Structural Imaging of the Transition Zone in Northern Cascadia Using Low Frequency Earthquake Templates

English Abstract:

Episodic tremor and slow slip occurs in warm subduction zones such as Cascadia and is thought to be influenced, directly or indirectly, by the presence of fluids. For example, the supply of free water by dehydration of the subducting oceanic plate through prograde metamorphism, serpentinization of the mantle wedge, and the transition in frictional regime at the plate interface from stick-slip to aseismic sliding due to high pore fluid pressures are processes that have been invoked to explain the distribution and nature of episodic tremor and slip. To elucidate the relationships between fluids, structure and seismic behaviour, we undertake a structural imaging study of the forearc structure below southern Vancouver Island in the northern Cascadia subduction zone. While the distribution and occurrence of regular earthquakes in the region is sparse, low frequency earthquakes (LFEs) that comprise tectonic tremor are recorded during several days every 14 months or so. From these LFE detections, we build high SNR LFE templates that approximate Green's functions for sources on the plate boundary. We have augmented an existing dataset of LFE templates for a total ~150 independent LFE template locations. We combine this catalogue with regular intraplate earthquakes and extract direct P- and S-waves traveltimes to perform joint tomography of the subducting oceanic plate and mantle wedge. In addition, we plan to characterize fine-scale discontinuous structure within the subducting oceanic crust and adjacent plate boundary using forward and back-scattered converted waves that have been previously identified within LFE templates.