Tectonic tremor source depths in relation to subduction zone structure in northern Cascadia

Abstract

Tectonic tremor occurs in frequent episodes throughout northern Cascadia, typically during slow slip events deep in the subduction zone. Adequately constraining tremor source depths is a challenge associated with using low-amplitude, emergent waveforms. In this study, we present a tremor catalogue with improved depth resolution computed using the differential traveltime Bayesian inversion method, where tremor sources are represented as 3D probability distributions. We examine tremor source depths throughout northern Cascadia in relation to low-frequency earthquakes (LFEs) and key features in the subduction zone imaged using receiver-function analysis. We show that the highest concentrations of tremor are 5–10 km shallower than LFEs everywhere in northern Cascadia. We also find that neither tremor nor LFEs localize consistently to the top of the oceanic crust, which is often interpreted to represent the subduction fault. Rather, tremor localizes throughout a volume corresponding to the deep accretionary complex at depths deeper than 15 km. Where the accretionary complex appears to be vertically-truncated by forearc terranes, such as the Olympic Accretionary Complex in Washington, the tremor depth distribution is similarly vertically restricted. Depth trends of tremor and LFEs may indicate deformation within and above the downgoing plate that is not represented in current models of the subduction zone. We suggest that tremor-generating processes may involve multiple mechanisms that are partly dependent on their distance from the downgoing plate and occur within the compositionally- and structurally-heterogeneous mélange of the deep accretionary complex.