PORT ANGELES — When the next 500-year earthquake hits the Cascadia subduction zone off the coast of Washington, Neah Bay residents will have a little more than a half-hour to hightail it to higher ground, a U.S. Geological Survey scientist said Thursday.

“If you’re down here in Port Angeles, it’s more like an hour and a half,” Brian Atwater, USGS geologist and University of Washington professor, said in a workshop in the Port Angeles City Council chambers.

“Up here in Port Townsend, it’s practically two and a half hours.”

The last magnitude 8.7 to 9.2 earthquake in the Cascadia subduction happened Jan. 26, 1700, Atwater said. The resulting tidal wave was probably 10 feet above the high tide line.

He warned that the 500-year interval is only an average.

“If one happens tomorrow, we shouldn’t be surprised,” Atwater said. “Some of the faults can work on schedules that are a lot longer than election cycles.”

Atwater joined state and local Emergency Management officials in a discussion about tsunamis, how they are generated and the destructive potential on the North Olympic Peninsula.

Although infrequent, tsunamis are a significant natural hazard to Clallam County.

Geological record

“There’s no place I know of in the state of Washington that has a richer geological record of tsunamis than Discovery Bay,” Atwater told the 10 people in attendance at the workshop.

“And you could treat it as, in some form or another, a Strait of Juan de Fuca tsunami record.”

Atwater focused on the “orphan tsunami” of Jan. 26, 1700, which caused a significant tidal wave along the present-day Washington coast and the Strait of Juan de Fuca. The wave traveled much farther.

Written accounts in Japan, combined with evidence in tidal flats and tree rings here, give geologists the exact date.

The average interval for a significant tsunami in Discovery Bay is about 300 years, Atwater said.

He said the Canadian part of the underwater subduction zone doesn’t always do what the American part does.

Small tidal waves

Small tidal waves are far more common.

“There are anecdotal reports — maybe more than anecdotal — of both the 1960 Chilean tsunami and the ’64 Alaska tsunami getting up onto the Highway 101 at Discovery Bay,” Atwater said. “Discovery Bay is this big signal amplifier for tsunamis.”

Warning systems such as the All Hazard Alert Broadcast sirens that are scattered along the Pacific coast, the Strait of Juan de Fuca and Puget Sound are designed for tsunamis generated far away.

A big earthquake in Alaska, for example, won’t be felt on the North Olympic Peninsula.

“That’s where it’s easy for the warning system, and essential for the warning system, to be of help,” Atwater said. “Warning systems were set up to avoid those kinds of surprises.”

Atwater said North Olympic Peninsula residents will feel it when the big quake hits the Cascadia Subduction zone.

The advice from emergency managers is to get to higher ground, preferably 50 feet above sea level, and wait for the “all clear” signal.

State emergency manager Dave Nelson outlined the four levels of tsunami alerts and what they mean.

They are:

■ Information statement: An information message used when no tsunami is generated.

■ Tsunami watch: Potential danger. Be alert and listen to weather radios.

■ Tsunami advisory: Possible strong and dangerous current. Be prepared to take action. Stay off the beach.

■ Tsunami warning: Danger. Run to higher ground and follow emergency instructions.

Brynne Walker, a tsunami assistant for state Emergency Management, displayed an emergency preparedness kit containing first-aid supplies, food, water, personal hygiene products, important documents and additional supplies such as flashlights and batteries, an emergency blanket, matches and a whistle.

Walker said everything in her backpack cost $20.

Earlier tsunami forums were held in LaPush and Neah Bay.

Another forum is planned for March 24 at the Sequim Community Church, 950 N. Fifth Ave., at 6 p.m.

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Reporter Rob Ollikainen can be reached at 360-417-3537 or at rob.ollikainen@peninsuladailynews.com.