Changes in deformation rates
Atmospheric Sensing with Continuous GPS
Measuring the Growth of the Southern Alps
Related sites
Continuous GPS measurements at permanent sites have
many advantages - and one disadvantage - over repeated observation
campaigns at a network of sites. The one disadvantage is that we
don't have enough GPS receivers available to be able to instrument
the thousands of points necessary to fully monitor New Zealand's
deformation. Some of the many advantages are described in the following
sections.
By installing GPS instruments at permanent sites we can measure the movement of these sites continuously. We are also able to process the data almost in real time so that we can have a continuous, nearly real-time measurement of motion at these sites.
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The
figure shows the continuous GPS stations currently operating in
New Zealand. Many of the North Island stations were installed by
GNS in 2001-2003 under a
Land Information New Zealand (LINZ) project. Another ten LINZ
stations will be installed by GNS in the South Island during 2003-2004.
In parallel with this effort, GNS will be installing 10-20 continuous GPS stations each year for the next 5-6 years as part of the New Zealand GeoNet project . This is a major effort to upgrade New Zealand's geological hazards monitoring capability. It is funded primarily by the Earthquake Commission (EQC), with additional support from the Foundation for Research, Science and Technology (FRST). The GeoNet continuous GPS installations will initially focus on the Hikurangi subduction zone beneath the eastern North Island, and parts of the Taupo Volcanic Zone including Mt Ruapehu.
Daily position solutions from the continuous GPS network are calculated routinely by GeoNet, and can be viewed on the GeoNet website. The daily solutions for the LINZ stations can be seen on the LINZ website.
Even with the growth of the continuous GPS network there are not
enough stations to provide a detailed picture of deformation across
the whole country. A combination of continuous GPS at widely separated
sites and occasional GPS surveys in dense regional networks will
be required to monitor New Zealand's deformation for the forseeable
future.
Occasional GPS measurements only give a snapshot of
the deformation at the time of each new set of measurements. Often
this is not a problem, as deformation rates are usually steady.
But there is evidence from
overseas, as well as hints from old 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.
Similar
events have been observed beneath the west coast of Canada .
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. Vice versa, 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. We operated a pilot system from 2000-2002 that produced near real-time estimates of water vapour from New Zealand's continuous GPS stations and displayed the results on a web site. This system is not presently running, but we expect it to be brought back on-line in the future.
Measuring the Growth of the Southern Alps
One of our continuous GPS projects is to measure the distribution
of uplift rates across the Southern Alps. This is a joint experiment
that started in January 2000, between the Massachusetts Institute
of Technology, Otago University, GNS, the University of Colorado
and UNAVCO.
We are operating 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.
If we can measure the distribution of uplift across the mountain range we will be 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 isw likely to help 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 is also being undertaken as part of the experiment.)
After three years of the experiment, we have achieved vertical rate measurements at the 1-2 mm/yr level of accuracy, and we are finding 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.
Related sites:
http://igscb.jpl.nasa.gov
International GPS Service
http://www.unavco.org UNAVCO,
Inc.
http://cddisa.gsfc.nasa.gov
Crustal Dynamics Data Information System
http://sopac.ucsd.edu/ Scripps
Orbit and Permanent Array Center
http://sideshow.jpl.nasa.gov/mbh/series.htm
Global GPS time series
http://www.linz.govt.nz/positionz
Land Information New Zealand
http://gsc.nrcan.gc.ca/geodyn/index_e.php
Geological Survey of Canada.
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