Chile’s huge earthquake occurred on a tectonic boundary that is strikingly similar to the one off the coast of the Pacific Northwest.
And, even though the tsunami generated by Saturday’s quake barely rippled the Oregon and Washington coasts, scientists say it should be a reminder to prepare for the subduction zone quake poised to strike the Pacific Northwest.
“When we see the destruction that these earthquakes can cause, it’s a good spur for us to make sure we are doing all we can to get ready,” said Bill Steele, seismology lab coordinator at the University of Washington.
In Chile, the quake occurred on the boundary where the offshore Nazca tectonic plate is grinding below the South American plate at a rate of about 80 millimeters per year, according to the U.S. Geological Survey. Ruptures in this zone have generated some of the largest earthquakes in recorded history, including a 9.5-magnitude quake in 1960 — the largest ever recorded.
Since 1973, there have been 13 events of magnitude 7.0 or greater in coastal Chile.
A similar process is occurring off the coast of Washington, Oregon and Northern California. Although not as frequent, herky-jerky movement between the North American and offshore Juan de Fuca plates is no less deadly.
The oceanic Juan de Fuca plate is diving — or subducting — below the North American plate in an area known as the Cascadia subduction zone. Unfortunately, the two plates are locked together. As pressure builds, scientists say it’s only a matter of time before the two plates unlock with catastrophic consequences.
This last occurred on Jan. 26, 1700.
Scientists know the precise date because of written historical records of an “orphan” tsunami inundating coastal Japan. Combined with carbon dating of tree trunks discovered in marshy areas along the Northwest coast, scientists pinpointed the date when upland forests snapped downward severely — exactly 310 years ago.
The unlocking plates roiled the Pacific Ocean like a bathtub.
Scientists now believe Cascadia subduction zone quakes have occurred every 250 to 800 years in the past.
“It could be tomorrow. It could be in five minutes,” said Evelyn Roeloffs, a USGS geophysicist based in Vancouver. “More likely, it will be longer than that.”
“The fault that broke in 1700 has been reloading for future Cascadia earthquakes,” according to “The Orphan Tsunami of 1700” published by the USGS and the University of Washington Press. “Sometimes the fault may break along its entire length; at other times it may break piecemeal.”
With new scientific awareness, Steele said, coastal communities are increasing their preparation with improved seismic monitoring and better evacuation options. Coastal engineers, for example, are thinking about reinforced concrete buildings where people could climb above the surging water in a pinch.
“We’re beginning to get a grip that this is going to be serious,” Steele said Saturday. “And we’re starting to plan realistically.”
In Vancouver and other inland cities, four or five minutes of ground motion will endanger older highway structures, brick structures and property on soft soils.
Although single-family dwellings probably would remain standing, emergency managers say it’s wise to plan for long-term disruption of basic supply lines. Everyone should keep at least a three-day supply of food and water, as well as extra clothes, shoes and toiletries.
Washington emergency managers provide a basic primer for emergency preparation at www.emd.wa.gov/preparedness/prep_prepare_year.shtml.
Cheryl Bledsoe, Clark County’s emergency manager, said concern about a widespread computer meltdown in the year 2000 had a positive effect in prompting many people to consider basic steps to being prepared.
“There was a deadline,” she said. “The earthquake is nebulous. It could happen today, or in 10 years or in 100 years. People think it won’t happen in our lifetime.”
