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) |