L.A. moves, but not in the way expected

Science News, August 25, 2001 by S. Perkins

Researchers monitoring small ground motions along faults in Southern California ended up detecting an altogether different phenomenon: the rise and fall of the ground as local governments pump billions of gallons of water into and out of the region's aquifers.

In some spots--particularly in the 40kilometer-long Santa Ana basin, southeast of Los Angeles--the ground rises and falls up to 11 centimeters over the course of a year. This periodic movement, which the scientists say isn't linked to Southern California's earthquake activity, hasn't been measured before, says Gerald W. Bawden, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif. Satellite-mounted instruments and a ground network of Global Positioning System (GPS) stations detected the rhythmic motion, which Bawden likens to breathing, during the past 6 years. He and his colleagues report their findings in the Aug. 23 NATURE.

More than 14 million people live in the metropolitan Los Angeles area. Add in all the golf courses, car washes, and industrial activity of a large city and you've got a recipe requiring a lot of water. Throughout the year, utility districts in the region pump rainwater, as well as water diverted from the Colorado River and sources in northern California, into aquifers.

When demand for water is low, the underground reservoirs recharge and swell like a wet sponge, says Bawden. In the summer months, the net withdrawal of water from these aquifers causes the overlying ground to compact and subside. Because the ground doesn't fully spring back when it's recharged with water, parts of the Santa Ana basin lose 12 millimeters, or about a finger's width, in altitude per year.

Natural settling in the area would typically amount to less than 1 mm per year, says Thomas K. Rockwell, a geologist at San Diego State University. However, exploitation of the region's water, oil, and natural gas has caused some sites to drop as much as 9 meters, or 9,000 times normal, in the past century, he adds.

Horizontal ground movements at locations with little or no vertical change show that the southwestern portions of the Los Angeles basin are creeping about 4.4 mm each year toward the mountains in the northeast, Bawden notes. However, areas that experienced large vertical motions due to the extraction and recharge of water in aquifers showed horizontal movements of up to 14 mm during the course of a year.

Minster hails the new measurement of the direction and size of tectonic ground movement in the Los Angeles basin as "a major step forward in assessing the seismic hazards" of Southern California.

"When scientists set out to measure small quantities like ground motion, they often find unexpected phenomena like this," says Bernard Minster, a geophysicist at the University of California, San Diego.