PORTLAND, Ore. (KOIN) — The last time a Cascadia Subduction Zone earthquake hit Oregon was on January 26, 1700, before it was even Oregon. It was an estimated 9.0 magnitude. A big one.

Geologists say a subduction zone earthquake happens about every 250 to 500 years. That means we’re due for one — but it could happen today, next week or in 100 years. That’s why it’s important to be prepared.

Thursday is the Great Oregon Shakeout, part of an international effort to get people to be more prepared for earthquakes — including The Big One. At 10:21 a.m., you’re asked to Drop, Cover and Hold.

But when an earthquake hits you might not always be in the most convenient location.

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Experts say everyone should have a preparedness plan for when the shaking stops. Cell service might be down. Roads may be closed. Check your emergency preparedness kit and make sure it’s stocked.

PSU Geology Professor Dr. Ashley Streig said we need to pay attention to earthquakes beyond just The Big One. There are other faults in the area that shouldn’t be ignored.

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Crustal faults, she said, are smaller cracks in the Earth’s crust that would cause an earthquake if they shifted. There are 3 nearby — the Portland Hills Fault, Gales Creek Fault and one zone near Mount Hood.

These quakes will last 15 to 30 seconds but will feel harder because they are so close. A subduction zone quake could last 5 minutes.

Students in Seaside show off new tsunami warning signs. (Courtesy photo)

Streig said bridges in Portland will likely fail. The only bridge in Portland built to withstand an earthquake is the Tilikum Bridge.

“The closer you are to the coast, kind of the greater the impact of that shaking because ground motions will be higher,” Streig said. The ground will drop a little.

“But you add that lower elevation to the expected tsunami that would be caused by the earthquake by disruptions of the sea floor. And so you have suddenly lower elevations, slightly lower elevations, and a tsunami that’s coming in,” she said.