The Cascadia margin is characterized by a young (<8-million-year-old) subducting slab stretching from British Columbia, Canada, to northern California, marking the convergent boundary between the North America plate and the oceanic Juan de Fuca and Explorer plates (Figure 1). Geodetic measurements over the past two decades have established that the shallow portion (depth <15 kilometers) of the interface between the subducting Juan de Fuca plate and the overriding North America plate is strongly coupled. The inevitable “unlocking” process can and will generate a megathrust earthquake with a magnitude as large as 9, as confirmed by the studies of paleoearthquakes in the region. Paleoseismic and tsunami data have indicated that the last Cascadia megathrust event (magnitude ∼9) occurred in 1700 [e.g., Satake et al., 2003].
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