The Southern California
Integrated GPS Network Education Module
Exploring the
use of space technology in earthquake studies
Concepts covered
Master list for Sections 1-4
Plate
tectonics
Structure of the Earth
Geophysical studies have uncovered much new information
about the Earth's interior, including its composition and mechanical
properties.
History of plate tectonics
Plate tectonic theory had its roots in Alfred Wegener's
erroneous "continental drift" theory (1915) but since the
1950's, geologic and geophysical evidence has helped it gain widespread
acceptance as a model of Earth processes.
Plates
Plate tectonic theory tries to account for the movement
of plates over time. According to this model, the lithosphere is divided
into 7 major and several minor plates which move in relation to one
another.
Plate boundaries
Plate boundaries are wide zones of deformation where
two or more plates interact. They are found at the edge of plates and
are characterized by three distinct motions.
Forces
Forces create stress, which acts to change the volume
and/or shape of a material. There are three different stresses which
act upon a material in three distinct ways.
Faults
Faults are surfaces along which rocks have been
fractured and displaced. There are three main types of faulting and
each type is associated with one of the three plate boundaries.
Hypercard resources
Animations of tectonic phenomena, courtesy of the
United States Geological Survey.
Earthquakes
What are earthquakes?
Earthquakes are the Earth's natural means of releasing
the stress that is built up due to plate motions.
Types of earthquakes
There are many different types of earthquakes and
each type is the result of different geologic and non-geologic causes.
Forces
A force can be thought of as a push or pull and
has both a magnitude and direction, so therefore it is a vector.
What causes stress?
Much of the stress in plates is caused by tectonic
motion, but some stresses occur as the result of non-tectonic processes,
especially in the interior of plates, far from plate boundaries.
Elasticity
Materials have different properties which allow
them to undergo elastic, plastic, or brittle deformation, depending
on the amount and type of stress applied.
Waves
Three types of waves are produced when stress is
released as energy in earthquakes. Each of these waves travel in different
directions and velocities within the Earth.
Detection and recording
Earthquakes can vary in size and various methods
are used to measure and record the energy released in earthquakes. Using
seismological instruments, the earthquake's size and location can be
determined.
Measurement
Energy released in an earthquake can be measured
in several ways. Intensity, magnitude, and seismic moment are three
common ways to describe an earthquake's energy.
A new type of measurement
GPS is another tool used by scientists to measure
the horizontal and vertical motion of the crust. It can be used to monitor
these motions both during and in between earthquakes.
GPS
What is GPS?
The Global Positioning System (GPS) is a navigation
and precise positioning tool developed by the Department of Defense
in 1973. It was originally used only by the military, but now has wider
uses including scientific and recreational.
How does it work?
The Global Positioning System is made up of three
main parts. These include the satellites (which send radio signals to
the Earth that describe their orbital position and time), the ground
station (comprised of an antenna and receiver) and the data center (where
ground stations are monitored and controlled and the data retrieved
and analyzed).
GPS in earthquake studies
The Global Positioning System can be applied to
scientific studies of earthquakes and faults. The Southern California
Integrated GPS Network (SCIGN) is an example of how scientists are using
GPS in order to study tectonic motion on a regional scale.
Using GPS to measure earthquakes
GPS (and more specifically SCIGN) does not measure
the actual ground shaking during an earthquake, but rather the final
displacement of the stations caused by the permanent deformation of
the Earth's crust during an earthquake.
Space
Technology at Work
What is SCIGN?
The Southern California Integrated GPS Network (SCIGN)
is a dense array of GPS stations in the Los Angeles Basin and surrounding
region which continuously records the millimeter-scale deformation of
the Earth's crust caused by tectonic motion. The data collected are
used by scientists to study mechanics of faults as well as to estimate
seismic hazard in the region.
Who runs it? Who has access?
SCIGN is operated by the Jet Propulsion Laboratory
(JPL), the US Geological Survey (USGS), and the Scripps Institute of
Oceanography (SIO) under the umbrella of the Southern California Earthquake
Center (SCEC). Data are freely available to the public and may be used
as long as the user acknowledges SCIGN and its funders as the source.
What happens to the SCIGN
data?
Data are continuously recorded by stations and are
processed daily at the three SCIGN data centers (at JPL, the USGS, and
Scripps) and are made publicly available via the world wide web or anonymous
ftp.
How are the SCIGN data used?
SCIGN data are used by scientists to study plate
motions, model faults and fault mechanics, identify blind thrust faults,
and to help determine the seismic hazard potential posed by faults in
Southern California.
More information about SCIGN
Explore these web pages to learn more about SCIGN
and its participants.
About
this module
Table
of Contents
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Last modified on 8/13/98 by Maggi Glasscoe (scignedu@jpl.nasa.gov)
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