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Geophysics Seminar: Prof. Pascal Audet, University of Ottawa
March 20, 2018 @ 4:00 pm - 5:00 pm
Prof. Pascal Audet, University of Ottawa
Title: Development of shear zone and fluid pressure along the megathrust fault at the Cascadia subduction zone
It has been the subject of numerous Hollywood movies, and is often referred to as “the big one” – a reference to the potentially massive earthquake and resulting tsunami that will, one day, affect North America’s west coast. But how does one measure the potential impact of an earthquake? One of the ways to constrain the properties of future earthquakes is to characterize the geological environment at the depths where they are expected to occur using seismic imaging techniques. In particular, we use the properties of seismic waves to refract and convert at sharp interfaces to probe the geological structures across the transition from seismic (i.e., where earthquakes happen) to aseismic (i.e., where rocks slowly creep) behaviour along the fault line. In a recent study that combines seismic data from both offshore and land seismograph stations, we image the down dip variations in the signature of a dipping layer characterized by extremely low seismic wavespeeds that coincides with the megathrust fault zone. By relating the low wavespeeds to the levels of fluids in the rocks, we infer that at shallow depths the fluids are free to circulate through the rocks and friction is elevated; at greater depth the fault becomes a shear zone and fluids become trapped in the rocks, decreasing friction and preventing earthquake propagation. The transition from high to low fault friction (or low to high fluid pressure) coincides with the coastline at the surface, which implies that the rupture will mainly occur offshore during a large earthquake. These findings help constrain the factors responsible for earthquake ruptures at subduction zones, and can be used as to help estimate the level of shaking during a large earthquake.