Here comes the big one.

The science community has murmured the maxim for years — the Pacific Northwest coastline is due for a catastrophic earthquake.

Despite a recent series of quakes that have rattled the ocean floor and one near Powers, geologists and disaster experts are no closer to predicting when the next massive quake will occur.

“An earthquake like this is going to shake the hell out of the coast,” said James Roddey, Earth sciences information officer for the Portland-based Oregon Department of Geology and Mineral Industries.

Known as the Cascadia earthquake, the quake is a recurring 9.0-average magnitude event — making it as powerful as the 2004 Indian Ocean earthquake — that occurs on an average of once every 200 to 1,000 years, Roddey said.

The cause is the Cascadia subduction fault, a tectonic snag where the Juan de Fuca crust plate is diving under the North American plate.

As one plate pushes down on the other, pressure builds until one plate snaps free. Then an enormous amount of energy is released along the entire 680-mile length of the fault line, which stretches from Northern California to Canada.

The motion of the Juan de Fuca plate — down and forward — displaces water and also means an earthquake above magnitude 8 would create a 30- to 40-foot tsunami, Roddey said. Scientists predict the first surge would push inland for about 30 minutes with smaller waves following.

“(The coast) is going to see a significant wall of water,” he said.

The last Cascadia quake occurred in 1700.

Today, DOGAMI identifies 10 million people from the coast to the Cascade Range between Cape Mendocino, Calif., and Vancouver Island as at risk for a major earthquake and tsunami.

Recent quakes are a result of the Blanco fracture zone — a strike-slip fault where the Gorda plate slides parallel to the Juan de Fuca — and are very routine.

“You don’t get any warning. The (Cascadia) earthquake is your warning,” he said.

Rather than predict the next big quake, DOGAMI is focusing on how geology can help prepare people for the inevitable.

The organization is completing detective work on coastal soil layers to map tsunami flooding zones.

DOGAMI paleoseismologist Rob Witter has finished surveying a site at Ecola Creek near Cannon Beach and has begun similar work at Bradley Lake in Bandon.

Using core samples from coastal lakes and rivers, Witter and a team of geologists found fossilized algae and other evidence in the 1990s that shows the coast has been submerged in marine water.

“(The coast) has an incredible record of tsunami events,” Witter said.

Scientists found 14 layers of marine debris deposited intermittently in a period of 4,800 years. That’s one tsunami every 343 years. The most recent layer coincides with records from Japan, which indicate a tsunami hit that nation in 1700.

Witter’s recent work on the tsunami flooding maps helped support the earlier findings.

“It builds on evidence from elsewhere that large tsunamis accompany great earthquakes along the Cascadia (fault),” he said.

Tsunami flooding maps, when complete, will help cities better organize evacuation routes and identify school, government, health care or other buildings in the path of a massive wave.

“There are a lot of people who are unaware that this can happen,” he said.

Coos Bay Fire Department Chief Stan Gibson said local emergency agencies are gearing up for a major drill on April 28.

The collapse of bridges surrounding Coos Bay and North Bend could cut off communities from aid, Gibson said, and since some parts of the cities are built on fill, it would mean major destruction.

“You would expect a number of buildings to be destroyed,” he said.

Gibson also said aid would be available from outside agencies, such as the American Red Cross, by boat or helicopter within a few days.

Roddey said earthquake and tsunami planning in Oregon communities is not perfect, but is improving. Seaside, which was identified as the most at-risk community on the coast, has set a 20-year plan to move its schools out of flooding zones.

A panel of geologists will be going on the road in April to touch bases with the public and local governments to spread awareness. They will be in Coos Bay on April 8 discussing how to cope with a Cascadia quake.

“That’s the ultimate goal at the coast,” Roddey said. “Everyone knows what to do.”