5.5.1: Commotion in the Ocean
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The Juan de Fuca Plate is entirely oceanic (Figures 2-7), 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. Its weakness means that it is subject to internal deformation where it interacts with the continental edge of North America. 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 (Figure 4-14). These seismically active regions are generally referred to as separate plates, the Explorer Plate off Vancouver Island and the Gorda Plate off northern California (Figure 5-1). The Juan de Fuca Plate between its northern and southern ends has few earthquakes, indicating that internal deformation is less important there.
The fact that the Juan de Fuca Plate is completely oceanic means that we are not able to measure its displacement rates directly but instead must rely on indirect geophysical evidence. All permanent seismic stations are onshore, resulting in considerable inaccuracy in locating earthquakes on the plate. However, in recent years, the declassification of the U.S. Navy’s hydrophone detection system has allowed scientists of the National Oceanic and Atmospheric Administration (NOAA) in Newport, Oregon, to study earthquakes using seismic waves (T-phase waves) that are transmitted through ocean water rather than through the crust beneath the ocean. They have been able to improve greatly the accuracy and detection threshold for earthquakes far from shore (Figure 4-14).
Mapping of the distribution of earthquakes shows that the spreading centers, the Juan de Fuca, Gorda, and Explorer ridges, generate low-level seismicity related to the movement of magma that rises to the surface and forms new oceanic crust. These earthquakes are small, most of them too small to be detected by ordinary seismographs onshore, although they are monitored through the SOSUS hydrophone detection system.
On the other hand, the Gorda Plate is cut by large strike-slip faults that rupture frequently to cause earthquakes (Figures 2-4, 5-2). The Gorda Plate west of Arcata, California, sustained an earthquake of M 7.3-7.6 on January 31, 1922, that was felt in Oregon and Nevada, and as far south as San Jose, California. Another earthquake of M 6.9-7.4 thirty miles west of Trinidad, California, on November 8, 1980, destroyed a bridge, liquefied the sand bar at Big Lagoon, and caused six injuries and $1.75 million in damage. In 1991, the Gorda Plate was shaken by an earthquake of M 6.9 on July 12, another of M 6.3 on August 16, and the largest one of M 7.1 on August 17, three hours after a crustal earthquake onshore. On April 26, 1992, one day after the M 7.1 Cape Mendocino Earthquake on the Cascadia Subduction Zone, two aftershocks of M 6.0 and M 6.5 struck the Gorda Plate twelve and eight miles, respectively, offshore. One of these aftershocks trashed the commercial district of the small town of Scotia. These were the largest of hundreds of aftershocks of the Cape Mendocino earthquake in the Gorda Plate, complicating the problem of whether that earthquake was mainly a subduction-zone earthquake or a Gorda Plate earthquake. Except for the 1980 earthquake and the two Petrolia aftershocks, these Gorda Plate earthquakes were far enough offshore that intensities on the coast did not exceed V or VI.
The Gorda Plate has accounted for more damaging historical earthquakes in northern California than any other source, including the Cascadia Subduction Zone and the North American Plate. However, it is incapable of producing earthquakes in the M 8 to 9 range, such as those expected on the Cascadia Subduction Zone.
The Explorer Plate off Vancouver Island is also shaken by frequent earthquakes (see Appendix A). But, unlike Gorda Plate earthquakes, these are far enough from populated areas that they do no damage and in some cases are not even felt onshore.