Capturing and comparing slow slip events in Cascadia and New Zealand using a novel, physics-based approach

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Over the last 15-20 years, GPS technology has revolutionized our ability to track spatial and temporal variations in rates of deformation at tectonic plate boundaries around the globe. Typically, these data are analysed in terms of (a) time-averaged site velocities; (b) discrete displacements; (c) temporal variations in displacement rates, or (d) converting GPS velocities into strain rates. Notably, continuously operating GPS instruments at plate boundaries around the world have led to the discovery of ?slow slip events?, which are a previously unknown form of fault slip that involves transient aseismic slip across a fault (lasting weeks to months) at a rate intermediate between steady plate boundary displacement and the slip velocity required to generate earthquakes. To push the boundaries on the limits of spatial and temporal resolution of deformation processes that can be obtained by GPS, this project will adapt a novel, physics-based approach that our team has recently developed to use continuous GPS (cGPS) data to objectively detect and characterize slow slip events at a higher spatio-temporal resolution than can be done using existing methods. Unlike conventional methods, our new approach (for the first time) obtains Vertical Derivatives of Horizontal Stress (VDoHS) rates from cGPS timeseries. VDoHS rates are the most tightly resolved surface expression of subsurface forces associated with these (and other) deformation events. We will implement our new method to characterize and detect slow slip events (SSEs) at the Cascadia subduction zone (Pacific Northwest) and the Hikurangi subduction margin (New Zealand), both of which are well-covered by dense continuous GPS networks. Moreover, due to the strong differences in the duration and magnitude characteristics of SSEs at Hikurangi and Cascadia, this project will allow us to investigate a broad spectrum of slow slip event types using the new method.The observation of SSEs and associated seismic phenomena at subduction zone plate boundaries worldwide has ignited one of the most dynamic fields of research in seismology today. To advance the overall understanding of SSE processes, our new method will (a) lower the threshold of geodetically detectable SSEs, increasing our knowledge of the full spectrum of SSE slip behavior; (b) enable objective, high resolution identification of SSEs, and reveal the relationship of this to temporal variations in megathrust locking, and (c) foster a detailed comparison of Hikurangi and Cascadia SSEs and their relationship to seismicity and tremor. We also expect that the adaptation of our method to cGPS timeseries to analyse transient deformation will pave the way for a much needed approach to incorporating cGPS data into time-dependent seismic hazard models.