article: Jul 16 2009 - Fiordland quake biggest for 80 years (updated Aug 6, 1:20 pm)

The earthquake which struck the Fiordland region in the evening of Wednesday July 15 was the biggest since the Buller and Hawke's Bay earthquakes of 1929 and 1931.

The earthquake occurred at 9:22 pm, and the epicentre was located in Dusky Sound at the south-west corner of the South Island. Its magnitude of 7.8 makes its size comparable with the Buller (or Murchison) earthquake of 1929 and the damaging Hawke's Bay earthquake of 1931. It is also the largest earthquake of 2009 anywhere in the world (so far).

What do we know so far?

Focal mechanisms for the earthquakes in Fiordland (the mainshock is shown in yellow and offset from its true location).

The mainshock was a large reverse faulting or thrust mechanism resulting from the Australian plate pushing (subducting) beneath the Pacific plate upon which Fiordland lies. This is the largest and one of very few examples of this type of earthquake in New Zealand, making it very important for our understanding of hazards here. The aftershocks are concentrated at either end of the faulting area and are mainly a mixture of reverse faulting and strike-slip faulting mechanisms reflecting the complexity of the tectonics in the area.

The earthquake rupture started at about 30 km depth and ruptured upwards and to the south, focusing energy offshore. The motion was more like a lurch than a snap (meaning the energy was released more slowly); this is typical for a subduction thrust event. This explains why the damage was much less than many people expected for this size of earthquake. The motion was slower (with lower frequency shaking) and "rolling" rather than the sharp (higher frequency shaking) movements that cause building damage; this also explains the low number of landslides.

This, and the remoteness of the epicentre, means that New Zealand has been very fortunate - if this earthquake had happened anywhere else it would have caused huge damage, even with the mitigating low frequency behaviour.

Horizontal motions caused by the Dusky Sound earthquake.

The deformation caused by the Dusky Sound earthquake resulted in the movement of the south-west of the South Island towards the west-south-west (in a direction somewhat to the south of Australia). The map to the right shows the horizontal motion of various points in south-west New Zealand. The blue arrows are observed permanent displacements due to the earthquake (there are only four of these on the map). The largest is about 350 mm of motion at Puysegur Point in south-west Fiordland (see below for how to observe the data); the red arrows are predicted displacements at other points.

Large earthquake occurrence in New Zealand has shown clustering in time (mid-1800s and late 1920s to early 1940s). Therefore, in general, the probability of an event is higher after a large earthquake than before. The occurrence of the Dusky Sound earthquake does not mean we have had the "big one" as more are always possible. It is also important to remember that large earthquakes can occur in many parts of New Zealand, not just Fiordland.

The stress on the southernmost (offshore) section of the Alpine Fault was increased by the Dusky Sound earthquake. We do not know how close the offshore segment of the Alpine Fault is to rupturing, so the effect of this earthquake is uncertain. If the offshore section of the Alpine Fault was to rupture, it is unlikely to rupture into the onshore section. The known geology of the Alpine Fault does not indicate that earthquakes on this fault have been closely clustered in time.

A detailed study using a model of the fault system and earthquake occurrence is required if we are to get a better understanding of the probability of a southern Alpine Fault earthquake being caused by the Dusky Sound earthquake; such a study would take many months.

Aftershocks of the magnitude 7.8 Dusky Sound earthquake on 15 July continue to be felt in the lower South Island. Yesterday three aftershocks were felt, including the largest, at magnitude 5.9, since 16 July. This earthquake was of a similar type to the original 7.8 earthquake but occurred further off the coast. Up to this time 380 felt reports from the magnitude 5.9 aftershock have been received on the GeoNet web site.

A team from GNS Science is currently in the region using GPS to measure the changes in the land caused by the Dusky Sound earthquake. This involves taking new measurements at a large number of sites which have previously been occupied. Radar measurements from Earth observing satellites have already shown measurable changes which this work will complement.  More information about this work can be found here.

GNS Science will continue to study this earthquake and its possible consequences.

Mainshock ("Dusky Sound" earthquake)

The above map shows the latest distribution of aftershocks in the area. The Dusky Sound mainshock is the largest circle in the centre of the map.

Peak Ground Accelerations recorded for the Dusky Sound earthquake.

Wednesday, July 15 at 9:22 pm, magnitude 7.8

Primary aftershocks to date

Wednesday, July 15 at 9:41 pm, magnitude 6.1
Thursday, July 16 at 1:50 am, magnitude 5.9
Thursday, July 16 at 11:41 am, magnitude 5.1
Thursday, July 16 at 12:24 pm, magnitude 5.3
Thursday, July 16 at 12:44 pm, magnitude 5.5
Thursday, July 16 at 2:13 pm, magnitude 5.1
Thursday, July 16 at 6:30 pm, magnitude 5.6
Thursday, July 16 at 9:36 pm, magnitude 4.4
Friday, July 17 at 4:13 am, magnitude 5.3
Friday, July 17 at 5:27 am, magnitude 5.6
Friday, July 17 at 10:18 am, magnitude 5.5
Friday, July 17 at 11:01 am, magnitude 5.2
Friday, July 17 at 6:39 pm, magnitude 5.6
Saturday, July 18 at 4:40 am, magnitude 5.1
Saturday, July 18 at 8:25 am, magnitude 4.5
Saturday, July 18 at 12:57 am, magnitude 4.9
Sunday, July 19 at 3:33 am, magnitude 5.4
Sunday, July 19 at 5:11 pm, magnitude 5.5
Sunday, July 19 at 8:48 pm, magnitude 5.2
Monday, July 20 at 7:04 pm, magnitude 4.5

Further aftershocks can be found at Recent Quakes.

