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Analysis of the Atypical 2018 and 2019 Episodic Tremor and Slip Events in Northern Cascadia

Episodic tremor and slip (ETS) events in Cascadia are characterized by a transient reversal of long-term crustal movement and low-frequency, semi-continuous seismic signals. These near-annual events are generally considered to be related to subduction processes; however, the depth distribution of ETS tremor does not appear to cluster around the subduction interface (Kao et al., 2009). This study examines the tremor and slip distribution of the 2018 and 2019 Northern Cascadia ETS events. These most recent events were of typical duration (4-6 weeks) but were each interrupted by a 1-2 month break during which minimal tremor activity occurred. To provide insight into whether each of the 2018 and 2019 ETS events should be considered a single interrupted event or two short, separate events, we determine the cumulative slip and computed seismic moments of each. Geodetic slip is estimated using GNSS displacements in Canada and the US and inverted using GTDef (Chen et al., 2009). The location of tremor is determined using a modified version of the Seismicity-Scanning based on Navigated Automatic Phase-picking (S-SNAP) method (Tan et al., 2019), which uses a combination of source scanning, kurtosis-based phase picking and the maximum intersection location technique to automatically detect and locate the source of seismic signals. The depth distribution of tremor in the two most recent events is compared with relocated tremor from past events and analyzed for its relation to the current interpretation of the plate interface along the subduction margin. Understanding the nature of tremor at depth may improve our understanding of the Cascadia subduction zone structure and its potential for megathrust earthquakes.

Presenting Author: Madison Bombardier

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