Experts say the next Cascadia quake, which could be magnitude 9 or greater, could strike any time.

COURTESY OF THE FEDERAL EMERGENCY MANAGEMENT AGENCY - A map of the Cascadia subduction zone shows how close it is to major population centers in Oregon, Washington and British Columbia."The Big One" is coming, scientists warn.

No one knows exactly when it will strike. It could happen tonight, as people sleep. It could happen during your morning commute sometime next month. It could happen a year from now — five years, 10 years, 20, 50, 80, 100, maybe more.

But geologists believe Oregon and Washington — along with parts of coastal British Columbia and California — were last shaken by an earthquake greater than magnitude 8 in the year 1700. They believe similar earthquakes, generated by fault activity in what is called the Cascadia subduction zone several dozen miles off the Pacific coast, tend to strike the region every few hundred years. Experts disagree on exactly how long the average interval between major quakes has been. By some reckonings, we are overdue.

While the challenges of confronting what would certainly be the worst natural disaster in the region's history are immense, requiring a concerted effort by federal, state and local authorities, the American Red Cross and other agencies offer advice and caution for residents to boost their own chances of survival when the great quake strikes.

The Red Cross is holding an event at Tigard High School next Tuesday, Sept. 26, called Prepare Out Loud to give people a sense of what a magnitude-9 quake would look like in western Oregon.

Warnings from the Oregon Office of Emergency Management, Oregon Health Authority, and Department of Geology and Mineral Industries are stark: An earthquake of that size, comparable to the Tohoku earthquake that devastated Japan in 2011, would be "catastrophic."

Among the buildings at risk of collapse would be many schools, hospitals and public safety buildings. Roads, railroad tracks, freeways and bridges would be torn apart, as would aging infrastructure for utilities, including water pipes and power transmission lines. Landslides would have devastating effects in and around Portland's West Hills and the Chehalem Mountains near Sherwood. Coastal communities, of course, face the prospect of a tsunami that could wipe them out entirely with just a few minutes' warning at best.

Residents of the Pacific Northwest, especially those living west of the Cascade Range, are advised to keep emergency supplies on hand — enough to survive for at least two weeks. Washington County warns on its website that power may be knocked out for as long as three months, water and sewer utilities offline for up to a year, and public health facilities for perhaps as many as 18 months.

While National Guard units would almost certainly be deployed and emergency services would be activated as soon as possible, experts generally advise that residents should not expect immediate assistance, as first responders will be overwhelmed.

Prepare Out Loud will describe what people living in the area can expect to experience during and after a Cascadia earthquake. It will also provide instructions on how to prepare, including how much food and water, among other supplies, residents should keep for in case disaster strikes.

For those curious about how earthquakes and faults work, the Prepare Out Loud presentation will also outline the science underlying the Cascadia subduction zone, as well as its history. Though the fault last ruptured in 1700, scientists believe it has ripped and caused major earthquakes and tsunamis many more times over the past millennia as well.

The event begins with a "Prepare Fair" from 5:30 to 6:30 p.m. The presentation, including a rundown of what people should know about the major quake and how to get ready for it, starts at 6:30 p.m. Following that will be another hour of the Prepare Fair from 8 to 9 p.m., after which the event concludes.

Tigard High is located at 9000 S.W. Durham Road.

For anyone not able to make it to Tuesday's event, the Red Cross will be hosting another Prepare Out Loud earthquake preparedness forum in the Portland area on Wednesday, Sept. 27, from 5:30 to 9 p.m. at Montgomery Park, 2701 N.W. Vaughn St. in Portland.

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