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Is the Cascadia Subduction Zone About to Blow?

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If an earthquake should erupt on the US Pacific West Coast, it would either hit the San Andreas Fault in California or the Cascadia Subduction Zone in the Pacific Northwest. The Cascadia Subduction Zone has produced 41 major earthquakes in the last 10,000 years at intervals of 240 years. The last one occurred in 1700, 311 years ago, and is overdue.

University of Colorado seismologist Roger Bilham reports in a column published by Bloomberg that this month’s megaquake in Japan is almost a template for the magnitude 9.0 earthquake scientists expect to rock the Oregon-Washington coastline.

Robert S. Yeats, a retired Oregon State University professor of geology, spoke about Northwest earthquakes to the Lebanon, Oregon Rotary Club on March 16. The Japanese earthquake was caused by movement along a subduction zone, where two great plates of the Earth’s crust meet, with one diving under the other. Another subduction zone sits off the Northwest coast.

In the Cascadia Subduction Zone, the Juan de Fuca (ocean) plate is forcing itself under the North American (land) plate. What Yeats referred to as the locked zone extends from northern California up to central Vancouver Island. If it all goes off at once, as we expect, it will produce an earthquake about the same as the one in Japan, Yeats told the group.

Because the Cascadia Subduction Zone is under water, it will produce a tsunami, he said. When Yeats arrived at OSU in 1977, some geologists thought there was no earthquake hazard in the northwest. Since that time, geologists have identified not only the subduction zone, but also deep and crustal faults throughout the area.

The Statesman Journal in the Oregon state capital, Salem, reported on March 21 that, due to the danger of a 9.0 earthquake and tsumani in the area, the Oregon Department of Geology and Mineral Industries(DOGAMI) had issued a report last year on the effects of an earthquake occurring in the Cascadia Subduction Zone, a 600-mile expanse, 50 miles off the coast, stretching from Northern California to British Columbia. The report says:

"The effects of a great (magnitude 9.0) CSZ earthquake will reach far inland. Shaking will be strongest on the coast but also strong in [the] Willamette Valley. We can expect up to five minutes of shaking. Prolonged shaking can cause structure collapse, landslides and disruption of lifeline services. If a CSZ earthquake generates a significant tsunami, Oregon can expect an estimated 5,000 fatalities and over $30 billion in damages."

"An earthquake could result in a tsunami inundating the Oregon Coast within 15 to 30 minutes," DOGAMI says. "Before that, ground shaking could cause liquefaction, landslides, and coastal subsidence."

Liquefaction occurs when shaking weakens water-saturated soils and sediments, creating conditions ripe for landslides, as well as ruptured pipelines and uprooted homes, buildings, and bridges. Amid widespread devastation caused by a massive quake and tsunami, relief efforts would be hampered because of the crippled status of "critical lifelines," such as roads, bridges, and power-transmission systems, according to the DOGAMI report.

"Oregon’s lifelines were constructed to standards well below current seismic safety specifications," it says. "In a major earthquake, many of our critical lifelines, such as major highways, fuel ports, electrical transmission, and telecommunication systems, will not be able to provide their intended services."

- More than half of Oregon K-12 schools are at high risk of collapse in a strong quake.

- An estimated 1,000 bridges along Oregon highways may fail in a powerful quake.

- Nearly a dozen coastal communities are within tsunami inundation zones. Some neighborhoods have no easy escape routes to high ground.