Dynamic Stress Modeling Applied to Dynamic Triggering Seismicity in Australia and Utah

Static stress changes can trigger nearby earthquakes that occur within a few fault lengths from the causative event. Transient stresses caused by the passage of surface waves commonly trigger events at remote distances; yet, the processes and stresses necessary for remote triggering are poorly documented and not well understood. To understand the causative stresses and environments behind remote, or dynamic, triggering, we must decipher the stresses caused by the passage of the surface waves in relation to the local stress field and fault conditions where the triggered events occur. In this study, we model the change in the stress field that the passing of Rayleigh and Love waves cause on a fault plane of arbitrary orientation relative to the direction of propagation of the waves, and apply a Coulomb failure criteria to calculate the potential of these stress changes to trigger reverse, normal or strike-slip failure. We compare these model results with data from dynamically triggered earthquakes in both the Australian Bowen Basin and the Utah region. In the Bowen Basin, a region located at the margin of a stable continental craton and with little background seismicity our modeling shows that surface waves arriving at 45 degrees from the average local stress field are the most likely to trigger local seismicity, this is confirmed with our data analysis. In Utah, we model two events with known well-constrained, normal focal mechanisms; which allows a direct comparison of the normal and shear stress generated by the triggering events. Our modeling shows that Love waves promoted failure on these normal faults, and allows us to discriminate between the axial planes of a first motion focal mechanism of a dynamically triggered event.