Exploring the influence of tidal stress changes on the generation of secondary slip fronts during slow slip events in Cascadia

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Energy within the Earth is released on faults that can either rupture abruptly, causing hazardous earthquakes, or slip slowly in largely aseismic events that can last from hours to years. Slow-slip phenomena occur frequently in Cascadia, responding to and modifying conditions along the plate interface beneath the locked region where the next mega-thrust earthquake will occur. The specific physical processes that govern slow-slip behavior also determine the state of the potential rupture surface nearest to the major population centers in the Pacific Northwest. Discriminating between the different rheological models and mechanical treatments that have been proposed to explain slow slip is challenging because many of them can reproduce the primary characteristics of slow slip events, such as propagation speeds, stress drops, and recurrence intervals. This makes all models that can reproduce these features equally plausible. The recent discovery of "secondary" slip fronts that occur in conjunction with slow slip provides a new opportunity to distinguish between competing models. Accordingly, this project will characterize secondary slip fronts by determining their spatial extents, slip velocities, and stress drops. These diagnostics will then be compared with modeled secondary slip fronts to determine which of the competing model formulations is able to reproduce the observations and provide a window into changing fault conditions beneath the Pacific Northwest. Slow-slip phenomena require the slip rate to increase to observed speeds, typically 10 to 100 times the plate rate, but refrain from accelerating fast enough to generate seismic waves. The specific physical mechanisms responsible for imposing this speed limit are in dispute. Our research combines observational and theoretical components to examine the influence of tidal stress changes on slow-slip processes, thereby offering an objective test of several competing model treatments that have succeeded in reproducing the first-order characteristics of slow slip (e.g. slip speeds, stress drops, etc.). The proposed observational effort will use Principle Component Analysis on a low-frequency earthquake dataset from Cascadia to systematically quantify the length scales, time scales, propagation speeds, and propagation directions of secondary fronts that immediately follow passage of the main slip front. The theoretical effort will incorporate shear and normal stress oscillations into slow-slip simulations that include rate-and-state formulations and dilatancy hardening. The observational catalog that we assemble will be used to test and refine model treatments that seek agreement between predicted and actual characteristics of secondary fronts that propagate along the Cascadia mega-thrust.