GPS provides information for earthquake loss reduction

The outer layer of the earth is composed of a dozen or so large pieces called tectonic plates. Most earthquakes result from distortion that occurs near the edges of these plates as they try to move past one another. The Global Positioning System (GPS) satellites provide precise measurements of this movement and distortion and, thus, provide very direct information about what happens during earthquakes and very directly feed estimates of the probability of future earthquakes.

Role of GPS in monitoring Deformation

GPS finds application in earthquake hazard reduction work because it gives a very precise measurement of a station location. Depending upon the way the signals from the GPS satellites are used, the resulting uncertainty in the site location can be as large as 100 m., or as small as 2 mm. In the simplest, and least accurate, application of GPS, a single receiver uses the distances to the satellites to determine the receiver's position, giving a position with about 100 meter uncertainty. This is called GPS Navigation. A more precise measurement can be made by parking one receiver at a known location, and monitoring the errors in the distances to the satellites. These errors can then be used to correct the distances measured at nearby receivers at unknown locations. This produces positions with an accuracy of 1 or 2 m., and is called Differential GPS. The most precision is obtained when satellite signals from two receivers are combined. Phase information in the signals can be used to determine the position difference between the sites with an accuracy of a few millimeters in the horizontal and a centimeter in the vertical. This Interferometric GPS is the only technique with sufficient accuracy to measure the small motions produced by earthquakes and all of the remainder of this discussion will refer Interferometric GPS.

Until about 1992, all of the GPS measurements used for earthquake hazard reduction work came from GPS receivers that were operated in "Campaign" mode. The receivers are placed at a site for a few hours or days then moved elsewhere.

NEHRP supports campaign-mode GPS surveys both inside the USGS and in the NEHRP external grant program. Current areas monitored with campaign-mode GPS include the Pacific Northwest, the San Francisco bay area, the Los Angeles metropolitan area and many other active faults in the western U.S. NEHRP supports some GPS campaign work in Alaska in a cooperative project with NASA. Some of the GPS funded by the NEHRP external grants program is done in cooperation with an NSF-funded consortium of GPS researchers called UNAVCO (University Navstar Consortium).

In the past few years, many sites have been instrumented with permanent GPS receivers. These "Continuous" GPS sites are almost always equipped with telemetry that brings the data back to some central location for rapid processing. There are a growing number of arrays of continuous GPS stations. Those of most relevance to NEHRP are discussed here.

• BARD--The USGS and the University of California, Berkeley jointly operate the Bay Area Regional Deformation network, an array of 18 continuous GPS stations in northern California. BARD provides an accurate velocity field from which to understand earthquake potential in this part of California.
• SCIGN--The Southern California Integrated GPS Network is a joint effort of the USGS, NASA/JPL, and the University of California, San Diego. SCIGN currently consists of about 40 GPS stations distributed throughout the greater Los Angeles metropolitan region. NEHRP/USGS, NSF, NASA, and the Keck Foundation have funded an expansion of the array to 250 stations. To achieve high precision, the sites are carefully monumented and all the GPS receivers operate continuously. The goals of the array are to provide an accurate and detailed velocity field from which to identify the deformation from known faults, test current models of the geologic structure, and make better estimates of the seismic potential in the populous parts of southern California.
• IGS--The International GPS Service is an international cooperative effort that collects data from about 200 stations worldwide and provides precise positions and satellite orbits that are needed for the highest precision local surveys.
• PANGA--The USGS, the Canadian Geodetic Survey, Central Washington University, the University of Oregon, Oregon State University, and the University of Washington, operate this Pacific Northwest Geodetic Array of 40 or more continuous stations to monitor earthquake hazard and study continental geodynaimcs in this area.
• CORS--The U.S. Coast Guard, the Army Corps of Engineers, the Federal Aviation Administration and others are building a nation-wide network of GPS receivers Currently, the Continuously Operating Reference Station network has 82 sites distributed fairly uniformly throughout the United States. This array is not designed for earthquake mitigation; rather, it is aimed at navigation and surveying users. However, the receivers are high quality and the data are available for other users. The network is a valuable source of data for reconnaissance earthquake studies on a national scale.
• Japan--The largest such array in the world today is located in Japan where about 1000 instruments have been installed throughout the country.

Other applications of GPS in earthquake mitigation

GPS is currently being used to monitor some structures. As part of the USGS continuous GPS operations in Southern California, Pacoima Dam is being monitored through a joint pilot study in collaboration with Los Angeles County and other researchers within the Southern California Integrated GPS Network (SCIGN). The project was begun in order to demonstrate the feasibility of monitoring dams and other engineered structures (such as freeway overpasses and high-rise buildings) using continuous GPS technology and infrastructure. This dam is a 120 meter tall concrete arch that was constructed in the late 1920's. Pacoima dam withstood, but was damaged by, very strong ground shaking in both the 1971 and 1994 earthquakes. Due to concern about the stability of this dam in response to potential earthquakes in the future, the County purchased GPS equipment, and with assistance from the USGS in Pasadena, they began monitoring the dam using continuous GPS on September 1, 1995.

In the future other structures may be monitored with GPS. GPS provides the absolute motion of the structure in ways that are not easily achieved with other techniques. There is discussion about monitoring the deformation of building during earthquakes with GPS instruments installed at strategic locations on the building. The difference between the motion of a GPS receiver on the roof of a building and one at mid level, and one at ground level can supplement the strong motion recorders that seismologists have used in the past to understand how to increase the building's resistance to earthquakes.

This discussion has focused on the role of high-precision GPS measurements in NEHRP. GPS is also used as a positioning system for many other NEHRP sponsored activities, such as geologic and topographic map making and locating seismic instruments.