A High-resolution Image of Tremor Migrations Beneath the Olympic Peninsula from Stacked Array of Arrays Data

Abstract:

Non-volcanic tremor is generally interpreted as the seismic manifestation of slow slip, and tremor locations have been used extensively to infer detailed behaviours of slow slip fronts due to higher spatial and temporal resolution over geodetic observations. Taking advantage of S-wave coherence among stations separated by 10-20 km, Armbruster et al. [2014] and Rubin and Armbruster [2013] obtained high precision tremor locations using “cross-station” cross correlations. However, in principle “cross-station” methods do not perform well when the response to an impulsive tremor source has coda with amplitude comparable to the main arrival. Based on the catalogs of LFE families in northern Washington from Royer and Bostock [2013], we stacked seismograms to obtain LFE templates for stations from the Array of Arrays (AofA) and observed large-amplitude coda in the waveforms. In this study, we attempt to eliminate non-coherent coda and noise by stacking seismograms at stations within individual arrays based on empirical time offsets determined from the templates. We compare 4-s windows of those stacks at 3 arrays, instead of seismograms at 3 single stations, after correcting for shear wave splitting, and rotating into the empirical shear wave polarization directions [Peng et al., AGU 2013]. Only detections within 8 km of the corresponding template are included such that there is little penalty associated with fixing array time offsets, splitting parameters and station rotations within each array. For each template, the 3-array detector obtains 3-10 times more LFE events than a conventional 3-station detector. We find numerous rapid tremor migrations propagating along the main front, one of which continue to propagate back into regions that have already ruptured. These observations are consistent with tremor migration patterns beneath southern Vancouver Island [Peng et al., AGU 2013]. We also observe several unusually fast back-propagating fronts originating from the main front, with propagation speeds of 700-1000 km/day.