Scientists trying to isolate which faults under Southern California present the greatest earthquake hazard have found that people have a bigger impact on local ground motion than tectonic activity does.

Groundwater pumping is apparently causing the Earth?s surface to rise and fall every year. Scientists made the discovery by comparing their data with space-based instruments designed to measure the subtle movements of Earth.

It turns out that the Los Angeles basin undergoes a tectonic squeeze, as drinking water is pumped out each summer, resulting in a drop as large as 100 feet in the water table. In winter and spring the ground bounces back, as water is pumped into the ground and stored in natural reservoirs.

Geophysicist Gerald Bawden of the U.S. Geological Survey says that seasonal groundwater pumping causes compaction in deep soil layers. The soil cannot be returned to its original thickness by a water infusion, meaning that the ground is permanently collapsing. The ground movement caused by this activity, about 15 millimeters of horizontal motion back and forth motion, is ?unprecedented anywhere in the world.? Mt. Wilson is getting closer to Palos Verdes by about 4.4 millimeters a year.

Bawden found that the is earth ?breathing? in the Santa Ana Basin, an area 25 miles long and a dozen miles wide, stretching from Santa Ana and Anaheim northwest to Downey and Long Beach. Santa Ana is literally sinking about a half inch each year. This could have an impact on underground pipelines, causing them to crack or become less efficient.

The good news is that ?There?s no correlation, there?s no link between groundwater pumping and earthquake activity,? says Bawden. California scientists are currently working on a way to get around the groundwater effect so they can accurately measure tectonic movements.

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