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The Cascadia subduction zone (also referred to as the Cascadia fault) is a convergent plate boundary that stretches from northern Vancouver Island to northern California. It is a very long sloping subduction zone fault that separates the Juan de Fuca and North America plates. The denser oceanic plate is subducting beneath the less dense continental plate offshore of British Columbia, Washington, Oregon and northern California. The North American Plate moves in a general southwest direction, overriding the oceanic plate. The Cascadia Subduction Zone is where the two plates meet. Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism of the Cascades. This volcanism has included such notable eruptions as Mount Mazama (Crater Lake) about 7,500 years ago, Mount Meager about 2,350 years ago, and Mount St. Helens in 1980.〔(【引用サイトリンク】title=Cascadia Subduction Zone Volcanism in British Columbia ) USGS〕 Major cities affected by a disturbance in this subduction zone would include Vancouver and Victoria, British Columbia; Seattle, Washington; and Portland, Oregon. ==History== In the 1960s, underground fractures were uncovered by oil companies in Puget Sound. These were believed to be inactive through the 1990s. In the 1980s, geophysicists Tom Heaton and Hiroo Kanamori of Caltech compared the generally quiet Cascadia to more active subduction zones elsewhere in the Ring of Fire. They found similarities to faults in Chile, Alaska, and Japan's Nankai Trough, locations known for megathrust earthquakes, a conclusion that was met with skepticism from other geophysicists at the time. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cascadia subduction zone」の詳細全文を読む スポンサード リンク
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