The Big One: Oregon Agency Assesses Prospects

July 17, 2008

A magnitude 9 earthquake could cause Oregon $30 billion to $40 billion in damage, a state geologist estimates, or about 100 times the damage blamed on last winter’s severe storms and flooding.

Oregon Department of Geology and Mineral Industries released an assessment of the damage such a major earthquake would cause in six Willamette Valley counties: 1,000 deaths, 13,000 injuries, and 23,000 families displaced and more than a quarter of all buildings damaged or destroyed.

The damage in the six-county area was estimated at $12 billion _ about the same as an estimate made 10 years ago for the entire state.

The six counties are Yamhill, Marion, Polk, Linn, Benton and Lane.

The 10-year-old statewide figure is based on outdated population and infrastructure records, said Yumei Wang, a state geotechnical engineer who worked on both assessments.

Taking into account population growth and new construction, she said, “the numbers would be much higher.”

A subduction zone earthquake that could reach magnitude 9 is one of the most serious natural hazards threatening the Pacific Northwest.

It would involve a rupture of the offshore subduction zone where one plate of the Earth’s crust slides beneath another and would probably also trigger a tsunami that would strike the Oregon coast.

Geologists estimate a roughly one-in-seven chance of such an earthquake in the next 50 years. It would be similar to the earthquake and tsunami that devastated the Indian Ocean region in 2004.

The project was funded by the Federal Emergency Management Agency and the state. Cities and counties that participated are eligible for additional federal disaster aid.

The risk of earthquake damage in much of the Willamette Valley is compounded by the region’s soils. River sediments that line much of the valley floor are subject to liquefaction, where soils turn to mush. Also, they can amplify the shaking.

City and county leaders said the new assessment would help them work to retrofit vulnerable buildings and raise public awareness of the possibility of a major earthquake. They said many people still have not taken basic precautions, such as assembling enough disaster supplies to support themselves for 72 hours without help.

“It’s easy to get complacent about earthquakes, because they don’t happen very often,” said Wes Hare, city manager in Albany. “We now know that these things can happen.”

Source: The Oregonian.

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