Engineers and emergency planners from northern California to British Columbia say the massive undersea quake and tsunami that recently
assaulted Japan gives clear warning about the danger that lurks just off the Pacific coast like a mad dog sleeping by the bed: A 630-mile-long
geologic feature that was identified in 1984 is believed to be very similar to the one that broke with such violence off the coast of Japan in March.
Evidence of violent breaks in the feature—called the Cascadia Subduction Zone, or CSZ—has been found in sediment layers left by prehistoric
tsunami. On April 25, 1992, a 7.2-Mw earthquake with an epicenter at the south end of the CSZ in northern California produced a modest tsunami
that struck shore. That event confirmed the zone's potential to produce not only strong earthquakes but locally sourced tsunamis that could be
ashore within as few as 10 minutes.
“The 1992 quake finally got everybody onboard,” says Lori Dengler, a geology professor at Humboldt State University in northern California.
“Before then, it was all paleo-tsunamic evidence. We were fortunate that our real live earthquake was not a 9 but a 7.2.”
Alarmed, Congress in 1995 directed the National Oceanic and Atmospheric Administration to form a federal and state working group called the
National Tsunami Mitigation Program. Much of its energy comes from officials in Alaska, Hawaii and the Pacific Northwest.
Based on fast-changing science, inundation zones have been modeled, maps drawn and revised, and warning systems deployed. “FEMA P646:
Guidelines for Design of Structures for Vertical Evacuation From Tsunamis” was published in 2008 to help planners find ways to move people to
safety within minutes.
A few West Coast vertical evacuation facilities are in conceptual design, but uncertainty persists about how to evaluate the risk or engineer for
tsunami loads. Knowledge about the dangers continues to evolve.
“We had been helping prepare for Cascadia for about 15 years,” says Jay Raskin, president of Ecola Architects PC, Cannon Beach, Ore. As a
Cannon Beach councilman and, for a time, mayor, Raskin advanced plans for a new city hall strong enough and tall enough not only to withstand
a 9.0 quake but to provide shelter from tsunamis on its upper floor and roof—for a few hours, at least—to the resident population of 1,700. The
problem of a possible additional 15,000 visitors who could be there on a summer day is not addressed by the plan.
“We always thought getting to high ground was enough—and we have a lot of high ground around here,” Raskin says. But each new tsunami in
the world generates new insights, leads to improved modeling and data, and usually ratchets up the appreciated risk. The March 11 quake off the
Tohoku region of the northeast coast of Honshu Island, Japan, is no exception. There is a greater respect for what is unknown about
subduction-zone behavior, experts say.
Geologists, engineers and policymakers were caught off guard by the power of the Tohoku event. They believed the geology that launched it
could produce an 8.2-Mw quake—but not a 9—and defenses were scaled accordingly, Dengler says. The March 11 event is a reminder of how
scant the historic record is on subduction zones, she says. The Tohoku quake itself could be considered unusual—except there is so little data to
say what “unusual” really is, she says.
Dengler says the Dec. 26, 2004, Indian Ocean quake was the greatest and longest subduction-zone rupture in recorded history—1,400
kilometers long with 20 meters of slip. By contrast, she says, models of the Tohoku quake suggest the slip was in a “patch” 100 km long by 300 to
400 km wide; the shift was 50 meters. “If I were at the tip of the plate, in a minute or two I would find myself 150 feet to the east. That's an
unbelievably huge slip,” she says.
Even though Japan invests a greater part of its gross domestic product in tsunami defenses than any other country in the world, Dengler says
experts on that coast had not prepared for an event of that magnitude, which had a wave run-up as high as 128 ft.
“The No. 1 failing in Japan was that they underestimated their hazard,” says Dengler, who just returned from a 10-day examination of the tsunami
wasteland. “When you underestimate the hazard, everything else fails—the engineering, the education, the policies, the response ... they fell so
short. It's discouraging.” She says her team is reexamining everything now in light of lessons from Japan.
Yumei Wang, a geotechnical engineer focused on risk management and the geo-hazard section leader at the Oregon's Dept. of Geology and
Mineral Industries, (DOGAMI ), is at the forefront of efforts to protect people in Oregon's tsunami zone. She says the Tohoku quake is “a brutal
reminder that we are not doing nearly enough.”
Wang says the southern half of Oregon's 300-mile coastline has high ground that can provide refuge for coastal residents. However, her biggest
concern is the central and north coast, a lower area with extensive flatlands. “I've identified 12 communities that should be considering vertical
evacuation,” Wang says. “They are so far from being ready.”
Just as in Japan, officials here on the West Coast warn that the only reliable way to escape a tsunami is to go to high ground on foot. Cars get
stuck in traffic. Roads rarely take the shortest route uphill. In the case of the Pacific Northwest, where the CSZ is so near the shore, a tsunami
could be on the beach in less than 20 minutes in most places.
Inundation maps throughout the CSZ coast are progressively being refined. Most now are based on modeling of dozens of runs of both
distant-source and near-source tsunamis, with partial slips or complete slips along the entire length of the CSZ. Models project across the local
bathymetry and topography. Predicted heights vary by location.
Raskin says the design tsunami for Cannon Beach is based on a DOGAMI study; the state has 95% confidence that it covers all possible tsunami events.
“The decision to keep with the 95% confidence level was based primarily on the fact that the worst-case scenario is a one-in-10,000-year
event,” Raskin says. The inundation for that 100% solution is 25 ft higher.
The conceptual plan for a new city hall calls for an elevated, 10,000-sq-ft building with deep concrete piles laterally braced by post-tensioned
grade beams, with a post-tensioned municipal office floor and rooftop terrace. Three sets of external stairs lead to the main floor, which is 15 ft
above grade, including one that is obviously a grand stairway to safety.
No Visible Damage
Raskin says Kent Yu, a seismic engineer in Portland and principal with San Francisco-based Degenkolb Engineers, is working on the city-hall
project. He says Yu recommended, based on his observations in quake-disaster zones around the world, the building be engineered to withstand
extreme seismic loading without showing damage, Raskin says. Frequently, people will not reoccupy buildings after a quake if they see damage,
even if the building is declared safe. “If we only have 20 minutes to get people in, there should be no hesitation at all,” Raskin says.
The building may cost about $4 million—twice the cost of a conventional solution, Raskin says. Hopes for a congressional appropriation are dead
for now, but the town is applying to get funding as a disaster mitigation project through FEMA. Raskin believes mitigation funding would be a good
solution and set a precedent that could benefit towns all along the coast.
Meanwhile, Yu has been hired to do a comprehensive study of Cannon Beach's resilience to disasters. “I was asked to look at the effect of the
earthquake and/or tsunami on their community and to look at government resilience, because we don't want to become another Haiti,
incapacitated by disaster,” he says.
“We need to look at the bridges, wastewater, hospitals and schools and get a general idea of what's going to happen,” Yu says, referring to a
major seismic and tusnamic event as an inevitability, as many planning officials in the region now do. “We need to look at it from an economic
perspective: What will be the effects of a disaster on recovery? Also, we need to look at the civic infrastructure—the social networks, the
non-governmental, community-based and faith-based organizations. These organizations provide very critical services to the community.”
Yu admits that such a study may seem a broad portfolio for a structural engineer, but he says, as a member of a state seismic safety policy advisory
commission, he is interested in promoting a safety policy to make the state more resilient to seismic events. He says, “When I do
engineering projects, I can only solve one thing at a time. When I look at the policy level, I can bring more impact to society.”
转自（Engineering News-Record）