‘The Big One’: potential local impacts of a earthquake or tsunami in the San Juans

The tsunami devastated the coast of Japan, washing away homes and killing 1,000 people on a winter’s night.

It was the year 1700.

Japanese history calls it “The Orphan Tsunami” because it seemed to birth itself; they felt no warning earthquake.

The Pacific Coast has no such comprehensive written history, and for a long time quake experts believed that the closest fault, the Cascadia subduction zone, was safely aseismic.

That was until they found mysteriously sunken Northwest forests that appeared to have been killed by salt intrusion – in the year 1700. And before scientists doing horizontal geodetic modeling discovered that the Olympics are creeping languidly toward Tiger Mountain.

It turns out that the eastward-moving Juan de Fuca tectonic plate is not sliding smoothly beneath the westward-moving North American plate; instead, it’s bunching up, building up tremendous pressure that scientists believe will eventually let loose in the space of a few minutes.

If the fault’s five segments all “go off” at once, a 9.0-plus magnitude megaquake could launch twin killer tsunamis, one toward the Pacific Coast and one toward Japan.

Seismologists now believe that’s what caused The Orphan Tsunami in 1700. When the plates finally slipped free, the pent-up Pacific Coast dropped by about five feet, submerging forests.

Within 35 minutes, tsunami waves had hit the outer West Coast, depositing sand layers that can be identified today. The westward-moving tsunami took about nine hours to reach Honshu.

State DNR Chief Hazards Geologist Tim Walsh held 80 Orcas Islanders spellbound with earth-shaking tales as he shared about earthquake potential in the Pacific Northwest at a Nov. 17 event hosted by the San Juan County Department of Emergency Management.

Predictions as to when the plates will roar again vary, but Walsh said six past events affecting Washington have occurred 500 to 550 years apart. In 2011, we’re at 311 years and counting. Canadian and U.S. experts have offered probabilities of 14 to 29 percent that the event could occur during the next 50 years.

How could a Cascadia subduction zone megaquake and tsunami affect the islands?

The National Oceanic and Atmospheric Association (NOAA) Center for Tsunami Research has been creating tsunami modeling to help Washington state communities prepare. They’re focusing on high populations first, but plan to model for the San Juans next year.

NOAA is modeling this particular size of quake based on the probability of such an event occurring during the normal lifetime of a building or person.

“What we’re trying to establish is a scenario that has a reasonable probability,” said Walsh. For perspective, he said, the massive tsunamis caused by the end-Cretaceous meteor that struck the Yucatan Peninsula 65 million years ago were an event likely to recur just once every 100 million years.

While specific information on local effects isn’t yet available, Walsh and Cowan shared some general predictions.

Walsh said quakes normally don’t fell trees, and that rocky areas don’t tend to destruct. Glacial sediments or soft, wet and sandy soils can potentially “liquefy,” causing buried tanks and pipes to float to the surface and wreaking havoc with infrastructure like septic, power and and water lines.

Walsh said buildings built before 2003, under the earlier Uniform Building Code “may be okay,” but newer buildings will fare better. The 2012 International Building Code will include increased precautions for the West Coast based on newer input.

As for what to do during a quake event, county emergency management director Brendan Cowan’s advice is to keep it simple: If you feel a quake, drop to the ground, take cover under sturdy piece of furniture, and wait for the shaking to stop. If you live on low-bank waterfront and a quake lasts minutes, as soon as the quaking stops, calmly walk to higher ground.

In NOAA’s simulation for Bellingham, a tsunami barrels down the Juan de Fuca Strait and slams into the west side of Whidbey Island before funneling north to slosh around the San Juans (potentially 90 to 180 minutes post-quake).

“Tsunamis are drastically affected by bathymetry, topography, and geography,” said Cowan. High resolution modeling for Bellingham and Anacortes gives preliminary estimates that waters could reach a height of 10 feet above mean high tide in some places, and up to 15 feet at the heads of long inlets or other constriction zones.

The tidal surge may or may not sweep through Eastsound, which rises from 15 feet elevation on Main Street. Areas of the Eastsound airport sit at 15 feet as well, with elevation reaching 30 feet at Mount Baker Road. The public schools are perched at 95 feet. Walsh said damage can vary greatly depending on water speed and wave length. He also said tsunamis often cause fires.

Sudden isolation

Cowan said for the 90 percent of county residents who live at higher elevations in wooden frame homes built on bedrock, impacts of a megaquake and tsunami could be “dramatic … but probably not catastrophic.”

“We are better off in the San Juans than almost anyone else in Western Washington,” he said.

The real problem will be sudden isolation.

“Seattle, Vancouver, Victoria and the I-5 corridor are going to be significantly incapacitated,” Cowan said. “We’re going to be cut off from the mainland.”

He said islanders should expect to be on their own for at least week, providing their own food, water and other essentials.

The department offers simple ways to be prepared for potential emergencies like this at sanjuandem.net.

For an older tsunami model showing potential water movement, see http://www.pac.dfo-mpo.gc.ca/science/oceans/tsunamis/medium-tsunami-moyenne-eng.htm (predicted wave heights have since increased.)

A NOAA model for Seattle is online here.