Song, Richards and colleagues studied waves generated by smallearthquakes in the southern Atlantic Ocean that passed through the core to a seismograph in Alaska where they were measured.
Study co-author Geoffrey Abers of Columbia University said that "there's something important about getting unexpectedly large earthquakes out of small systems that we have discovered here, " adding that "the risk of humans inducing large earthquakes from even small injection activities is probably higher" than had been believed.
There was a ripple of fear in Britain when test-drilling for shale gas in north-west England set off small earth tremors, but the evidence so far suggests that earthquakes should not be a worry.
Shale-gas producers also face opposition from greens, who object to the industry's heavy water usage and a small risk that fracking could lead to contamination of aquifers and even to earthquakes.