The role of Episodic Tremor and Slip (ETS) in Cascadia: Slip budget, time-dependent behavior, and variability in recurrence, slip, slip rate, depth, and durations of events

Abstract:

We have created a new, self-consistent, continuous time-dependent inversion of ETS slip rate in Cascadia for the period 2006-2019 using the Network Inversion Filter applied to daily GNSS solutions from the University of Nevado - Reno geodetic laboratory . We use heterogeneous elastic Green’s functions computed using the PyLith finite element software (Aagaard et al., 2013, 2019), with density (and by extension, elastic properties) estimated via empirical relationships with V_P and V_S (Brocher, 2005), using the velocity model of Stephenson (2007). We use this model to automatically detect ETS events based on criteria of a minimum slip rate for a minimum period of time in some region. From these detections we build a catalog of events including properties such as depth, updip limit of slip, peak slip rate, total slip, recurrence interval, and event duration. We explore how these properties vary in time and space, including whether any of these properties show consistent evolution with time. Some modeling studies predict slow slip events may become smaller and more frequent, and/or the updip limit may become shallower as the megathrust approaches the time of the next great earthquake (e.g. Luo and Liu, 2019). It is therefore important to create a self-consistent way of measuring these properties to monitor for changes. We also show that geodetically measured slip rate and seismically detected tremor from the Pacific Northwest Seismic Center catalog (Wech & Creager, 2008; Wech, 2010) track together extremely well in every detected ETS event for within the time period covered by the tremor catalog, similar to what has been shown previously for individual ETS events (Bartlow et al., 2011; Wech and Bartlow, 2014). There is no consistent leading or lagging of tremor relative to slip, confirming that that they are two aspects of the same process. Lastly, we compare our results for cumulative slip from this time-dependent model to those of Bartlow (2020), which used a static approach and discuss the role of ETS in the overall slip budget in Cascadia. We show that total slip from the time-dependent approach is consistent with the results of Bartlow (2020), but with considerably less spatial resolution, illustrating the trade-off between spatial and temporal resolution in ETS modeling.