QUESTION: If an earthquake on the San Andreas fault shifted the land five meters northward at the San Diego-Tijuana border, what would be done with the physical objects at the surface? Would the United States own the land that was previously on the Mexican side?
– Richard Kurylo,
North Park
ANSWER: If the border fence and all the objects along the border shifted in unison, the U.S.-Mexico border would simply be five meters farther north. If the land shifted unevenly, negotiations between the two countries would probably be necessary to define the new border.
Of course, geological changes really do affect borders. The Pacific Plate is moving northwest two to three inches per year, shifting the U.S.-Mexico border. In addition, geomorphological changes – alterations in the features of the Earth's surface through processes such as erosion – can occur even more quickly. Many borders are drawn along rivers, which can alter their course significantly over time.
Many border disputes exist around the world, but none stems from geological or geomorphological changes, according to Yehuda Bock, a Scripps Institution of Oceanography geodesist – someone who studies the shape of the Earth and how its surface deforms over time. These studies provide the mathematical underpinning for mapmaking.
This may seem surprising, but Bock points out that maps vary depending on which reference system is used to draw them. Differences between maps dwarf the changes in the Earth's surface due to earthquakes and erosion.
Most maps of the U.S. are currently based on either the North American datum 1927 or the North American datum 1983. A datum is a mathematical model of the shape of the Earth and a reference point on the Earth's surface used to make measurements and assign coordinates. Switching datums can change the assignment of a point on the Earth's surface by hundreds of feet.
We are only beginning to draw highly accurate maps based on a combination of GPS and satellite data. Bock is also the director of the California Spatial Reference Center, which has the mission of determining how any point in California is moving over time. Researchers at the CSRC are collecting information in real time from hundreds of GPS stations all over the state.
They are using mathematical modeling to describe how the Earth's crust shifts due to earthquakes and subsides as a result of the extraction of water and oil. The CSRC's work will be useful in addressing any border or boundary disputes that may arise from earthquake or other geophysical processes.
Sherry Seethaler has a doctorate in science education and is a UCSD science writer and educator. Send your scientific questions to her at Quest, San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.
