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5.4: Cascadia Subduction Zone

  • Page ID
    5925
    • 5.4.1: Discoveries Beneath the Sea
      It is late summer of 1989. Chris Goldfinger and Bruce Appelgate, graduate students at Oregon State University, and electronics technician Kevin Redman of Williamson and Associates, are in the science lab of OSU’s research ship Wecoma, looking on the TV monitor at side-scan sonar imagery taken as the Wecoma cruises above the base of the continental slope along the Cascadia Subduction Zone off central Oregon.
    • 5.4.2: Earthquakes in the Estuaries
      It was in the bays and estuaries along the coastline that the most conclusive evidence for great earthquakes was found by Brian Atwater and his American and Canadian colleagues, as stated in the Introduction. From Port Alberni, at the end of a deep fjord on the west coast of Vancouver Island, to Sixes River in southern Oregon, and at many bays and estuaries in between, the sediments give evidence of sudden drops in the land level.
    • 5.4.3: The Bad News
      The new discoveries went against the long-held view that the Cascadia Subduction Zone was not a seismic hazard. Most subduction zones around the world are shaken by frequent earthquakes, some of magnitude 9 or greater. But not Cascadia, which has been as seismically quiet as Kansas. At first, it was believed that the apparent absence of recorded earthquakes might be because the Juan de Fuca Plate is no longer subducting beneath North America.
    • 5.4.4: Instant of Catastrophe or Decade of Terror?
      After Atwater’s discovery at Willapa Bay, other scientists found evidence of marshes buried by sudden subsidence accompanying earthquakes at South Slough near Coos Bay in southern Oregon, at Salmon River near Lincoln City, Oregon, at Nehalem Bay and Netarts Bay in northern Oregon, at the mouth of the Copalis River in Washington, and at Port Alberni and Ucluelet on the Pacific coast of Vancouver Island.
    • 5.4.5: A Japanese Tsunami from Cascadia; A Detective Story
      The difficulty in figuring out the maximum size of a Cascadia earthquake, of course, is the lack of local historical records at the time the last great subduction-zone earthquake struck the Pacific Northwest. But there is one last chance. Suppose the earthquake generated a tsunami that was recorded somewhere else around the Pacific Rim where people were keeping records. This leads us to Japan, the first country in the Pacific Ring of Fire to develop a civilization that kept written records.
    • 5.4.6: Native Americans Were Making Observations, After All
      Could there be confirmation in the oral traditions of Native Americans living along the coast at that time? Garry Rogers of the Pacific Geoscience Centre in Sidney, B.C., found in the provincial archives at Victoria a tradition that an earthquake had struck Pachena Bay on the west side of Vancouver Island during a winter night. It was discovered the following morning that the village at the head of the bay had disappeared.
    • 5.4.7: M 8 or M 9? Where Do We Stand?
      Participants in a scientific conference held at Seaside, Oregon, in June 2000 were asked their opinions on M 9 vs. M 8 for the 1700 earthquake. The vote was overwhelming for an earthquake of M 9, indicating that another paradigm change had taken place. The evidence from the Japanese tsunami of A.D. 1700 and the constant number of post-Mazama turbidites from submarine channels from southern Oregon to Washington confirmed the tectonic model of Hyndman and Wang and carried the day.
    • 5.4.8: Northern California; It’s Not the Same South of the Border
      The subduction zone in northern California is different from the rest of Cascadia. Off Oregon and Washington, it lies at the base of the continental slope but in northern California, it turns toward the southeast and angles up the continental slope, headed for the Triple Junction of the North American, Pacific, and Gorda plates beneath the village of Petrolia.
    • 5.4.9: Recurrence Intervals and the Next Subduction Zone Earthquake
      Atwater and Hemphill-Haley analyzed an earthquake record spanning 3,500 years at Willapa Bay in southwestern Washington, identifying seven earthquakes, including the A.D. 1700 event. As shown in Figures 4-9 and 4-21, these earthquakes struck at irregular intervals. At the Redtail site, events S, U, and W struck within a five-hundred-year interval, whereas about nine hundred years elapsed between Event S and the previous Event N.
    • 5.4.10: Subduction-Zone Earthquakes That Don’t Quake
      Herb Dragert of the Pacific Geoscience Centre in Sidney, B.C., was checking the GPS records of tectonic strain accumulation in southern Vancouver Island when he caught what appeared to be an error in one of the measurements. Since 1992, the Pacific Geoscience Centre and the Pacific Northwest Geodetic Array (PANGA) GPS networks had been recording the slow accumulation of elastic strain in the North America Plate as the oceanic Juan de Fuca Plate drives northeastward beneath it.
    • 5.4.11: Summary
      Despite its low seismicity, the Cascadia Subduction Zone has been revealed as a major seismic source. The last earthquake was a 9, based on modeling of a tsunami that struck Japan in January 1700. The Native Americans who were living here in A.D. 1700 reported the earthquake in their oral traditions handed down from generation to generation. Some of the earlier earthquakes may have been smaller since the subsidence accompanying them was less than the subsidence in 1700.