Nonlinear dynamics, granular media and dynamic earthquake triggering
Résumé
The 1992 magnitude 7.3 Landers earthquake triggered an exceptional number of additional earthquakes within California and as far north as Yellowstone and Montana1–3. 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 Mine1 and the 2002 magnitude 7.9 Denali4 earthquakes—and in the near-field as aftershocks5. The physical origin of dynamic triggering, however, remains one of the least understood aspects of earthquake nucleation1–5. 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 1026 and wavelength 1m corresponds to a displacement amplitude of about 1027 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 surrogate6,7. From these we infer that, if the fault is weak8–10, 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.