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5.5: Earthquakes in the Juan de Fuca Plate

  • Page ID
    5962
    • 5.5.1: Commotion in the Ocean
      The Juan de Fuca Plate is entirely oceanic, with a thin crust made up of basalt. No part of it is above sea level. The crust is nowhere more than a few tens of millions of years old, which means that it is relatively shallow, weak, and hot. At its northern and southern ends, where the spreading center is closest to the base of the continent, and the oceanic crust is youngest, the weak oceanic plate is being actively deformed internally, a deformation that is marked by frequent earthquakes.
    • 5.5.2: Offshore Transform Faults; The Northwest’s Answer to the San Andreas Fault
      In Chapter 2, we considered two types of plate boundaries: ocean ridges or spreading centers, where new oceanic lithosphere is created as plates move away from each other, and subduction zones, where oceanic lithosphere is recycled back into the interior of the Earth as plates move toward each other. The Juan de Fuca and Gorda ridges are examples of spreading centers, and the Cascadia Subduction Zone is an example of two plates converging.
    • 5.5.3: Slab Earthquakes in the Juan de Fuca Plate Beneath the Continent in the Puget Sound Region
      The greatest amount of seismicity generated by the Juan de Fuca Plate itself (not including the Explorer and the Gorda plates) is beneath western Washington, where it is being subducted beneath North America. These are called slab earthquakes or Benioff zone earthquakes. Most of the damage and loss of life in the Pacific Northwest has been as a result of these earthquakes, including the largest known historical shocks to strike either Washington or Oregon.
    • 5.5.4: Northern California
      What about the onshore Gorda Plate in northern California? An earthquake of M 6.75 on November 23, 1873, on the thinly settled Oregon-California border may have been a slab earthquake. After this earthquake, cracks in the ground appeared on the trail between Crescent City and Gasquet in the Smith River Valley, and all the chimneys were knocked down. The highest intensity recorded was VIII, in a limited area in the northwestern corner of California.
    • 5.5.5: Discussion and Summary
      Why should seismicity within the subducting oceanic plate be concentrated in the Puget Sound region? Oddly, this lower-plate seismicity does not extend very far south into Oregon. If subduction is taking place all along Cascadia, why should seismicity be concentrated only in Washington? To answer this question, we look at the contours of the subducting Juan de Fuca Plate and observe that the plate has an eastward-convex bend in Washington.