Nonlinear dynamics, granular media and dynamic earthquake triggering

Trigger action The Landers earthquake of 1992 triggered a series of earthquakes in other parts of California, focusing attention on the mechanism of remote triggering. Laboratory experiments on granular material under pressure suggest that triggering is a result of seismic waves impinging on a fault and inducing elastic nonlinear behaviour of the fault core, accompanied by instantaneous weakening and failure. The resulting ‘softening-to-weakening’ model fits in well with field observations of real earthquakes. The 1992 magnitude 7.3 Landers earthquake triggered an exceptional number of additional earthquakes within California and as far north as Yellowstone and Montana 1 , 2 , 3 . Since this observation, other large earthquakes have been shown to induce dynamic triggering at remote distances—for example, after the 1999 magnitude 7.1 Hector Mine 1 and the 2002 magnitude 7.9 Denali 4 earthquakes—and in the near-field as aftershocks 5 . The physical origin of dynamic triggering, however, remains one of the least understood aspects of earthquake nucleation 1 , 2 , 3 , 4 , 5 . The dynamic strain amplitudes from a large earthquake are exceedingly small once the waves have propagated more than several fault radii. For example, a strain wave amplitude of 10 -6 and wavelength 1 m corresponds to a displacement amplitude of about 10 -7 m. Here we show that the dynamic, elastic-nonlinear behaviour of fault gouge perturbed by a seismic wave may trigger earthquakes, even with such small strains. We base our hypothesis on recent laboratory dynamic experiments conducted in granular media, a fault gouge surrogate 6 , 7 . From these we infer that, if the fault is weak 8 , 9 , 10 , seismic waves cause the fault core modulus to decrease abruptly and weaken further. If the fault is already near failure, this process could therefore induce fault slip.