Connecting near-field and far-field earthquake triggering to dynamic strain

Any earthquake can trigger more earthquakes. This triggering occurs in both the classical aftershock zone as well as the far field. These populations of triggered earthquakes may or may not be distinct in terms of triggering mechanism. Here we look for a distinction between the populations by examining how the observed intensity of triggering scales with the amplitude of the triggering strain in each. To do so, we apply a new statistical metric based on earthquake interevent times to a large data set and measure earthquake triggering as a function of dynamic strain amplitude, where strain is estimated from empirical ground motion regressions. This method allows us to identify triggering at dynamic strain amplitudes down to 3 × 10^-9, orders of magnitude smaller than previously reported. This threshold appears to be an observational limit and shows that extremely small dynamic strains can trigger faults that are sufficiently near failure. Using a probabilistic model to transform measured interevent times to seismicity rate changes, we find that triggering rates in the far field scale with peak dynamic strain. This scaling, projected into the near field, accounts for 15%-60% of earthquakes within 6 km of magnitude 3-5.5 earthquakes. Statistical seismicity simulations validate the interevent time method and show that the data are consistent with the number of far-field triggered earthquakes being linearly proportional to peak dynamic strain. We interpret the additional near-field component as reflecting either static stress triggering, more effective dynamic triggering at higher frequencies, or a concentration of aftershock nucleation sites very near main shocks.