Sam Wormley
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 Posted: Wed Oct 15, 2008 10:21 pm Post subject: Using GPS to Track Tectonic Plates |
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Utilities and Communications — October 2008
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My drive home from work takes me through a canyon that is planted with a long-legged
futuristic metal tripod that looks like a mechanical sci-fi spider. It is part of a
network of GPS stations that measure steady or slow movements of the deformation of the
earth>s crust.
Oct 14, 2008
By: Janice Partyka
Using GPS to Track Tectonic Plates
I live in earthquake country. As a San Francisco Bay Area resident, I was shaken by the
Loma Prieta earthquake, and later, here in Los Angeles, I also survived the Northridge
quake. As frightening as they were, earthquakes can get much worse, as we saw in Sechuan
last May. High precision GPS has an important but not well-known role in understanding the
strains in the earth>s crust that cause quakes.
Like my neighbors, my fixation has been on the earthquake>s magnitude. As soon as the
shaking stops and we>ve checked on our family, we scamper to the radio to learn the
magnitude of the quake. The magnitude is derived from data from seismometers that detect
ground motions caused by seismic waves – and can do so from far away. GPS plays a
different role.
My drive home from work takes me through a canyon that is planted with a long-legged
futuristic metal tripod that looks like a mechanical sci-fi spider. It is part of a
network of GPS stations that measure steady or slow movements of the deformation of the
earth>s crust. These movements can signal strain on a fault. After an earthquake,
measurements can show release of the strain and post quake deformations in the crust.
The GPS net in Southern California was one of the first, and now nets based on the same
equipment set-ups are used throughout the world, including New Zealand, Japan, Sumatra,
Nepal, China, Taiwan, and Chile. Japan>s system was first and is now the largest, and
includes 1,400 stations.
"There was a continuously operating GPS net in the city of Sechuan, we hear, mostly with
GPS antennas mounted on the tops of buildings," explains Ken Hudnut of the U.S. Geological
Survey (USGS). "In addition, survey monuments had been observed with GPS before the
earthquake and again afterwards that will show ground shifts. These number around 200 and
will provide a complete picture of the deformation caused by the earthquake."
With recent completion of the Plate Boundary Observatory (PBO), the system in Southern Cal
now includes 465 stations and uses L1 and L2 differential phase data measured between
ground stations. This enables accuracy to the millimeter. The GPS receivers used in
Southern Cal and around the world are indicative of when the system was installed. Many
have a combination of equipment that was purchased over the years. Southern California
stations now use the Trimble NetRS dual frequency semi-codeless receiver that also tracks
L2C at all PBO stations.
Data are transmitted from the GPS stations back to servers via dial-up telephone,
dedicated cell modems and radios, and, in rare cases, satellite data telemetrics.
Scientists use several years of data to compute a long-term movement rate – the plate>s
velocity – for each point. Land surveyors, airborne imagery providers, and others also
rely on the stations' data. Each month people retrieve more than 50,000 files from the
Southern California GPS net. Data can be found at www.scign.org and pboweb.unavco.org.
USGS>s Ken Hudnut (second from right) in New Zealand, along with (from left to right)
Anders Crofoot, owner of Castlepoint Station; John Beavan of New Zealand>s GNS Science;
and Takeshi Sagiya (far right) of the Research Center for Seismology, Volcanology, and
Disaster Mitigation at Japan>s Nagoya University.
In the United States, GPS stations are concentrated in Southern Cal because of the
combination of large earthquake hazard and a dense urban population. Maps provided by the
USGS show movements of the Pacific Plate and the North American Plates, squeezing LA from
the north and south. By reputation, LA is a spread-out city that stretches beyond reason.
Some day, plate movements may squeeze us back together.
Not one to let an opportunity pass by, I asked Ken Hudnut to evaluate my danger relative
to the nearby Sierre Madre fault, a half mile up a hill. I learned it is a thrust fault
and my living on the downside, as opposed to the top side, is much preferred. Even more
preferred would be to live in Kansas.
Regulatory Update
The U.S. Federal Communications Commission (FCC) released a notice seeking comments on the
proposals of NENA, APCO, AT&T Wireless, and Verizon Wireless to have wireless E9-1-1
location accuracy requirements measured at the county level. Comments were due on October
6 and reply comments on October 14.
The order can be found at here; the submitted proposals can be found here.
The U.S. Department of Transportation>s National Highway Traffic Safety Administration
(NHTSA) and the U.S. Department of Commerce>s National Telecommunications and Information
Administration (NTIA) announced a notice of proposed rulemaking (NPRM) regarding a
proposal for more than $40 million in grants to help states improve their 9-1-1 systems.
The grants were authorized by the ENHANCE 911 Act. |
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