The San Andreas fault is part of the boundary between
the Pacific and North American tectonic plates
Extracting water for human activities
is increasing the number of small earthquakes being triggered in California. A new study suggests that the heavy use of ground water for pumping and
irrigation is causing mountains to lift and valleys to subside. The scientists say this depletion of the water is increasing seismic activity
along the San Andreas fault. They worry that over time this will hasten the occurrence of large quakes.
The San Andreas fault runs for almost 1,300km through
the western part of California and marks part of the boundary between the
Pacific and North American tectonic plates. Seismologists have mainly focussed on the movements of these plates as the
critical factors in the build up of stress that can lead to large earthquakes,
such as the one that
destroyed San Francisco in 1906. The paper examines another factor - the impacts of humans on the Earth's
surface.
The researchers have used the well developed GPS system in the western US to
analyze small lifts and dips in the topography of the San Joaquin valley. San Joaquin is part of California's central valley, one of the most
productive farm regions in the US. That productivity is based on access to
ground water, extracted and pumped to irrigate crops. So great is the demand that scientists estimate twice as much water is being
consumed as is being returned through rain and snow.
The researchers have taken data on the topography
from hundreds of GPS stations like this
All this extraction is having a significant impact on the shape of the Earth.
The floors of the valleys are subsiding, the researchers found, while the
surrounding mountains are on the rise.
"We are removing a weight from the Earth's crust and it is responding by
flexing upwards and literally moving mountains," said lead author Dr Colin Amos
.
"It seems as though these small stress changes that happen on a yearly basis,
are causing more small earthquakes to occur on portions of the fault."
Dr Amos and his colleagues stress that there is a natural pattern to these
tiny rises and falls along the mountain ranges - the extraction of water is a
small but significant impact that researchers haven't recognised in this area
before.
In a commentary on the research, Dr Paul Lundgren from the California
Institute of Technology (Caltech) says the movement of the mountain serves to
unclamp and increase the sliding on the San Andreas fault system.
Irrigation canals like this are used to channel
billions of litres of water to farms in the central valley
"There is both a seasonal variation in and long term promotion of seismicity
associated with the water extraction," he writes.
"The latter may hasten the occurrence of future large earthquakes in the San
Andreas fault system."
In another part of the region along the southern Sierra Nevada mountain
range, scientists had believed that the crustal uplift was due to tectonic
forces. This new research indicates that it too is partly a consequence of
groundwater depletion.
Dr Amos believes the study shows that we need to think more broadly about the
impact of our actions in relation to nature.
"Human activities are changing things that we hadn't appreciated before - its
a wake up call to the far reaching implications for the things that we are doing
that may affect systems that we didn't know that we could affect."
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