Spatial Distribution of Tremor Episodes From Long-Term Monitoring in the Northern Cascadia Subduction Zone

Abstract

Large bursts of non-volcanic tremor (“major” tremor episodes) correlated with geodetic deformation recur regularly in the Cascadia subduction zone and are often called episodic tremor and slip (ETS). Minor episodes of tremor between ETS are ubiquitous but have been understudied. This paper assesses time-invariant characteristics of tremor episodes of all sizes within northern Cascadia. We derive a catalog of tremor episodes ranging in size from 10 to >13,000 tremor events using the results of 17 years of tremor monitoring. Minor episodes represent ∼96% of all 896 tremor episodes and their occurrence varies on 10-km scales. Using estimates for the depth of the forearc Moho and subducting slab, we observe an association between the location of the forearc mantle corner (FMC) and tremor occurrence that leads to along-dip modality. Bimodality, present in southern Washington and Vancouver Island, represents the segmentation of major and minor episodes up-dip and down-dip of the FMC, respectively. Unimodality, present in Puget Sound, results when the FMC is located near the down-dip edge of the ETS zone and no segmentation occurs. We also use our extensive tremor episode catalog alongside three-dimensional regional tomographic velocity models to reassess the relationship between tremor activity and low Vp/Vs signatures in the forearc. We do not find a correlation between tremor episode recurrence intervals and Vp/Vs, contrary to some previous work, suggesting that controls on silica precipitation in the forearc crust are not dominant controls of tremor episode recurrence, or that the association is not widely observable.