Slow Slip Event Detection in Cascadia Using Vertical Derivatives of Horizontal Stress Rates

We present a new approach assuming elastic behavior at the Earth's surface to invert continuous GPS time series for Vertical Derivatives of Horizontal Stress rates. This approach enables detection of transient slow slip events at the Cascadia subduction zone. Vertical Derivatives of Horizontal Stress rates are the most spatially compact expression of surface deformation due to subsurface deformation sources (compared to GPS displacement/velocity fields, and strain rates), and enable identification of transient slip events and temporal evolution of locking on the interface throughout the slow slip cycle. Slow slip events (SSEs) identified by our method coincide spatially and temporally with tremor episodes, and agree well with SSEs identified by previous geodetic studies. We present daily slip models that fit the Vertical Derivatives of Horizontal Stress rates, showing phases of locking, unlocking, and SSEs on the subduction interface for the 2009–2015 period. Similar to previous studies, our results highlight along‐strike variations in SSEs and locking behavior from northern Cascadia to southern Cascadia. Between events, the SSE source zone in northern Cascadia appears to be completely locked. In contrast, we suggest that some fraction of plate motion in the SSE zone in central and southern Cascadia is accommodated by steady creep (or very small, frequent SSEs) between large SSE events. We observe rapid relocking of the interface (within weeks) following the end of most SSEs, with implications for the time scales of fault healing following deformation events. Our results present a promising approach to transient deformation detection, with a fine temporal and spatial resolution of the surface expression of deformation. Key Points A new time‐dependent approach with elasticity constraints at the Earth's surface captures Cascadia slow slip and locking We confirm strong along‐strike variation in Cascadia ETS and locking behavior The subduction interface typically relocks within weeks of the end of slow slip