Hundreds of aftershocks are being recorded, but only those expected to be felt are being posted to the website; the early ones are listed above. You can see them all by clicking on the above aftershock map. This will start to fill in over the coming weeks as the even smaller earthquakes come up for routine analysis by our team at the GeoNet data centre.

How many aftershocks have there been?

It all depends on your definition of an aftershock: earthquakes below about magnitude 2 are usually not able to be located because they aren't recorded on enough seismographs. We prefer to see the same earthquake detected on at least four seismographs before we attempt to determine its location, otherwise the uncertainties are too great for it to be useful for research. In the past, we have usually located several thousand aftershocks over a two to three month period for earthquakes in the Fiordland region.

How big will the aftershocks be?

A rule of thumb is that the biggest aftershock is about 1 magnitude unit less than the main shock. This earthquake has not caused the same impact as was expected by a 7.8 shake; its effects have been observed to be less than the 7.2 earthquake of August 2003. This suggests that the largest aftershocks may be up to magnitude 6. However, nature is no respecter of statistics!

Map showing the location of portable seismograph deployments (red) and permanent stations (green).

The installation at Last Cove, secured against weather and keas.

GNS Science Response

A team of scientists and technicians are visiting the area to carry out observations and install further monitoring equipment to better understand this earthquake and its place in New Zealand's seismological history:

• An overflight of the area was undertaken on the morning of Thursday, July 16 to see what damage had happened to the landscape. There were expected to be numerous landslides in Fiordland as a result of this earthquake, but that did not prove to be the case, one of the pointers to the style of earthquake described above.
  
• Six portable seismographs were deployed in the vicinity of the mainshock on Friday, July 17 (see map at right). Data recorded by these instruments will enable scientists to locate aftershocks more accurately and so determine which part of the crust has ruptured, and how it might affect nearby faults.
• A repeat GPS survey will also be carried out to evaluate the extent of any movement or uplift in the region. The permanent GPS network data has already shown some large offsets since the earthquake; they may be seen using the GPS Time Series application. It is most apparent on PYGR, MAVL, BLUF and LEXA, where the latest point(s) have jumped away from the previous long-term trend. The earthquake has caused permanent displacements of approximately 2 cm at Alexandra (LEXA), 3 cm at Bluff (BLUF), 5 cm at Mavora Lakes (MAVL) and 30 cm at Puysegur Point (PYGR).

How big was this earthquake?

The earthquake was originally reported by our website as being magnitude 6.6. This is a rapid estimation which is only reliable for earthquakes of up to magnitude 7.0. Perversely, it is easier to get a better estimate of magnitude from further away, so when an estimate of 7.8 was received from the United States Geological Survey, the website was updated with that more reliable figure. This in turn was a revision from their initial estimate of over 8.0. They currently feature three estimates of magnitude for this earthquake, 7.6, 7.7 and 7.8, with a preferred value of 7.6. GeoNet released details with a magnitude of 6.6 in order to get some preliminary information out as soon as possible, with the revision to the magnitude following this an hour later.

New Zealand shares its seismograph data in real time with the data centres in several other countries, including the United States, to ensure that our own information services have backup from other nations. New Zealand in its turn receives feeds from seismographs throughout the south-west Pacific region.

Did I feel it in the North Island?

You may have, but in the midst of all the activity down south, a deep magnitude 5.5 earthquake occurred south of Taranaki. It was felt in Wellington at about 9:40 pm, a quarter of an hour after the 7.8 earthquake in Fiordland. So we will be working through the hundreds of felt reports and trying to fit them to the correct quakes!

Reported Damage

No deaths or injuries were reported.

Cracks in exterior walls were reported from one property in Invercargill, and another at nearby Otatara reported the piles were now distorting their lounge floor. There are also reports of ground slumping causing some property damage in that area. There have been lots of reports of articles falling from shelves and items being ejected from cupboards throughout Southland, as far north as Wanaka and Hawea and east to Dunedin, and isolated reports of power and telephone outages at Gore, Glengarry, Scotts Gap, Orepuki, Otautau, Otatara, Te Anau, Invercargill, Merrivale and Queenstown. A water main was reported to have broken at Winton, north of Invercargill.

Tsunami recordings

A small tsunami was generated by this earthquake, with the tide gauge at Jackson Bay, near Haast, recording a wave of 1 metre (peak to trough). An Australian gauge located out at sea, south-west of New Zealand, also detected a small wave some time after the main earthquake, which may have been generated by a landslip into the sea or an undersea landslide.