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.