5.5.2: Offshore Transform Faults; The Northwest’s Answer to the San Andreas Fault
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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 (Figures 2-6 and 5-1). We also considered the third type of plate boundary where plates neither converge nor diverge but instead move past each other without destroying or creating the lithosphere. These are called transform faults because they transform plate motion between two spreading centers. They involve the entire lithosphere and not just the Earth’s upper crust.
The San Andreas Fault is a transform fault in which continental rocks of the North America Plate move past continental rocks of the Pacific Plate (Figure 2-8, top diagram, and animation, Figure 2-9). Transform faults in the Pacific Northwest, on the other hand, are found in the deep ocean floor, where they form linear topographic features called fracture zones. The Blanco Fracture Zone separates the Juan de Fuca and the Gorda ridges, and the Sovanco Fracture Zone separates the Juan de Fuca and the Pacific plates (Figure 5-1). The Mendocino Fracture Zone separates the Gorda and Pacific plates and is the northwest continuation of the San Andreas Fault. These are typical transform faults. The grinding of one plate past the other causes many earthquakes on these fracture zones. Barring the next subduction-zone earthquake, they and the interiors of the Gorda and Explorer plates have the highest instrumental seismicity in the Pacific Northwest, onshore or offshore. Large earthquakes on the Mendocino and Blanco fracture zones are felt frequently every year in northern California and southern Oregon.
At first glance, the Blanco Fracture Zone resembles a left-lateral strike-slip fault because of the apparent left offset of the Juan de Fuca and the Gorda ridges (Figure 5-1). But this apparent left offset would only be true if these ridges had once been a continuous unbroken ridge that was later separated along the Blanco Fracture Zone. This is not the case. Remember that the Juan de Fuca Plate is moving away from the Pacific Plate at these spreading centers. Imagine yourself standing on the Pacific Plate looking northward across the Blanco Fracture Zone at the Juan de Fuca Plate. The Juan de Fuca Plate moves from left to right along the Blanco Fracture Zone with respect to your position on the Pacific Plate. This means that the transform fault on the Blanco Fracture Zone is a right-lateral, not a left-lateral, fault.
As another thought experiment, imagine two jigsaw puzzle pieces that lock together by a tab that projects from one piece into the other. Now pull the pieces slowly apart. They are difficult to separate because the sides of the tab resist being pulled apart. In the same way, the Pacific and Juan de Fuca plates are being pulled apart, with molten rock welling up along the spreading centers as the plates are separated. Along the Blanco Transform Fault, the crustal plates push past each other, generating friction and producing earthquakes. These earthquakes could be as large as magnitude 7 or even larger, but probably not 8. The crust is too warm and therefore too weak to generate such large earthquakes. Accordingly, despite the high instrumental seismicity on the Blanco Transform Fault, including many earthquakes felt onshore, it does not constitute a major hazard to communities along the coast, in part because the earthquakes are many miles offshore and in part because these offshore earthquakes are not large enough.
Earthquakes on the Mendocino Transform Fault are frequent. The first recorded major earthquake was felt on May 9, 1878, causing chimneys to fall in Petrolia, California, at the Triple Junction (Appendix A). A larger earthquake, of M 6.5-7.3, struck close to Cape Mendocino on January 22, 1923, resulting in intensities of VIII and damage to buildings in Petrolia. Other earthquakes include a magnitude 6 in 1922 and smaller earthquakes in 1932, 1936, and 1951. Other earthquakes with magnitudes greater than 6 struck in 1954, 1960, and 1984. The 1984 earthquake of M 6.6, 166 miles west of the coast, was felt from Oregon to San Francisco, but it produced intensities of V or less because of its great distance from shore. On September 1, 1994, an earthquake of M 6.9-7.2 struck the Mendocino Transform Fault 88 miles offshore, the largest earthquake to strike the United States that year, larger even than the Northridge Earthquake of the preceding January. Because it was so far offshore, it did no damage, but it was felt from southern Oregon to Marin County, California.
Like the Blanco and Mendocino faults, the San Andreas Fault is also a transform fault, separating the Gorda Rise from a spreading center in the Gulf of California of northwest Mexico (Figure 2-8, top diagram; Fig. 2-9 animation). The offshore transform faults differ from the San Andreas in involving relatively hot oceanic crust and mantle, whereas the San Andreas cuts across colder continental crust for most of its length. For this reason, the San Andreas generates significantly larger earthquakes than does the Blanco, up to at least M 7.9. So, fortunately for the Pacific Northwest, the Blanco and Mendocino are the weaker relatives; they generate many earthquakes, but no giant ones.
Two transform faults lie off the coast of Vancouver Island: the Sovanco Fracture Zone that separates the Explorer Plate and the Pacific Plate, and the Nootka Fracture Zone that separates the Explorer Plate and the Juan de Fuca Plate (Figures 2-8, 5-1). Like the Blanco, these fracture zones are characterized by high seismicity but are not believed to generate very large earthquakes. In the next chapter, we will consider the possible relation between the oceanic Nootka Fracture Zone and two large historical earthquakes in the continental crust of central Vancouver Island.
Northwest of the Explorer Plate, the Pacific Plate grinds against the North America Plate along the Queen Charlotte Fault, located at the base of the continental slope. On August 22, 1948, this fault was the source of an earthquake of M 8.1, larger than any historical earthquake on the west coast of the United States south of Alaska. On October 27, 2012, this fault was the source of the Haida Gwaii earthquake of M 7.8 (using the First Nations name, Haida Gwaii, for the Queen Charlotte Islands). This fault had a focal mechanism of reverse faulting rather than the expected strike-slip. These earthquakes are evidence that the Queen Charlotte Fault poses a hazard to the thinly populated coast of British Columbia north of Vancouver Island, including the Queen Charlotte Islands. However, the region is so thinly populated that it is not considered as a major earthquake threat in Canada.