Earthquake triggering provides a fundamental clue into earthquake nucleation. If an observational relationship could be established between seismicity rate changes and the triggering strain of an earthquake, then we would have a basic constraint on the nucleation process. Here we use a novel statistical approach and an abundance of modern data to measure the number of earthquakes triggered for both nearby and distant earthquakes. The preliminary results indicate that the intensity of triggering scales continuously as a function of dynamic strain as measured by seismic wave amplitude. The trend is consistent regardless of distance. However, some regions, like California, are more prone to triggering than others, like Japan. The lack of discernible crossover in triggering behavior at small distances implies that dynamic triggering is sufficient to explain local aftershocks.
This result points the way to a new methodology for earthquake forecasting based on the amplitude of the observed seismic waves.
This proposal presents a plan of study to test, analyze and apply these preliminary results. Specifically, we will: 1) Evaluate the robustness of the relationship between triggering rate and strain by: a) Varying the choices of parameters in the measurement scheme. b) Improving the proxies for dynamic and static strain by using observed seismograms corrected to depth and measured focal mechanisms. 2) Constrain the physics of earthquake triggering by: a) Determining whether immediate triggering or delayed triggering is a stronger signal using filtered waveforms and the distribution of the normalized inter-event times. b) Comparing the tendency to trigger across different tectonic regimes in New Zealand and globally. c) Comparing the results to standard models of earthquake triggering. 3) Apply the results to hazard mitigation by: a) Combining the interpolated ground velocity from an earthquake with the triggered rate function to predict triggered seismicity in the next hour.
Any earthquake can trigger more earthquakes. This triggering occurs in both? the classical aftershock zone and the far field. The triggering mechanism of these populations of triggered earthquakes may or may not be distinct. In this work, we ?looked for a distinction between the populations by examining how the observed intensity of triggered earthquakes scales with the amplitude of the triggering strain. To do so, we developed a new statistical metric based on earthquake interevent times to a large data set and measured earthquake triggering as a function of the dynamic strain of the perturbing seismic waves. This method allows us to identify triggering at dynamic strain amplitudes down to 3 × 10−9, which is orders of magnitude smaller than previously reported. This new threshold appears to be an observational limit and shows that extremely small dynamic strains can trigger faults that are sufficiently near failure. We also found that triggering rates in the far field are a function of the peak dynamic strain. The data is consistent with the number of triggered earthquakes being linearly proportional to peak dynamic strain. The new relationship provides a method to predict future earthquake rates locally following a distant, large earthquake. The observed dynamic triggering scaling, projected into the near field, accounts for 15%–60% of earthquakes within 6 km of magnitude 3–5.5 earthquakes. We interpret the additional near?field component as reflecting a combination of static stress triggering, more effective dynamic triggering at higher frequencies, and/or a concentration of aftershock nucleation sites very near main shocks. In addition, we examined the possibility that the recent magnitude 8 earthquakes were linked globally by dynamic triggering. We found that the hypothesis is physically reasonable, but the current offshore earthquake data is too limited to provide a definitive test. We calculated specific requirements for offshore instrumentation that will allow future evaluation of the possible links between subduction zone mega-earthquakes. The broadest impact of the work is the new method to predict future earthquake rates based on the measureable, observable amplitude of seismic waves arriving from a distant earthquake. The intellectual merit includes the development of a new statistical metric to measure triggering, the elucidation of the connection between near and farfield triggering and the discovery that the number of earthquakes triggered is directly related to the amplitude of the perturbing waves.
