Big earthquakes cause other quakes far away at least 9 per cent of the time. That鈥檚 the conclusion of a new statistical analysis, one that will add to the debate about whether big quakes can trigger damaging tremors elsewhere, and the mechanism that could allow that to happen.
A spate of huge earthquakes in recent years 鈥� in Sumatra, Indonesia, in December 2004, Chile in February 2010, and Japan in 2011 鈥� caused many to question whether one large quake can cause another on the other side of the world.
Tom Parsons of the US Geological Survey in Menlo Park, California, surveyed catalogues of seismic activity on every continent except Antarctica, going back to 1979. Over the period, there were 260 earthquakes of magnitude 7 or greater. Twenty-four were followed by small earthquakes on separate fault systems, as the seismic waves passed through distant lands. 鈥淚t鈥檚 a small hazard, but there is a risk,鈥� he says.
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His data suggest that any given region will be hit by an earthquake as a result of 2 per cent of big earthquakes elsewhere in the world. Parsons presented his results today at the Seismological Society of America annual meeting in Salt Lake City, Utah.
He was keen to see if any quakes of magnitude 5 or larger 鈥� which can cause serious damage 鈥� were the result of remote seismic activity. He found seven events that might qualify, but they all happened at least 9 hours after the seismic waves had passed through those regions, making it hard to say what caused them. 鈥淲e still can鈥檛 say whether they鈥檙e linked,鈥� says Parsons.
鈥淚f there鈥檚 always a persistent time lag,鈥� he adds, 鈥渕aybe that tells you something interesting about these earthquakes. Maybe you always have to wait at least nine hours.鈥�
A recent study of the 2012 Indian Ocean megaquake linked it to a fivefold increase in magnitude 5.5 or greater quakes the world over, for up to a week following the tremor. The activity was so unusual that the researchers could make a convincing case that they were caused by the megaquake, says David Hill, also at the USGS in Menlo Park.
So far, geologists can only speculate about how one quake might trigger another with such long time delays.
Messing up the plumbing
One idea is based on the fact that earthquakes can change the fluid plumbing in a distant region. For instance, 鈥淵ou can have an earthquake in Alaska and someone鈥檚 well in Florida will dry up,鈥� says Parsons. We also know that changes in subterranean water pressure can affect the friction on a fault, so it鈥檚 conceivable that this could help remotely trigger earthquakes with a delay of hours or days.
Hill notes that geothermal and volcanically active areas like The Geysers, a geothermal development in California, are particularly susceptible to rumbling after the waves of distant earthquakes have passed through.
Parsons says his next step will be to investigate the 24 quakes that caused far-off events and see if there is anything special about them. 鈥淪o far they look fairly ordinary. So we鈥檙e going to have to really dig into them,鈥� he says.
