Unshaken complacency: Unprepared for quake

o Oct. 12, 1877: Estimated magnitude 6.7, shakes the Portland area, felt in Clark County, no casualties.

o Dec. 14, 1872: Estimated magnitude between 6.8 and 7.4, shakes Washington, Oregon, Idaho.

o Jan. 26, 1700: Magnitude of about 9, centered off the Northwest coast.

o 1310: Evidence of magnitude 8 or higher in the geologic record.n 810: Evidence of magnitude 8 or higher in the geologic record.

o 400: Evidence of magnitude 8 or higher in the geologic record.

o 170 BC: Evidence of magnitude 8 or higher in the geologic record.

o 600 BC: Evidence of magnitude 8 or higher in the geologic record.

Largest earthquakes since 1900

o May 22, 1960: Magnitude 9.5, Chile. About 1,655 killed by quake, and about 230 killed by tsunami across the Pacific.

o March 27, 1964: Magnitude 9.2, Prince William Sound, Alaska. About 15 killed by quake, 113 by tsunami.

o Dec. 26, 2004: Magnitude 9.1, Northern Sumatra. About 227,898 people killed, missing or presumed dead from quake and tsunami.

o March 11, 2011: Magnitude 9.0, Japan. At least 15,703 people killed, 4,647 missing from quake and tsunami.

o Nov. 4, 1952: Magnitude 9.0, Kamchatka, Russia. No reported deaths, but tsunami damage reported across the Pacific.

Deadliest earthquakes in world history

o Jan. 23, 1556: Magnitude 8, China, more than 830,000 deaths.

o Jan. 12, 2010: Magnitude 7, Haiti, 316,000 deaths.

o June 27, 1976: Magnitude 7.5, China, 242,769 deaths officially reported, could be as high as 655,000.

o Aug. 9, 1138: Magnitude unknown, Syria, 230,000 deaths.

o Dec. 26, 2004: Magnitude 9.1, Northern Sumatra, 227,898 deaths.

SOURCE: U.S. Geological Survey

Average number of earthquakes globally each year, by magnitude:

o Mag. 8 and up: 1

o Mag. 7 to 7.9: 15

o Mag. 6 to 6.9: 134

o Mag. 5 to 5.9: 1,319

o Mag. 4 to 4.9: 13,000 (estimated)

o Mag. 3 to 3.9: 130,000 (estimated)

o Mag. 2 to 2.9: 1,300,000 (estimated)

SOURCE: U.S. Geological Survey

People often remember the calm, the quiet, how normal everything seemed before a disaster.

In Clark County, they might remember grabbing a cup of coffee at Starbucks by Esther Short Park, hanging out on the patio at Beaches by the waterfront, taking a bike ride along the Salmon Creek Trail — before the shaking started.

During the long seconds of a magnitude 9.0 Cascadia earthquake, the soft loose soils along the Columbia River could quickly convert to the consistency of liquid or quicksand.

Rock from the dipping, or subducting, plate melts as it moves under the continent, feeding the volcanic arc that includes Mount St. Helens, Mount Hood and Mount Rainier.

Pressure also builds up along the fault. The plates don’t move smoothly but tend to stick and lock against one another, resisting movement until the fault suddenly slips, creating deep and potentially very deadly earthquakes.

There’s no way to predict exactly when the fault will move again. The last time it happened was just over 300 years ago — when the entire 700-mile stretch slipped in the span of about five minutes, creating a magnitude 9.0 earthquake and a massive tsunami recorded in Japanese history as occurring Jan. 26, 1700.

Geologists have uncovered evidence of similarly sized quakes in the region in 1310 AD, 810 AD, 400 AD, 170 BC and 600 BC. There may have been more, but it can be hard to find evidence of earthquakes in the rock record.

Because of that, scientists continue to debate how often the fault ruptures. Some think it happens about every 500 years; others think it’s more like every 250 years.

