ABSTRACT
Global Positional
System (GPS) devices are used to monitor and record data of the movement of the
Earth's crust due to earthquakes. GPS
satellites provide signals that can be used by a GPS
receiver to compute position, velocity and time. The position and velocity of ground movements
can be gather by tracking embedded GPS devices in the
Earth’s crust. GPS earthquake systems do
not gather information about vibrations on the ground; they gather information
about ground displacement due to an earthquake.
Several GPS earthquake systems are in place around the world specially
areas with high seismic activity. An
example of GPS earthquake system is SCING, the Southern California Integrated GPS
Network, that is designed to monitor ground movement caused by the San
Andreas Fault and other faults in the LA Basin.
INTRODUCTION
The
Global Positional System (GPS) can be used to measure vertical and horizontal movements of the
Earth’ crust. GPS
devices are used to monitor and record data of the movement of the Earth's
crust due to earthquakes. The data helps
in creating maps that show how fast and in what direction the crust is moving
due to plate and fault movement. Earthquake
monitoring with GPS is already in use in places like Japan,
Mexico, Canada
and the United States. An example of GPS earthquake technology is SCING,
the Southern California Integrated GPS Network, that is designed to produce detailed information
about ground movements in Southern California. GPS in earthquake analysis can help in
determining which areas are at greater risk of earthquake damage. This technology can be used in conjunction
with other earthquake analysis technologies to reduce damage to buildings,
freeways and homes, and to improve emergency preparedness and response. GPS systems, measurement of earthquakes with
GPS, and use of GPS earthquake systems in the world and specifically in Southern
California will be explained in the following sections.
WHAT IS GLOBAL POSITIONING SYSTEM (GPS)?
The Global
Positioning System (GPS) is a navigation system
made up of a network of twenty-four satellites orbiting the earth. The satellites provide signals that can be
used by a GPS receiver to compute position,
velocity and time. GPS
has a wide range of military, consumer and commercial applications. GPS
is used in transportation navigation, remote monitoring, environmental analysis
and other positioning and navigational services.
USING GPS TO MEASURE EARTHQUAKES
Earthquakes can be
monitored by using GPS devices that are placed in different locations in the
Earth’s crust. If
segments of the Earth move, the embedded GPS devices located in that segment
will move as well. The position
and velocity of ground movements can be gather by
tracking the embedded GPS devices. GPS
earthquake analysis does not measure shaking magnitude; it only provides information
about ground displacement and velocity.
GPS earthquake technology is used all over the world specially
in places with frequent seismic activity.
Several GPS earthquake technologies are used in the United
States.
For example in California
we have the Southern California Integrated GPS
Network (SCIGN) that is constantly tracking the ground movement and posting the
data on their website.
How can GPS measure
earthquake activity?
Several
methodologies have been developed to analyze earthquakes. One method is the analysis of the energy
release in an earthquake by measuring the amount of shaking of the ground. Seismographs are instruments that can
accurately measure this vibration in the ground. Another method is by examining the total displacement
of the ground after an earthquake. GPS devices
are used to measures the total amount of displacement by recording the position
of the ground layers before and after an earthquake. A correlation exist
between the displacement of the ground and the magnitude of the earthquake. By using the correlation a GPS earthquake
system can measure the size of an earthquake (Glasscoe).
Limitations of GPS in
earthquake measurements
GPS technologies
used in earthquake analysis do not measure the amount of shaking in the
ground. The data gather from the devices
is collected at certain intervals of time.
The time interval may be longer than the duration of an earthquake so
collection of data from shaking of the ground is not possible using this
technique. The data collected at larger
time intervals has grater accuracy when using GPS devices. Because shorter time intervals introduce
greater percent error in the analysis of the earthquake, GPS devices are not
used to measure vibrations during an earthquake (Glasscoe). The limitation of a GPS system is that it can
not gather data of ground movement during an earthquake, but it can be used
with a seismograph to obtain a better picture of earthquake behavior. Seismographs are designed to make accurate
readings of ground vibrations.
GPS earthquake systems in the World
GPS plays an important role in earthquake study
in areas of high seismic activity.
Countries like Japan and the United States count with sophisticated system that report
GPS data instantaneously through the internet. GPS stations continue to grow as the GPS
technology and software keeps improving.
Japan went from 110 stations in 1994 to 1000 in 1997 (Sagiya). In the Western North America there are hundreds of stations covering
earthquakes from Alaska to Mexico. GPS sites continue to expand
throughout the world given a better picture of ground movement with respect to
time. Figure 1 shows the velocity of
tectonic plates around the world. The
velocities were calculated by the Jet Propulsion Laboratory and the California
Institute of Technology. They used data compiled
from different GPS systems located around the world (GPS Time Series).
GPS earthquake systems
in Southern California
Several GPS
earthquake monitoring systems are located in California
due to the high seismic activity. The
major system is the Southern California Integrated GPS
Network (SCIGN) that is composed of more than 250 stations. Figure 2 shows a map of the location of the
SCIGN stations. SCIGN is an array of GPS stations distributed
throughout Southern
California used to
monitor ground movement caused by the San Andreas Fault
and other faults in the LA Basin. The
devices are able to detect movement in the millimeter scale. SCIGN
provides regional coverage in Southern California for estimating earthquake potential, for identifying thrust faults, for
discovering variations of strain rates and for measurement of crustal deformation.
Some of these measurements can not be detected by conventional
earthquake measuring techniques so SCIGN provides a tool to further analyze the
mechanical properties of faults.
The data and analysis produced by SCIGN stations is available for free at the SCIGN website. The number of SCIGN stations keeps growing as
well as the GPS earthquake technology that will allow SCIGN to provide
improvements in analysis of earthquake behavior in Southern California
(Hudnut).
CONCLUSION
Although GPS earthquake systems do not analyze the
magnitude of an earthquake, they do a better job in tracking the displacement
of the Earth’s crust compare to conventional earthquake measuring technologies. GPS earthquake technology has been also used
in monitoring the integrity of structures by embedding GPS devices in them. GPS earthquake systems are not a substitute
for conventional earthquake technologies; they are an additional tool to better
understand earthquake behavior. GPS
earthquake systems will become more useful with expanding stations and
improvements in GPS technology.
WORKS CITED
Glasscoe, Maggi, et
al. “Using GPS to
Measure Earthquakes.”
The Southern California Integrated GPS Network Education Module.
September 8, 1998. May 23,
2004
<http://scign.jpl.nasa.gov/learn/index.htm>.
GPS
Time Series. Jet Propulsion Laboratory.
May 23, 2004.
< http://sideshow.jpl.nasa.gov/mbh/series.html>.
Hudnut, Kenneth, et al. Southern California Integrated GPS Network (SCIGN)
April 2001. May 23,
2004 <http://www.fig.net/com6_orange/pdf/Session%20IV_Paper%201.pdf/>.
Sagiya, Takeshi.
“Continuous GPS Array of Japan
and its Application
to Crustal
Activity Modeling.” Geographical
Survey Institute, Tsukuba
May
23, 2004
<http://www.quakes.uq.edu.au/ACES_WS1_proc/PDF/4.1_1.pdf>.