CASCADIA ETS IN TIDAL RECORDS 1980 TO PRESENT

Tide gauge data has previously been used to estimate vertical displacements, uplift rates between events, and net uplift rates spanning 12 episodic tremor and slip events (ETS) on the northern Cascadia Subduction Zone (between 1997 and 2010) [Alba et al., 2019]. Dense GPS networks have thus far been the primary means to characterize displacements and inter-ETS strain rates associated with ETS that occurred beneath the Olympic Peninsula, WA [McGuire & Segall, 2003; Szeliga et al., 2004; Melbourne et al., 2005; Wang et al., 2008; McCaffrey, 2009; Schmidt & Gao, 2010]. We are exploring the use of tidal records to extend the observations of Cascadia ETS back into the pre-GPS era. By searching GPS-era hourly water level records from 4 NOAA tide gauges (Neah Bay, Port Angeles, Port Townsend, and Seattle), a clear signal of periodic transient water level changes corresponding to the phase and period of known ETS events is apparent. We have established that events with a characteristic uplift pattern can be found using a cross-correlation and estimated periodicity (from tidal records alone). The result of a search of the GPS-era data was an estimated recurrence interval of 14.6 ± 1.1 months, nearly identical the interval calculated from GPS-derived dates. In a similar search of pre-GPS (1980-1995) water levels we do not find a clear pattern corresponding to a recurrence interval of 14.6 months, suggesting that the time interval, phase, or average displacement pattern of Cascadia ETS has changed since 1980.