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Nonlinear Dynamics, Granular Media and Earthquake Triggering



The 1992 Landers earthquake was a seminal event in earthquake studies. As seismic waves propagated northward from the epicenter, earthquakes occurred as far north as Yellowstone and into Montana coincident with the arrival time of seismic waves (or soon thereafter).

The association in space and time of the events and seismic waves led many to speculate that the events were triggered by the seismic waves. Since this remarkable observation, there has been a concerted effort amongst researchers to determine if dynamic triggering of events actually takes place.

Studies of the 1999 Hector Mine earthquake confirmed that dynamic wave excitation (seismic waves) triggered seismicity rate increases on the Landers, Hector Mine and Denali Faults, among others. These studies confirm that dynamic and static stress fields are important in triggering in the near field, and dynamic induced wave triggering dominates at larger distances.

The physical origin of dynamic triggering remains one of the least understood aspects of earthquake nucleation. The dynamic strain amplitudes from a large earthquake are exceedingly small once the waves have propagated more than several fault radii, of order 10-7-10-6. The question is how do such small strains trigger earthquakes?


 
 

In our work we hypothesize that the dynamic, elastic-nonlinear behaviour of fault gouge perturbed by a seismic wave may be responsible. We base our hypothesis on recent laboratory dynamic experiments conducted in granular media, a fault gouge surrogate. From these we infer that, if the fault is already in a weakened state, seismic waves cause the fault core modulus to abruptly decrease and weaken further. If the fault is already near failure, the process could induce fault slip. Supporting our hypothesis are recent stick-slip laboratory experiments where elastic wave excitation is applied.

Our experiments indicate that elastic waves of strains of order 5x10-5 can cause shear weakening. Recent seismic observations show that a "triggering threshold" exists at strain amplitudes just over 10-6, corresponding to where we observe the onset of elastic nonlinear behavior in the laboratory. Long rang goals include continued testing of the hypothesis with laboratory stick-slip experiments and seismic observations.

Work being conducted in collaboration with Joan Gomberg of the USGS and Chris Marone of the Geosciences Department at the Pennsylvania State University

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