Segmentation of Episodic Tremor and Slip (ETS) in Cascadia and the possibility of ETS triggered earthquakes

Abstract:

Evidence is growing that the main mechanical control on the Episodic Tremor and Slip (ETS) region in Cascadia is the presence of high pore fluid pressure (e.g. Audet et al., 2010; Hyndman et al., 2015). This contrasts with an earlier hypothesis that ETS lies at the transition between the locked plate interface and continuous creep at depth, which frictional properties are conditionally stable (e.g. Schwartz and Rokosky, 2007). Friction laws and laboratory experiments still predict slow slip in this transition zone (e.g. Leeman et al., 2016). Here we present geodetic evidence of a possible detection of a second ETS zone at this frictional transition, the presence of which is important for understanding the potential role of ETS in earthquake triggering. We also present modeling of stress changes applied by ETS events to the base of the locked zone and discuss the role of ETS stress in modulating earthquake probabilities. In addition, Cascadia ETS displays strong segmentation, including 3 main segments and subsegments within those. Segmentation of the role of ETS in the overall slip budget, which ranges from almost 0 to 100% or more of the budget, lines up well with segmentation in tremor recurrence intervals (Brudzinski and Allen, 2007) and paleoseismic events (Goldfinger et al., 2012). Stresses applied by ETS to the base of the locked zone, and thus the potential role of ETS in earthquake triggering, is segment dependent. Paleoseismic evidence indicates most frequent rupture of the southern segment, which is also where ETS slip and thus ETS stress changes are the greatest. This may hint at a triggering relationship between ETS and megathrust earthquakes.