“Nothing is for sure,” said Tim Walsh, chief hazard geologist at the Washington State Department of Natural Resources. “But we’ve gone past that 250-year time scale already.”

The risks

The Clark County Hazard Identification Vulnerability Analysis, put out by Clark Regional Emergency Services Agency, called the threat of a dangerous earthquake “the hazard of greatest risk to Clark County,” more threatening than a flood, wildfire or volcanic eruption.

The analysis ranks the 25-year probability, vulnerability and risk rating for a strong quake — if not a full 9.0 Cascadia quake — as high.

There are three categories of earthquakes, all of which occur here.

• Shallow or crustal quakes happen along faults near the surface, up to about 10 miles deep. Such faults include the Mount St. Helens Seismic Zone, the Lacamas Creek Fault and the Portland Hills Fault. Shallow faults can trigger by themselves, or could be triggered by deeper earthquakes created through plate tectonics.

• Interplate quakes happen when one geologic plate affects another, such as parts of Juan de Fuca melting and scraping beneath the North America plate. Those quakes tend to be deeper, perhaps 30 miles below Earth’s surface.

• Subduction zone earthquakes happen when plates stick and then suddenly slip against one another at plate boundaries, such as the Cascadia fault.

Every year, the Pacific Northwest Seismograph Network records about 2,000 earthquakes in Washington and Oregon. Most are shallow quakes with magnitudes of less than 3.0.

Larger shallow quakes are far less frequent but can be dangerous. The strongest shallow quake recorded since white settlers came to the region was an estimated magnitude 7.4 back in 1872. It was felt in Oregon, Idaho and Washington. More recently, the “Spring Break Quake” on March 25, 1993 — a shallow magnitude 5.6 centered southeast of Portland — caused $28 million in damage.

Interplate quakes can cause even more damage. On Feb. 28, 2001, the magnitude 6.8 Nisqually earthquake centered 32 miles beneath the Puget Sound region killed one and injured 700, creating between $1 billion and $4 billion in damage.

Scientists think both of those types of earthquakes happen at a rate of about one per 50 years in the Pacific Northwest.

“It’s incredible because there’s a huge whopping fault that stretches from North California to Canada, and it wasn’t until the late ’80s that scientists reached a consensus on the hazard,” Wang said. “It wasn’t until about 2000 that scientists first reached a consensus that the (potential) earthquake could be a magnitude 8 or 9.”

And while that information was under debate, buildings, pipelines and other structures continued to go up, based on construction standards that said the region was relatively geologically stable.

“Our understanding of things has improved,” Wang said. “But a lot of our systems that were built before we knew there was a big fault here can be vulnerable.”

As information about the Cascadia fault has improved, so have building codes. And many structures — schools, prisons, churches, bridges — have been retrofitted to protect human life in the event of a strong earthquake.

But as scientists learn more about how structures withstand earthquakes around the globe, those codes continue to evolve — and the expense of meeting them continues to grow.

“We keep upping the ante, we keep making buildings stronger and stronger, and then another earthquake comes along, things are destroyed and we have to change everything again,” said Eric Lanciault, a Clark County architect.

Cascadia, because the recurrence rates are highly debated and a similar quake hasn’t happened since the area was developed, is another story, he added.

“Nobody’s code anticipates Cascadia,” Lanciault said.

In Clark County

The Department of Natural Resources has a series of hazard maps for counties across the state that it updates periodically. The most recent maps for Clark County show large swaths — especially along the Columbia River and Vancouver Lake — that are particularly susceptible to strong shaking.

Loose soils, like those along rivers and lakes, can be especially vulnerable to a process called liquefaction. In that process, the shaking mixes soil and sand grains with water, creating a land surface that temporarily acts like quicksand.

“River sediments, which a lot of Vancouver is founded on, are susceptible to liquefaction,” Wang said. “If you have a building built on soil that temporarily gives way, it can sink by inches up to a few feet.”

Another process, called lateral spreading, could also create hazardous conditions. That happens when liquefied soils move down a slope, spreading the ground surface.

