Researchers at the Earth Institute at Columbia University say such triggering of minor quakes by distant events could be precursors to larger events at sites where pressure from waste injection has pushed faults close to failure. In such wells, treated waste water is disposed of by injecting it into the ground between impermeable layers of rocks.
One site studied was a set of injection wells near Prague, Okla., where a huge earthquake in Chile Feb. 27, 2010, triggered a mid-size quake at the Oklahoma site less than a day later, followed by months of smaller tremors, they said.
Earthquakes off Japan in 2011, and Sumatra in 2012, similarly triggered mid-size tremors around injection wells in western Texas and southern Colorado, the study found.
"The [waste] fluids are driving the faults to their tipping point," lead author Nicholas van der Elst, a Columbia postdoctoral researcher, said. "The remote triggering by big earthquakes is an indication the area is critically stressed."
Writing in the journal Science, the researchers say the study is the first to link the effect of naturally occurring earthquakes to subsequent activity at faults that have been weakened by human activity.
Passing surface waves from the big events can cause small pressure changes on faults, triggering small earthquakes, they said.
"These passing seismic waves are like a stress test," study coauthor Heather Savage, a Columbia geophysicist, said. "If the number of small earthquakes increases, it could indicate that faults are becoming critically stressed and might soon host a larger earthquake."