Technical description of the project: Although advances are constantly being made in our understanding of the physics governing earthquake rupture, many questions remain. One fundamental question is ?Does a stress threshold exist (i.e., amplitude, frequency, stress orientation, or duration of shaking) for triggering earthquakes?? We propose to test two hypotheses related to earthquake triggering: (1) There exists a minimum dynamic stress threshold (i.e., in amplitude, frequency or both) required to trigger remote earthquakes; and (2) The stresses induced by the passage of seismic waves must align favorably with the local stress field and orientation of faults in order to trigger earthquakes. To test these hypotheses, we will use data collected by stations in EarthScope?s USArray Transportable Array (USArray TA) network, supplemented by local seismic network data when available. The uniform spacing of the USArray TA stations across the contiguous USA will allow us to examine characteristics of remote triggering within a variety of tectonic provinces, background seismicity rates, and within regions of both documented cases of triggered earthquakes and areas of no known triggered earthquakes. For each TA station we will create a catalog high-frequency detections (HFD). We will apply three statistical tests (Binomial, Kolmogorov-Smirnov and Wilcoxon Rank-sum) to determine the significance of HFD rate changes at each station following the surface waves (or S-arrival for local events). Working in tandem with Product Specialists at IRIS, we will augment Ground Motion Visualization (GMV) movies of the temporal evolution of the seismic wavefield to include spatial/temporal markers denoting high-frequency triggers and the orientation of theoretical great-circle paths. We will qualitatively examine these visualizations for signatures of systematic remote earthquake triggering. We test our hypotheses by: (1) Determining the peak dynamic stress and dominant frequency of the passing seismic waves coincident with each of the high-frequency triggers; and (2) Comparing the orientation of the passing seismic waves with the local fault structures and stress field.
Non-technical explanation of the project's broader significance and importance: A growing body of scientific studies has demonstrated that the passage of transient signals, or seismic waves, from large earthquakes can remotely trigger small earthquakes thousands of kilometers from an epicenter of a large earthquake. We will systematically investigate triggering using data collected from EarthScope?s USArray Transportable Array (USArray TA) network for global and regional earthquakes, which could provide fundamental insight into the physical mechanisms of earthquake rupture. We will provide the tools and expertise to effectively mine the large USArray dataset (>1,000 stations of which >400 are concurrently active; >89,000 earthquakes) for currently unrecognized triggered earthquakes. We will catalog these triggered events by station, thus having a spatial/temporal record for further analysis. We will also develop interactive visualization tools for comparing the spatial and temporal distribution of triggering to the seismic wavefield. The methodology we will develop, and the number of mainshocks systematically processed, have the potential to be transformative in terms of understanding earthquake triggering processes. The University of Texas at El Paso is a Hispanic Serving Institute (HSI) and we will actively recruit undergraduate and graduate students, plus a postdoctoral scholar from underrepresented groups to participate in this project.
