Imperceptible to the human senses, California's tectonic plates deform every day at a rate comparable to the growth of fingernails. Convection in the mantle under the Earth's solid crust forces continental-size "tectonic plates" to move slowly in different directions. Where plates meet, the motion between them is accommodated by slow deformation, but when the crust is no longer strong enough to withstand the stress buildup, it breaks as an earthquake at weak points we call "faults."
The boundary of the Pacific and North American plate runs right through California, as evidenced by a swath of faults. One such fault broke near the town of Landers on June 28, 1992, producing a magnitude 7.5 earthquake. A series of ruptures tore 70 m across the Earth's surface down to approximately 15 km depth in a matter of seconds, producing destructive seismic waves as the crustal stress was released. This was followed 3 hours later by the nearby Big Bear earthquake, magnitude 6.5. Reacting to the sharp reduction in stress, southern California rearranged itself, like a taut elastic sheet that is torn in the middle.
Using a global network of ground-based receivers to track ranging signals from the twenty-four-satellite Global Positioning System (GPS), geophysicists can now measure distances across North America with a precision of a few millimeters. The Permanent GPS Geodetic Array (PGGA) has operated in California since spring 1990 as a collaborative project involving the Jet Propulsion Laboratory, the Scripps Institution of Oceanography, the Massachusetts Institute of Technology, and several other universities and agencies.
Recently, before the Landers earthquake occurred, a global network of GPS receivers had become fully operational under the auspices of the International GPS Service for Geodynamics. This was fortunate, since it provided a stable "reference frame" that allowed us to accurately determine the positions of the GPS satellites and observe the motion of the California PGGA with respect to the rest of the world.
Investigators at the above-mentioned institutions performed independent analyses using different software to estimate the motion of receivers in the California PGGA. The results are in excellent agreement and are shown in Figure 1 along with contour lines depicting the modeled deformation.
The site JPLM in Pasadena is shown moving approximately in a northwesterly direction by 1 cm. We saw a slight but significant disparity between the results and earlier models of the earthquake that were based on seismological and field observations, which allowed us to improve our model of the rupture details. This illustrates the potential for future interdisciplinary research that involves seismology and GPS geodesy.
Although unexpected, the Landers earthquake provided an early demonstration of the utility and accuracy of permanent GPS networks in geophysical research. The operation and processing of such networks are now almost completely automatic.
Permanent GPS networks are quickly becoming recognized as an economically attractive research option. Regional GPS networks similar to the California PGGA have been or will soon be implemented in Japan, Canada, Norway, Sweden, and Australia.
Blewitt, G., Measuring the Earth to Within an Inch Using GPS Satellites, Geophys. News, 19-20, 1991.
Blewitt, G., M. B. Heflin, K. J. Hurst, D. C. Jefferson, F. H. Webb, and J. F. Zumberge, Absolute far-field displacements from the 28 June 1992 Landers earthquake sequence, Nature, 361, 340-342, 1993.
Bock, Y., et al., Detection of crustal deformation from the Landers earthquake sequence using continuous geodetic measurements, Nature, 361, 338-340, 1993.
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