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Seismic velocity structure under Vancouver Island from travel time inversion : insight from Low Frequency Earthquakes Savard, Geneviève

Abstract

The Cascadia subduction zone is a warm subduction zone where Episodic Tremor and Slow Slip (ETS), a phenomenon with debated governing physical mechanisms, is observed. The eventual development of an accurate physical model of ETS relies on a comprehensive set of seismic and geodetic observations. Low Frequency Earthquakes (LFE) are one of the seismic signatures that accompany slow slip and possess discernible phase arrivals that can be used for traveltime inversion methods. I first address the issue of detecting LFEs by developing an automated algorithm that exploits cross-station waveform similarity. The algorithm detects thousands of events per ETS episode in southern Vancouver Island that reveal pronounced spatiotemporal clustering and complex propagation patterns that compare favourably with independent ob- servations in Cascadia. This method allows us to build a significantly improved catalogue of LFE templates that we next employed for local double-difference earthquake tomography with the aim of resolving structures in the vicinity of ETS and metamorphic controls on its generation. In southern Vancouver Island, tomographic images reveal high Poisson’s ratios associated with a dipping low‐velocity zone (LVZ) inferred to be overpressured, upper oceanic crust of the Juan de Fuca plate where LFEs and other slow‐slip phenomena occur. We also observe a low Poisson’s ratio anomaly ( ∼0.225) in the forearc continental crust above the mantle wedge coinciding with high Vp and high levels of clustered microseismicity. We develop a conceptual model where quartz concentration is produced by metasomatism in the forearc crust and catalyzed by a focussed ingress of slab-derived fluids at high pore pressure. In northern Vancouver Island, we also observe the LVZ and a low Poisson’s ratio anomaly in a similar position, but no associated microseismic activity. Distinct, parallel lineaments that trend slightly oblique to the strike of the megathrust are resolved within seismicity concentrations of the Nootka Fault Zone immediately offshore Nootka Island. We speculate that these features may represent faulting developed as a result of high strains associated with slab bending, amplified in the vicinity of the newly formed plate boundary. We also find possible evidence for a deep-seated, seismogenic fault near the Explorer Plate edge. Supplementary materials available at: http://hdl.handle.net/2429/68898

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