This seismologist was well acquainted with subduction earthquakes and the tsunamis that so often accompanied them because they were common occurrences in his home country of Japan. If we were to imagine a line descending directly downward into the earth from these mountains we would encounter the subducting ocean crust from the Juan de Fuca Plate at a depth of about seventy miles. The undersea Cascadia thrust fault ruptured along a six-hundred-mile length, from mid Vancouver Island to northern California in a great earthquake, producing tremendous shaking and a huge tsunami that swept across the Pacific. Water went as far as the pine trees and returned to the ocean very fast. However, because the fault is offshore, it is not the greatest earthquake hazard faced by major west coast cities.
It was obvious that this part of the coast had once been below high water, then above it, and then below it again before coming to its present state with topsoil in the uppermost eight inches. These were the questions.By the 1980s refinements in our knowledge of plate tectonics, particularly awareness of the much greater power in plates that are subducting when the distances from their spreading ridges are short as they were with the Juan de Fuca Ridge, persuaded scientists to search for evidence of past quakes in the Northwest. As recently as 1989 a gigantic rock avalanche crashed down on the north side of the mountain.

Samples of plant material and bits of wood from the top peat layer were collected and their age calculated using the carbon-14 technique. This earthquake caused a shaking of the houses of the Cowichan people on Vancouver Island as we know from their stories. Geological evidence indicates that thirteen great earthquakes have occurred in the last 6,000 years. The Cascadia subduction zone is thought to generate a huge quake every 200 to 530 years. These floods occur during periods of unusually hot weather in summer or early fall. This causes tremendous strain and deformation of the earth’s crust in the coastal region and causes ongoing earthquake activity. Ocean crust from the Juan de Fuca ridges descends slowly so it has traveled some distance inland before it gets down to a depth of seventy miles. The Cascadia fault is the boundary between two of the earth’s tectonic plates: the smaller offshore Juan de Fuca Plate that is sliding under the much larger North American Plate.The shaking of the earthquake collapsed houses of the Cowichan people on Vancouver Island and caused numerous landslides. An inland magnitude 6.9 earthquake in 1995 in a similar geological setting beneath Kobe, Japan caused in excess of $200 billion damage. These events are recorded in the oral traditions of the First Nations people on Vancouver Island. Seawater was known to have covered land as if it had been high tide. The Cascades run from Mount Garibaldi north of the Canadian border all the way to Mount Lassen in northern California and each peak has its own unique history; some have a very violent past while others either took shape quietly or we do not know enough about their past. Steam and gas emissions from it were common during the 1970s. Careful analysis of these historic tsunami records indicated that several coastal villages were damaged. Because these inland earthquakes can be much closer to our urban areas and occur more frequently, they represent the greatest earthquake hazard. Mount Hood is the fourth highest peak. During the 1990s, Mount Rainier was selected by the United States Geological Service for intense study as one of three places that might cause major damage over the next decade or two. It was the remaining portion of an older Pacific spreading ridge, namely the Juan de Fuca Plate that caused the massive 1964 earthquake in Alaska, but that same plate is subducting beneath the Pacific Northwest. Because the way the tide came in was so unusual, people were advised to move to higher ground. The tsunami swept across the Pacific also causing destruction along the Pacific coast of Japan. Japan, being an older civilization, has records of earthquakes and tsunamis going back many hundreds of years. The undersea Cascadia thrust fault ruptured along a six-hundred-mile length, from mid-Vancouver Island to northern California in a great earthquake, producing tremendous shaking and a huge tsunami that swept across the Pacific. The Cascadia Fault is the boundary between two of the earth’s tectonic plates: the smaller offshore Juan de Fuca Plate, formerly the Pacific Plate before the North American Plate had overtaken it, now the one that is sliding under the much larger North American Plate.

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