Groundwater being pumped by Los Angeles city authorities is causing deformations up to five times as large as the seismic movements monitored by geologists.
Researchers from the US Geological Survey first identified the problem in a network of 250 seismic stations it set up in the Los Angeles basin after the 1987 and 1994 earthquakes. Position data from some stations showed unexpected movements.
To trace their origins, Gerald Bawden of USGS compared a series of elevation profiles compiled by satellite-based radar at different times. He found that a 20 by 40 kilometre region was moving up and down by between 10 and 11 centimetres each year. In addition to this annual cycle, the surface was sinking by 12 millimetres a year.
Bawden figured out that the annual motion is caused by city authorities buying up water and storing it in underground aquifers in preparation for the dry summer months. The continual pumping in and out of water progressively compacts sediment at the bottom of the aquifer, so it can no longer hold as much water, says Bawden. This makes the surface sink a bit more each year.
“It’s essentially noise in their signal,” says Devin Galloway of the USGS. “When they account for this noise, it’s going to improve their monitoring for tectonic hazards.”
But a fault also acts as a barrier, blocking water flow, so only one side swells. The change is easily visible as a sharp line in the radar images, Bawden told New Scientist. “It’s a tool to recognise faults that we had no idea existed.”
Journal reference: Nature (vol 412, p 812)