A team of researchers with Second University of Naples has developed a model that may help explain how it is that some earthquakes set off other earthquakes up to a thousand miles away.
Earthquakes happen when compressed rock slides against other rock along a fault line – but what researchers have struggled to understand is how or why some earthquakes seem to cause other earthquakes to occur, that are too far away to be chalked up to mere ground vibrations. In their paper published in Physical Review Letters, the team describes how they constructed a physical model.
To better understand what happens, the researchers set up some rocks in their laboratory, with some round granules between them – then applied pressure. As the rocks finally slipped, mimicking a real earthquake, the team recorded what happened with the grains between them. They found that they emitted acoustic waves.
Their model consisted of two rough plates pressed together with a bed of spherical grains in between. Under an applied shear stress, the plates exhibited occasional slips, just as in real faults. But when the team perturbed their system with externally produced acoustic waves, they found a process occurred they call “lathering” or fluidization, occurred earlier.
As predicted by acoustic fluidization, this “triggering” occurs at a resonant frequency corresponding to waves bouncing back and forth inside the fault. The researchers found that these same resonant waves appeared spontaneously in unperturbed systems a short time before each slip, suggesting that acoustic waves may internally arise inside a fault and trigger a slip.
The results beg the question: Why don’t all earthquakes set off lots of other earthquakes in distant places? The answer, the team notes lies in the frequency of the acoustics waves – lathering only occurs for any given fault line within a certain narrow frequency range, and the impact they have will depend on the state of other fault lines – they have to be near ready to slip on their own.
The idea put forth by the team should not be too difficult to study in real situations, as acoustic recordings that occur in the vicinity of earthquakes could be compared with those captured at distant sites that have what appear to be, related seismic events.