Uncovering the Physical Controls of Slow Slip Along Cascadia and Other Global Active Tectonic Margins

Abstract

The discovery of slow slip, an intermediary form of strain release characterized by slow slip rates, has been one of the most important advances in seismology in recent decades. We attempt to further our understanding of the physical controls of slow slip in subduction zones using three different approaches. We generate and train statistical learning models through supervised classification with the goal of determining the relationship between five subduction zone parameters and slow slip observations. These parameters include subducting plate age, relative plate velocity, sediment thickness entering the trench, slab dip, and seafloor roughness. We find that young subducting lithosphere is strongly correlated with slow slip in the case of short-term events but not in the case of long-term events. We use the trained models to predict slow slip observations in regions that are sparsely instrumented. Slow slip is predicted to occur in many subduction zones globally, most notably in South America where it is expected to be widespread. In our second approach, we generate thermal models along strike in Cascadia to estimate fluid flux rates near the depths of observed slow slip. We observe a correlation between shorter recurrence times for slow slip and higher fluid flux rates near the location of the mantle wedge corner. This provides support for models where the recurrence interval is dependent on the rate of processes that generate and trap fluids. Finally, we use teleseismic receiver functions to characterize seismic anisotropy in the Cascadia mantle wedge corner. We examine the potential role that widespread serpentinization may have in generating the conditions for slow slip. We observe seismic P-to-S velocity ratio (Vp/Vs) values that are consistent with 40-50% serpentinization. A slow axis of symmetry is estimated, which corresponds with subparallel foliation consistent with a serpentinized shear zone being deformed by the downgoing slab. Overall, these results provide further constraints on the physical controls of slow slip, and thus stress accommodation in subduction zones as a whole. As subduction zones are the site of some of the most devastating earthquakes on Earth, furthering our understanding of subduction zone processes is critical for improving earthquake hazard mitigation efforts.