THE mystery of how one earthquake can trigger another on a different fault
line may be solved if the ideas of a researcher in Germany turn out to be
true.
Rongjiang Wang has found evidence that earthquakes can change the composition
and pressure of groundwater thousands of kilometres away. This could force the
water into other fault lines, separating them and helping tectonic plates to
slip past one another, he says.
Wang, from the GFZ research institute in Potsdam, developed his idea after
monitoring the water within an artesian well in Kajaran, Armenia. Within an hour
of a quake 1400 kilometres away, he noticed a sharp drop in the electrical
conductivity of the water. Between 1996 and 1999, he and his colleagues saw this
happen at least four times.
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Seismic energy from earthquakes can rattle through the Earth and transmit
stress to other faults. But Wang says that by itself, the tiny strain caused by
these distant quakes is too small to change the properties of water in the well,
which is usually a mixture of fresh water and saltier water from deep
underground. He thinks that an influx of fresh water is what makes it conduct
less.
Wang believes that the shaking from a far-off quake is making carbon dioxide
bubble out of groundwater held within the rock surrounding the well. As the
bubbles of gas expand, they increase the pressure of water held in the rock’s
pores. This forces more fresh water out of the rock.
The shaking may also be forcing water into distant fault lines in the same
way as it does into the well, says Wang. If the shaking really is increasing the
pore pressure of the groundwater, then it could trigger a quake. Scientists have
known for decades that injecting extra water into the ground can force apart the
two plates of a fault, reducing friction and making it easier for them to slip
past each other
(91av, 30 June, p 34).
But others caution that it might not be a universal phenomenon. “It might be
a bit specialised to that one area,” says Uisdean McLeod Michie, a geologist at
UK Nirex, a consultancy firm that specialises in the deep burying of radioactive
waste. “But it would be interesting to see what happens in other tectonic
environments. It’s worth investigating.”
