The northern third of the Cascadia subduction zone is marked by a change in margin orientation near 48°N in Washington state that extends to the northern half of Vancouver Island. Here, the Juan de Fuca (JdF) and Explorer plates converge with North America at rates near 45 and 20 mm/yr, respectively (DeMets et al., 2010;McCaffrey et al., 2007). Convergence is manifest by seismicity both within the descending oceanic plate(s) and overriding crust, as well as near the JdF plate interface in the form of low frequency earthquakes (LFEs) and tectonic tremor (Bostock et al., 2019;Wech, 2010). Megathrust events (M > 9) originate along the shallow (<20 km) segment of the JdF plate interface but have not been recorded by modern instruments in Cascadia. Several lines of evidence suggest that they occur with an average repeat time around 600 years, with the most recent one having occurred