Cascadia's Fault - Jerry Thompson [141]
“Hopefully we’ll be able to use that videotape or the simulations to maybe wake up some folks in the local area,” said Barker, “to show them—give them an idea of what to expect.”
“If we could communicate that intensity to everyone,” added Lence, “we might have a better chance at being prepared in these emergencies.”
Wave-tank models and digital tsunamis were not the only kinds of experiments being conducted to anticipate the effects of Cascadia’s next violent outburst. Coastal inundation zones are clearly not the only concern. By the spring of 2009, a new question had arisen: what would happen to the urban cores of major cities from Victoria and Vancouver to Seattle and Portland? These four cities are built on land that lies well to the east of the main fault, and some experts had suggested the rupture zone was far enough away that damage in the urban areas might not be as catastrophic as first thought.
When Roy Hyndman and Kelin Wang published their study of the locked part of the subduction zone in 1995, they calculated that the stuck and truly dangerous area of the fault lay nine miles (15 km) underground to the west of Vancouver Island and the beaches of Washington, Oregon, and California. The “landward limit” or leading edge of the locked zone extended “little if at all beneath the coast,” which “limits the ground motion from great subduction earthquakes at the larger Cascadia cities that lie one- to two hundred kilometres inland.”
With this in mind many emergency planners in the Pacific Northwest have worked on the assumption that Cascadia’s magnitude 9 event would not be the worst scenario. Any of the much shallower faults in the continental crust could generate a magnitude 6 or 7 rupture directly beneath or very near an urban area, and this might actually cause more severe, localized damage. But in 2009 Timothy Melbourne and his colleagues suggested the locked zone was considerably nearer to the big cities, perhaps within 50 miles (80 km) of Seattle, for example. And if that’s true, then we’re right back where we started with Mexico City in 1985: what do we do about heavy shockwaves hitting high-rise buildings?
From a civil engineer’s point of view, the elephant in the room has always been how an urban forest of tall towers, long bridges, freeway overpasses, and hydro dams would respond to the much longer duration of low-frequency seismic shocks from Cascadia’s fault. The way it was explained to me, going from a magnitude 7 to a magnitude 9 means the intensity of the shaking doesn’t change so much as the length of time it lasts.
Instead of forty-five seconds or a minute of shaking in a magnitude 6 or 7, the seismic shockwaves from a magnitude 9 could go on for four or five minutes. Like they did in Alaska. And nobody really knew how well tall buildings would stand up to that much horizontal motion, slamming side to side, undulating like metronomes fifty or a hundred times, flexing every beam of steel, stressing every welded joint, every slab of concrete, every pane of glass to the limits of endurance. All anyone could do was speculate about the outcome because there has never been a magnitude 9 in a city full of skyscrapers. Neither the Chile quake of 1960 nor the Alaska rupture of 1964, nor even the Sumatra disaster of 2004, shook a large, modern city with high-rise towers. Mexico City gave us only a hint of what might happen.
To oversimplify things just a bit, building codes in North America generally require engineers to design tall structures in earthquake country to survive the forces imparted by temblors up to magnitude 7. Specifications for a magnitude 9 don’t exist because there have been so few of these megathrust