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* Figure 1
| Figure 1. The Juan de Fuca plate is produced by spreading at the Juan de Fuca ridge several hundred kilometres offshore. It moves landward and beneath the continent at an average rate of 40 mm/yr. Great earthquakes occur on the thrust fault contact. Cascade volcanoes such as Mt. Garibaldi and Mt. St. Helens are produced by melting above the downgoing plate where it reaches a depth of about 100 km. |
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* Figure 2
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| Figure 2. Earthquakes produced by most fault zones follow the Gutenberg-Richter relation. The av erage number of earthquakes per year decreases with increasing magnitude. The Queen Charlotte transform (strike-slip) fault zone along the coast to the north of Vancouver Island is typical. The Cascadia thrust fault is unusual in that it produces n o small earthquakes, only infrequent very large events. |
* Figure 3
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| Figure 3.The Juan de Fuca plate lies offshore between Vancouver Island and northern California. Complex sub-plates are present at its north and south ends. The San Andreas transform fault extends to the south and the Queen Charlotte fault to the north. The barbed line marks where the subduction thrust fault approaches the sea floor. The Cascadia volcanoes (triangles) are limited to the region where the Juan de Fuca plate is underthrusting the continent. The stars mark M~7 historical earthquakes. |
* Figure 4
| Figure 4. Earthquakes are common in the continental crust of the northern Cascadia coastal region at depths less than 35 km, which corresponds to a maximum temperature of about 350°C. "Wadati-Benioff " earthquakes occur to a greater depth within the downgoing oceanic plate (to about 750°C) . However, no earthquakes have been detected on the subduction thrust fault. The circle sizes are proportional to the earthquake magnitudes. |
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* Figure 5
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| Figure 5. A trench cut through a coastal intertidal marsh exposes a peat layer, the remains of a former, now buried, marsh. The marsh abruptly subsided 1/2-1 m in a great earthquake about 300 years ago. The sand above the buried peat layer was swept into the subsided coastal region by the waves of the resulting great tsunami (after Clague and Bobrowsky, 1994a). |
* Figure 6
| Figure 1.A 6 m long core sample taken from the deep sea floor shows fine grained mud layers alternating with sandier layers. The latter are interpreted to have been deposited from submarine landslides triggered by g reat earthquakes. The mud layers formed by the slow continuous rain of finer sediment settling from the ocean. The volcanic ash at the bottom is dated as 7700 years old (after Adams, 1990). |
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