequence (shown in red) are primarily reverse faulting mechanisms resulting from the Australian plate pushing (subducting) beneath the Pacific plate upon which Fiordland lies. A good description of fault plane solution representations (or beachballs) and the faulting type are to be found at the United States Geological Survey. (Note that they use the term thrust for reverse faulting in this latter link.)

The fault plane solutions are scaled according to magnitude as shown in the legend, with the main shock having a moment magnitude (M_w) of 6.7. Moment magnitude is derived from the seismic moment which is a direct measure of the physical size of an earthquake. Moment magnitude is considered to be the best magnitude estimate for earthquakes but is more difficult to calculate than other types of magnitudes.

Fault plane solutions for past earthquakes (shown in blue) are quite varied and show that the tectonics of the region are quite complex. However, the largest earthquakes have been primarily reverse faulting mechanisms with the exception of one normal faulting mechanism.

This report will be updated as more information comes to hand. Last updated October 25, 4:00 pm.

Poison Bay, Fiordland. A temporary instrument was deployed south of here at Rugged Mountain.

Kea-proofing the seismograph near Mount Tanilba, the southernmost temporary instrument.