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LINZ & GNS Give a Shakedown of the Fiordland Quake
LINZ and GNS Science are working together to better understand the impact of the major earthquake that shook the lower South Island in July. The earthquake, with an estimated magnitude between 7.6 and 7.8, is the largest in New Zealand for more than 70 years.
One of the key datasets used in analysing the effects of the earthquake comes from LINZ’s PositioNZ network, which monitors movement of New Zealand’s landscape. The network consists of over 30 continuously operating Global Navigation Satellite System (GNSS) stations around the country.
Shifts in the location of five PositioNZ stations show that the earthquake, centred in south-west Fiordland, caused a large area of the South Island to move in a southwesterly direction. Puysegur Point shifted by 30 cm, Mavora Lakes by 6 cm, Bluff by 3 cm, Alexandra by 2 cm, and Dunedin by 1 cm. It is expected that continued movement will take months or even years to tail off.
“PositioNZ data, combined with seismic data from GNS Science, is being used to develop a model demonstrating the extent the Earth’s crust has been deformed by the earthquake,” says LINZ National Geodesist, Graeme Blick.
A preliminary model has already allowed analysts from LINZ and GNS to predict the spread of the earthquake’s impact where there are no PositioNZ stations to collect data. For instance, Te Anau is predicted to have shifted by about 10 cm.
“The mainshock was caused by the Australian tectonic plate pushing under the Pacific tectonic plate beneath Fiordland, in a process known as subduction,” explains Dr John Beavan of GNS Science. “This is the largest ‘subduction thrust’ earthquake that has occurred in the written history of New Zealand.
“LINZ’s PositioNZ data, along with other measurements of the earthquake, will enable us to construct a ‘dislocation model’ that describes the size and shape of the fault that broke during the earthquake, and how much motion occurred between the two sides of the fault. Having an accurate dislocation model will allow us to assess the true extent of the quake’s impact, and will help us understand the impact a quake like this might have if it struck a more developed and populated area.”
The dislocation model is expected to be ready near the end of September.
Reshaping New Zealand
The significant impact of the earthquake means adjustments need to be made to the datum that mathematically describes New Zealand’s shape and allows for coordinates to be created. One of the principal objectives of the PositioNZ network is to monitor this datum—the New Zealand Geodetic Datum 2000 (NZGD2000).
NZGD2000 incorporates a National Deformation Model (NDM), which takes into account the normal changes in New Zealand’s geography caused by tectonic movement. The Fiordland quake is the first earthquake to require the NDM to be updated since its creation in 1998.
LINZ geodetic surveyors have teamed up with scientists from GNS Science to resurvey Fiordland and Western Southland during a one-week field expedition. They used GPS to collect deformation survey data, which will be incorporated into the NDM.
What impact do these shifts have on surveying?
For the most part, survey marks in a very local area affected by the quake will have moved about the same distance relative to each other.
“Because of this, and with much of the area being reasonably undeveloped, the impact on surveying is negligible in most cases,” says Graeme.
“For surveys carried out over large distances, however, there are likely to be inconsistencies between the surveys' coordinates and those contained within Landonline and the geodetic marks database.”
With the ongoing movements, coordinates could eventually be out by up to half a metre in western Fiordland and as much as 20 cm in Western Southland. Until the NZGD2000 is updated to account for the quake’s effects, surveyors will need to tie surveys into local control points to ensure discrepancies can be reliably identified and accounted for.