Active Tectonics Project
This is a question that people often ask us at UC-Berkeley . It's a good question! Unfortunately, most people think it's too complicated for them to understand. It isn't!!
After the 1906 Great San Francisco earthquake, some ranchers were surprised to see that their fences weren't going to keep their cattle on the range. Some fences were ripped apart by as much as 40 feet! When surveyors saw that the fences had moved, they realized that they would have to survey the area again. After all, property lines had just moved and landmarks used by ships to navigate along the coast had moved too!
Not long after this, geologist began looking closely at what happened. In 1910, H. F. Reid proposed the elastic rebound theory. He proposed that the earth's crust was elastic, like a rubber band (well, maybe not exactly). When two sides of a moving fault get stuck, the movement "stretches" the earth's crust around the stuck area. This stretching is called strain. Some of the strain deforms rocks and builds mountains and hills (plastic strain). The rest is stored and released like the snap of a rubber band (elastic strain). When the stuck area can't hold anymore, the accumulated elastic strain is released as seismic wave energy or simply earthquakes.
Well, we can actually see the earth's crust stretch in response to movements on faults! This is how . . .
Suppose we know the locations of four survey stations and the angles and distances of the lines in between them. If the fault is moving in the direction of the arrows, then over a period of time the distances and angles between the stations will change.
Lines A-B and C-D will stay about the same, since they are located on the same side of the fault. But lines A-C, A-D, and B-D will get longer and line B-C will get shorter! The change in the line length divided by the total line length is the computed strain. But where is the fault stuck?
If we put a line of stations across the fault and wait, we may see this . . .
If the line of stations indicates a gradual change in movement, we know the fault is stuck in that area and building up strain. After the earthquake, the strain is release and the earthıs crust returns to it near its pre-strain state. Some deformation is permanent, though, and we recognize this in folded rocks and uplifted mountains. The San Andreas fault in the San Francisco Bay Area, and portions of the Hayward and Calaveras faults in the East and South Bay Area show this behavior.
If the line of stations shows a sharp difference, then the fault is freely slipping or shows active creep. The San Andreas fault south of San Juan Bautista, the Hayward fault in the East Bay, and the Calaveras fault in the South Bay in Hollister shows this behavior.
Unfortunately, all active faults donı t behave the same. Some faults creep, while other are stuck, while others have both types of behavior.
Yes! It's amazing that simply measuring the locations, distances, and angles between survey stations can tell us so much about how earthquakes happen. It's one of the tools that help us better understand our earth. Hopefully, as technology and knowledge improve, we may be able to predict earthquakes.
In the past, we used visual or o ptical methods of surveying. The results were good enough to lay out property lines, but not good enough to use for high-quality earthquake studies. As technology developed, we began to use laser-transmitting equipment to measure the locations of survey stations. A laser beam would be shot from one station to a mirror located on another station. Since the speed of the laser was known, the time it took to travel to the other station and back would give the surveyor the distance. This method gives us highly precise data, but it is expensive and time consuming. Therefore, extensive studies were only done every few years.
|In the old days, optical surveying was time consuming, labor intensive work. Now, GPS technology provides better precision at a fraction of the time.|
Now, the Global Positioning System (GPS) gives scientists a quick way of surveying. The UC-DAVIS Active Tectonics Group uses satellite technology to pinpoint the location of survey stations down to a fraction of an inch. We set up a tripod-mounted antenna over a survey station and attach a data receiver to the antenna. The antenna picks up U.S. Department of Defense satellite signals and stores the information in the data receiver. After a couple of days, the set-up is taken down and we download the data onto computers for processing. After processing, the data is used to determine the station locations and the strain across active earthquake faults .
|The U.S. Department of Defense originally created GPS to track movements of ships, tanks, planes, troops, etc. Now GPS is available and affordable to the scientific community and the general publi c. GPS technology is currently used for surveying, boating and plane navigation, and its even available in certain luxury cars (perfect for those who don't like to ask for directions!).|
We try to set up our GPS units over survey stations or benchmarks. These are usually circular brass disks set in concrete on stable ground. A lot of the stations are located on hilltops, since it was necessary to be able to see from one station to another when optical or laser surveying is done.
|The survey stations are located on both public and private land. Some are located in parks and some along highways. But most are located on private property -- sometimes on ranches, and sometimes in people's backyards. We are grateful to every private landowner who allows us access to their property so we can continue this research. We hope that they realize that they are playing an important part in the study of earthquakes.|
This informational brochure has been prepared by the University of California at Berkeley, Active Tectonics Group. If you have any questions, you can reach us at:
Department of Earth and Planetary Science
University of California at Berkeley
307 McCone Hall
Berkeley, CA 94720-4767 http://seismo.berkeley.edu/~burgmann
|A typical GPS antenna set-up over a survey monument. This unit was located near the Pigeon Point Lighthouse.|