David Yamaguchi, who was then studying the eruptions of Mount St. In 1987, Atwater mounted another expedition paddling up the Copalis River with Dr. The event had happened so quickly that the top layer of sand sealed away the air, thus preserving centuries-old plants. This finding was evidence that the ground had suddenly sunk under sea level, causing saltwater to kill the vegetation. Under a top layer of sand, he uncovered a distinct plant- arrowgrass-that had grown in a layer of marsh soil. Large tree stump protruding from beach sandĭuring low tide one day in March 1986, paleogeologist Brian Atwater dug along Neah Bay with a nejiri gama, a small hand hoe. Major cities affected by a disturbance in this subduction zone include Vancouver and Victoria, British Columbia Seattle, Washington and Portland, Oregon. This volcanism has included such notable eruptions as Mount Mazama ( Crater Lake) about 7,500 years ago, the Mount Meager massif ( Bridge River Vent) about 2,350 years ago, and Mount St.

Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism of the Cascades. The North American Plate itself is moving slowly in a generally southwest direction, sliding over the smaller plates as well as the huge oceanic Pacific Plate (which is moving in a northwest direction) in other locations such as the San Andreas Fault in central and southern California. The Explorer, Juan de Fuca, and Gorda plates are some of the remnants of the vast ancient Farallon Plate which is now mostly subducted under the North American Plate. The zone varies in width and lies offshore beginning near Cape Mendocino, Northern California, passing through Oregon and Washington, and terminating at about Vancouver Island in British Columbia. It is a very long, sloping subduction zone where the Explorer, Juan de Fuca, and Gorda plates move to the east and slide below the much larger mostly continental North American Plate. The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States.

Area of the Cascadia subduction zone (left) and a USGS scenario ShakeMap for a M9 eventĬoordinates: 45°N 124°W / 45°N 124°W / 45 -124

Cascadea subduction