Home / Our Science / Earth Science / Plate Tectonics / Project Examples

Project Examples

Deformation rates not always steady

There is evidence from overseas, as well as New Zealand survey data, that deformation rates are not always steady.

Deformation rates definitely change for a time following large earthquakes, and there are several overseas examples of deformation rates changing in between large earthquakes. A "holy grail" of deformation measurements would be to detect deformation changes that might signal the occurrence of future earthquakes.

A dramatic example of a rapid change in deformation rate was observed on the LINZ continuous GPS station near Gisborne only a few months after it was installed. The change took place over a period of about 10 days (which is rapid in geological terms!) during October 2002. It is believed to have resulted from part of the Pacific plate beneath north-eastern New Zealand slipping downwards and westwards by about 20 centimetres. The slip was similar to what happens in an earthquake, except that it took place very slowly over days, rather than rapidly over seconds. Because it was so slow it did not cause any of the strong ground shaking that is normally associated with an earthquake.

Atmospheric Sensing with Continuous GPS

The radio signals from the GPS satellites pass through Earth's atmosphere on their way to the GPS receiver. The radio waves are slowed slightly depending on the amount of air that they pass through, and the most variable part of the delay is due to water vapour in the atmosphere. This slowing of the radio signal can be measured as part of the analysis of the GPS data. In turn this lets us estimate the amount of water vapour in the atmosphere above the GPS receiver.

The distribution of water vapour is very important in weather forecasting. As a simple example, if there is a small amount of water vapour, or the air is dry, rain is unlikely. Alternatively, if the air is damp or there is a large amount of water vapour, there is possibility of rain. There are a number of orbiting satellite systems that attempt to measure atmospheric water vapour by looking down from above, but we expect that the ground-based water vapour measurements from GPS may be a useful addition to these systems, to assist weather forecasters in the future.

Measuring the Growth of the Southern Alps

One continuous GPS projects aimed to measure the distribution of uplift rates across the Southern Alps. This joint experiment was between the Massachusetts Institute of Technology, Otago University, GNS Science, the University of Colorado and UNAVCO.

We operated six continuous GPS stations in an approximately linear array from Karangarua (30 km SW of Fox Glacier) in the west to Mt John (near Lake Tekapo) in the east. Five other stations are operated for a few months at a time, with the higher elevation stations generally being operated in the summer and the lower elevation ones in the winter.

We were able to interpret these observations in terms of what is happening in the top 30-50 km of the Earth's crust to cause the mountains to grow. This helps our understanding of mountain growth not only in the Southern Alps but in other mountain ranges worldwide.

From previous research the present-day uplift rates across the Southern Alps were expected to be 10 mm/year at most. To measure the distribution of uplift rates across the mountains we therefore need measurements with an accuracy of 1 or 2 mm/year or better. Continuous GPS is one of only two techniques available that may be able to measure present-day uplift rates with this level of accuracy over a reasonably short time frame (say, 5 to 10 years). (The other technique is absolute gravity measurement, which was also undertaken as part of the experiment.)

We achieved vertical rate measurements at the 1-2 mm/yr level of accuracy, and found that the maximum uplift rates are about 7 mm/year and are located about 8 km to the north-west of the highest peaks of the mountains. As more data are collected, the accuracy of these vertical rates should improve.