Geology: An 8.5 magnitude earthquake would cause 6,800 casualties and cost $11.8 billion, a recent analysis indicates.

For years, scientists have warned Oregonians of a big and bad earthquake in their future. Experts have long predicted that a magnitude 8 or 9 quake caused by slippage of offshore plates could leave Western Oregon rocking and rolling. They peg the odds of such an earthquake at 20% within the next 50 years.

But few would hazard a guess about the costs of a killer quake. Now, thanks to a recently completed state study, we have some idea what an 8.5 magnitude quake off the coast would cost Oregonians. 

Some numbers:

• $11.8 billion in economic losses, structural and nonstructural, including lost income.
• 6,800 deaths and injuries. 
• 54,000 buildings destroyed or extensively damaged, with 12,400 people needing shelter. 
• $490 million damage to highways and airports. 
• 29% of communications systems down. 
• 9.3 million tons of debris.

The study done by Yumei Wang, a geotechnical engineer in the Oregon Department of Geology and Mineral Industries, assumed a 300-mile-long plate slippage area. “This is the first study that has quantified what’s going to happen in terms that the public can relate to,” Wang says. Her work relied in new software that was developed by a California company at the direction of the Federal Emergency Management Agency.

Wang emphasizes that the study undoubtedly underestimates the loss of life and property that would result from such a large quake. For example, it doesn’t consider the deaths and damage caused by the tsunami waves -- up to 30 feet high -- that would likely follow such a quake.

And the computer model doesn’t consider how unreinforced masonry structures -- largely brick buildings -- would fare. Many of the buildings would probably crumble, typically killing one in five people inside, Wang says. Such details will be included in future studies, and the corresponding predictions will become more accurate, Beaulieu says. And the department is now working on earthquake hazard maps charting quake effects on more than 30 Western Oregon communities, including the Eugene-Springfield area. (CFF Editor’s Note: see CFF’s Tsunami Page for Florence Inundation Map)

Computer models of tsunami effects already exist, Beaulieu says, but it may be a year or more before that information can be integrated into the earthquake models. Although it’s not perfect, the current study gives Oregonians a valuable broad-brush look at the havoc that would follow a major subduction quake, says Don Hull, state geologist and director of the Department of Geology and Mineral Industries. “It gives the magnitude of the risk we live with and illustrates the importance of preventive measures to reduce these casualties and damages,” he says.

Communities should use the report to pinpoint the kind of structures that will need strengthening to withstand an expected quake, Hull says. Beaulieu adds that the predictions underscore the wisdom of tougher building standards aimed at making new structures more earthquake-resistant. Some changes were made in 1993, and the latest rules affecting construction on the south and central coast took effect in October.

The state plans to publish the entire study in January. Preliminary results are detailed in the current issue of the geology department’s magazine, Oregon Geology. Although the study is certainly sobering, Beaulieu says it also makes clear that such a quake wouldn’t necessarily lead to a “doomsday,” against which people are helpless. “This is manageable if we know what we have and work toward a solution,” he says.

Coos County would be hammered worst by the quake, according to the study. Lisa Wampole, Coos County’s emergency management coordinator, hasn’t seen the report, but she thinks it would help the county carry out its already developed earthquake response plan. Some people might be persuaded by the study to become better prepared for surviving a quake, Wampole says. “It might serve as a wake-up call,” she says. During a subduction quake, the coast would experience the most violent shaking, and some sandy soils in low-lying areas would turn to something like quicksand, Beaulieu says. Some buildings would fall, while others would be knocked off their foundations. Roads would crack and slide, bridges might fall, and utilities would be damaged. “Just a whole array of bad things will happen,” he says.

Moving inland, the severity of the quake would lessen, he says, with shaking in the Willamette Valley probably no worse than quakes that have struck the valley in recent decades. Eastern Oregon would hardly feel anything.

The last subduction zone quake hit the coast about 1700; experts believe such quakes occur every 300 to 600 years. That means another one could happen tomorrow, or in a century -- still only an eyeblink in geologic time. Oregon has had fewer earthquakes in the last 200 years than its geology indicates is normal, leading Beaulieu to conclude that quakes could come in unpredictable cycles. “It may be that when the subduction (quake) event goes, it so changes the stress structure of the state that a whole bunch of other faults that have been locked up start to go.” Beaulieu says.