Experts say the region is not remotely ready for the kind of big earthquake they now know is likely!
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.
Buried water mains and sewer lines could crack, separate, or float to the surface, spilling their contents across roads, landscapes and waterways. Downtown, older brick-and-mortar buildings could shift and shake, shedding bricks and rooftops in piles of debris.
In the flash of a few minutes, pretty much all of Clark County would be likely to find itself without power, without reliable roads and without safe water. And it could stay that way for months.
On a normal late fall day in Vancouver, such devastation might seem unreal. But it’s happened before — across the globe, and also right here.
What is a Cascadia quake?
Just off the Pacific coast — about 50 miles out to sea, and stretching from Northern California to British Columbia — a 700-mile fault marks where the Juan de Fuca geologic plate is sliding under the North America plate. The process, which started about 20 million years ago, is pushing North America over Juan de Fuca at a rate of about 1.5 inches a year.
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 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.
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.
Then there’s a Cascadia quake, which would be a subduction zone earthquake. It’s hard to tell how much damage a 9.0 quake like that would cause, but a report by the Department of Natural Resources suggests the damage could go well into billions of dollars and injure or possibly kill thousands of people.
“Big picture for a Cascadia (earthquake) … we’re definitely looking at years before we’re made whole again,” said John Wheeler, emergency management coordinator at CRESA. “If you look at (Hurricanes) Katrina and Sandy, at the Japanese earthquake — the damage from those is very similar to what we could experience in the Pacific Northwest.”
- Unshaken complacency: Unprepared for quake (columbian.com)