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Figures
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Fig. 1. Tectonic setting of western British Columbia and Washington state. The oceanic Juan de Fuca plate is moving beneath the continental North America plate at a rate of about 4 cm/year. Great earthquakes occur along part of the boundary between the two plates. |
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Fig. 2. Schematic diagram illustrating interseismic and coseismic deformation associated with a subduction thrust fault. Top: elastic deformation builds up between great earthquakes if the thrust fault is locked; the edge of the overriding plate is dragged down and a flexural bulge forms farther landward. Bottom: during a great earthquake, the leading edge of the plate is uplifted and the flexural bulge collapses. |
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Fig. 3. Source zone for Cascadia subduction earthquakes. This zone includes a completely locked section and a section transitional to free sliding along the thrust fault to the east. |
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Fig. 4. Excavation through a tidal marsh near Tofino, British Columbia, exposing a buried peaty soil (SO), the remains of a former marsh that subsided 0.5-1 m during an earthquake about 300 years ago. The peaty soil is sharply overlain by a sand layer (S), deposited by the tsunami that immediately followed the earthquake. The sand, in turn, is overlain by tidal mud (M) and peat (P) of the present-day marsh. |
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Fig. 5. Origin of the main coastal features that provide evidence for subduction earthquakes in the Pacific Northwest. (a) A peaty soil is buried by tidal mud after an earthquake lowers the land into the intertidal zone. (b) A sheet of sand is deposited on a coseismically subsided surface by a tsunami shortly after an earthquake. (c) Liquefied sand moves upward through cohesive sediments and is ejected onto a subsided surface as a result of earthquake shaking. |
