Analysis of the 2018 and 2019 Episodic Tremor and Slip Events in Northern Cascadia Using Novel Location Methods

Abstract:

Episodic tremor and slip (ETS) in Cascadia is characterized by a transient reversal of long-term crustal movement and low-frequency, semi-continuous seismic signals. These near-annual episodes are generally considered to be related to subduction processes, however the depth distribution of Cascadia ETS tremor does not appear to be localized to the subduction interface (Kao et al., 2009). This study presents a modified version of the Seismicity-Scanning based on Navigated Automatic Phase-picking (S-SNAP) methods (Tan et al., 2019) which are used to examine the tremor source distribution of the 2018 and 2019 Northern Cascadia ETS episodes. The modified S-SNAP methods include a sequence of the Source Scanning Algorithm (Kao and Shan, 2004), cross-correlation-based arrival times, and the maximum intersection location technique (Font et al., 2004) to automatically detect and locate the source of seismic tremor. The efficacy of these methods are verified by independently locating the catalog of tremor events in the 2003 and 2004 ETS episodes determined by Kao et al. (2009). This relocated catalog is then compared with the location of tremor events from the two most recent ETS episodes. The depth distribution of tremor is 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.