During the 2011 quake in Tokyo, the ground spread by 5 feet in spots and moved a river, she said.

“If you have a soft soil, you can amplify the ground shaking (and cause liquefaction or lateral spreading),” Wang said. “And Vancouver doesn’t have a lot of hard rock.”

Clark County also has its share of unstable slopes, which could be susceptible to landslides. Risk assessment maps show swaths of land around Hazel Dell, central Vancouver and La Center with soft soils that could be vulnerable to landslides or other hazards from amplified shaking.

“A lot of Clark County is on gravel,” which isn’t the most stable surface, said CRESA’s Wheeler. “And of course there’s more (relatively stable) bedrock in the least populated parts of the county (like Yacolt).”

“I can’t tell you what’s most likely to collapse, but I’ll tell you which buildings to run to,” Lanciault said. “There are some really, really good buildings in Clark County and Vancouver, even though older buildings probably won’t do so well.”

Holy Redeemer Catholic Church, 17010 N.E. Ninth St., which Lanciault designed, could be one of the safest buildings in town, he said.

“I designed it, but I’m not being biased,” he said. “That is a very safe building. I’d tell my family to head there in an earthquake.”

Newer construction at the Washington School for the Deaf, 611 Grand Blvd.; City Hall, 415 W. Sixth St.; the county’s two hospitals; and many newly built county school buildings should hold up fairly well also, he said.

“You just have to scan for whatever’s been recently built,” he said. “Especially government buildings.”

Emergency responders will have to assess damage throughout the county before they prioritize their efforts. If your home is habitable after a quake, it’s likely that you’ll be asked to return there to lessen the load on shelters.

“You have to realize that 911 might not be there, and fire agencies will have to do what we call ‘windshield surveys’ before responding to individual incidents,” Wheeler said. “They’re going to have to prioritize what they do, and a lot of people may need to wait before they get help.”

In the aftermath of a Cascadia quake, it’s likely that power could be out for weeks, perhaps months. Bridges could be similarly out of service, roads and train tracks damaged. It’s possible Clark County would be completely cut off from neighboring Portland for a time, Wheeler said.

“It’s not unreasonable to expect that we could be isolated in Clark County as a whole community,” he said.

It’s a good idea to prepare an emergency kit with several days’ worth of food, a gallon of water per day per person, flashlights, blankets, a first-aid kit and even a generator, Wheeler said.

“What people really need to be thinking about is how to be as self-sufficient as possible,” Wheeler said. “It’s very fair to point at what’s happened with Hurricane Sandy. You need to have a plan and be ready to be an island unto yourself.”

Some parts of New York and New Jersey that were hit by Superstorm Sandy in late October are still without power. That’s a lesson if you think the standard suggestion of a three-day supply of food and water is more than enough, Wheeler said.

“I say at least three days,” Wheeler said. “If you look at Sandy, several days past landfall and there were still people who couldn’t get basic supplies.”

Walsh, at the Department of Natural Resources, has several emergency kits stashed in various spots.

“In my car I have one with thermal blankets, packets of water, a first-aid kit, flashlights and stuff like that,” Walsh said. “At home I have enough food and water for several days and a generator.”

Water is especially important, even if you have a well, he added.

“One thing that happens in earthquakes is that they change the groundwater system,” Walsh said. “They can increase or decrease the flow in a well. In 2001 in the Nisqually quake, several wells in Eastern Washington dried up.”

Another bit of advice is to make sure your gas tank is always half-full. It’s good for the car, but it also gives you some mobility if all the infrastructure fails, Wheeler said.

“Unfortunately it’s usually a minority of people that prepare,” Wheeler said. “The lesson with Hurricane Sandy, with Katrina, with Japan, is that it could happen to us. It’s not just on TV. It’s a reality in the Pacific Northwest.”

Sue Vorenberg: 360-735-4457; http://twitter.com/col_SueVo; sue.vorenberg@columbian.com.