Séminaire ISTeP - Michael Bostock

(EOS, UBC, Canada)

LVZs and LFEs: New Perspectives on Slab Structure and Slow Slip in Northern Cascadia

Landward dipping low-velocity zones (LVZs) that align approximately with intraplate seismicity are seismically detected in virtually all subduction zones. I review studies that constrain the properties of the LVZ in Cascadia where it has been extensively studied for over a quarter century. A model in which the LVZ is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal Layer 2 is consistent with available data and reconciles previously conflicting interpretations. In this model, the upper oceanic crust is pervasively hydrated through circulation at the ridge and becomes overpressured upon subduction as a result of dehydration reactions combined with impermeable plate boundary above and low-porosity metagabbroic Layer 3 below. Near-lithostatic fluid pressures within the LVZ lead to unusually high Vp/Vs ratios and low S-velocities, and conceivably alter the rheological properties of the plate boundary region where episodic tremor and slip occur. Using network detectors, we have assembled low-frequency earthquake (LFE) templates from tremor in 4 regions within Vancouver Island and Washington state. These templates represent band-limited empirical Green's functions and I will demonstrate how they can be exploited to further improve our understanding of tremor locations, mechanisms and LVZ structure.

13/11/2013 à 16h, Salle Fourcade