Amassive earthquake that could one day devastate Jackson County will take center stage on Thursday, Oct. 17, when local students and other residents participate in the "Great Shakeout."

Amassive earthquake that could one day devastate Jackson County will take center stage on Thursday, Oct. 17, when local students and other residents participate in the "Great Shakeout."

At 10:17 a.m. on that day, Oregonians will "drop, cover and hold" for one minute during an earthquake drill designed to simulate a Cascadia Subduction Zone quake that could hit 9.0 on the Richter Scale, though a real one could last up to 10 minutes.

As of Thursday, 6,761 residents in Jackson County — mostly college and high school students — had signed up to take part in the drill. Throughout Oregon, 188,000 said they planned to participate.

In Jackson County, Rogue Community College, Helman Elementary in Ashland, South Medford High and Applegate Elementary have agreed to participate.

The area is overdue for a big earthquake, and state geologists predict Southern Oregon has a 37 percent chance of having one within the next 50 years.

The subduction zone starts 50 to 75 miles off the coast, where one tectonic plate slides under another. The zone extends under the Cascade Mountain range.

"You will definitely feel the shaking there," said Althea Rizzo, geologic hazards program coordinator for the Oregon Office of Emergency Management.

The amount of shaking will depend on the type of soil in the area. Bedrock will shake less than river-bottom soil — the kind of soil that is prevalent in many communities of Jackson County.

"It shakes like a bowl full of jelly," Rizzo said.

Based on geological data, a Cascadia quake typically starts in Southern Oregon and makes its way northward.

"Southern Oregon has the highest risk," Rizzo said.

She noted that a subduction zone quake would be felt as far east as Idaho.

State emergency officials suggest residents check out the following website for more information:

Emergency officials warn that a major earthquake could seriously disrupt food, water, transportation and other vital services.

The Oregon Resilience Plan, prepared for the Oregon Legislature, predicts that for this area electricity could be knocked out for up to three months, drinking water and sewers might not be available for up to one year, priority highways could be out of commission for up to a year, and health care facilities could take up to 18 months to be operational.

Emergency officials have previously suggested that people keep 72 hours worth of supplies such as food and water on hand. Now they say people should plan for at least two weeks.

On the coast, essential services could be knocked out even longer.

A family emergency plan should include establishing a meeting place in advance. Cellphones might be knocked out when an earthquake occurs. The kids could be at school, the grandmother at home and the parents at work.

If you're inside a building, drop, take cover and try to hold onto something until the shaking stops.

Some residents might want to seek higher ground if their homes could be flooded if a dam breaks.

Neighborhoods should devise individual plans so people can work together to help each other.

Harry Smedes, a geology professor at Southern Oregon University, said the average amount of time between major quakes is about 240 years. "We're now at year 313," he said. "The experts like to say, 'We're nine months pregnant.' "

Smedes teaches students about volcanoes and earthquakes, and particularly about the subduction zone off the coast.

"The ocean floor is plunging down beneath us," Smedes said.

The land mass on the continental U.S. gets stuck as the ocean floor goes down.

"Before long, it gets unstuck and the coast surges westward 20 to 30 feet," he said. "This thing is going to shake for six or eight minutes at least, not the usual 20 seconds or so for most earthquakes."

Smedes said people shouldn't try to get outside a building when the shaking starts because there will be up-and-down movement as well as side to side.

"You will feel like a drunken sailor," he said.

Smedes said he lives in an area near Jumpoff Joe Creek in Josephine County, and he anticipates major landslides could affect his home.

"I can't really comprehend how bad it's going to be," he said.

Reach reporter Damian Mann at 541-776-4476 or