Ready for the Really Big One?
Posted: Thursday, April 09, 2009 11:46 AM by Alan Boyle
Kim Olsen / SDSU |
A color-coded computer simulation charts ground
shaking caused by a 9.0 quake in the Pacific
Northwest. Click on the image for more information.
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The magnitude-6.3 earthquake that struck villages in Italy was horrible enough, but can you imagine what would happen to a city like Seattle if it were hit by a magnitude-9 shocker? That's exactly what Caltech's Thomas Heaton and Jing Yang try to do in a new series of simulations - and the picture isn't pretty.
In many of the simulations, high-rise buildings suffered severe damage. In some of the simulations, they collapsed altogether. Do those simulations reflect reality? The bottom line for Heaton and Yang, as for many other researchers looking into the potential effects of megathrust earthquakes, is that we just don't know.
"We can make some guesses, but they're just that: They're educated guesses," Heaton, who heads the Earthquake Engineering Research Laboratory at the California Institute of Technology, told me this week. "Depending on how we set the parameters, we can end up with shaking where it looks like current [building] codes can ride through it, or we can set parameters where it looks really bad."
The problem is that scientists haven't been able to analyze the effects of truly monstrous quakes on urban areas. Sure, there was the 9.0 quake and tsunami that devastated Sumatra in 2004 on the day after Christmas. But that was a different situation.
"They were unfortunate in all the tsunami issues, but they were fortunate that they didn't have any high-rise buildings that swayed in resonance," Heaton said.
Then there was the 8.1 quake that hit Mexico City in 1985. "All the crummy little buildings that existed in Mexico City were completely undamaged," Heaton said, "but the high-rise buildings, which were the pride of their construction industry, many of them collapsed. It wasn't just a matter of poor construction. It was a case of the wrong buildings being in the wrong place at the wrong time."
The kind of monitoring equipment needed to get a good fix on the shaking involved in that quake just weren't in place, Heaton said. As a result, the best that scientists can come up with are computer models based on the data they're able to gather from a variety of seismic situations.
Heaton and Yang combined data from the 2004 Sumatran quake and Japan's magnitude-8.1 Tokachi-Oki quake in 2003 to develop their models. They also factored in engineering data about the structure of modern steel buildings ranging in height from six to 20 stories, as well as geological data about Seattle and the Cascadia subduction zone.
Why pick on Seattle? It's not because they've got something against the Mariners: Rather, it's because the Pacific Northwest is known to be prone to infrequent but powerful earthquakes, including a 9.0 temblor that sparked tsunami waves as far away as Japan in 1700.
The findings, presented today at the Seismological Society of America's annual meeting in Monterey, Calif., indicate that such an event could pose a catastropic risk, due to the way high-rise buildings might sway during, say, four minutes' worth of low-frequency motion.
"In general, high-rise buildings behave very differently from low-rise buildings," Heaton explained. "They're primarily designed to be flexible - and in sharp, rapid shaking, during a moderate-size earthquake, high-rise buildings perform extremely well."
It's a different story for large-size, low-frequency quakes, particularly in places like the Seattle basin, Heaton said. "You put a typical house or low-rise construction on that kind of ground, and the accelerations are not very big. They make you feel a little dizzy. You just ride with the motion," he said. "But a tall building can begin to sway in time with the resonance. That can be very dangerous."
The risk would be significantly higher for buildings that don't come up to the construction standards set after the 1994 Northridge earthquake. Before then, the typical materials and procedures used for welding buildings together weren't as stringent, Heaton said.
Heaton emphasized that he and Yang were working with hypothetical 20-story structures, and not specific buildings. (Seattle's tallest skyscraper is the 76-story downtown Columbia Center, completed in 1985.) And he said his concerns aren't limited just to Seattle.
"We did a simulation of what the same buildings might do in a 1906 earthquake repeat, and to be honest with you, I think the threat in San Francisco is probably at least as severe as Seattle," he said. "This is not just a Seattle issue. These long-period motions only show up in the largest events, and the largest events don't happen very often. But they do have to happen: They're the main actors in plate tectonics."
So what's a person to do?
"I'm just one professor in a university, and these decisions about the best course of action are rarely made by an individual," Heaton said. "One of the obvious things to do is to repair the old welds in buildings, although it tends to be quite expensive to repair those welds. My feeling as a professional is that those welds need to be dealt with sooner or later, and you might as well deal with them sooner."
Earthquake experts in Washington state say they've been dealing with seismic concerns for many years - and particularly since the 6.7 Nisqually quake of 2001. "No one's drawn a final line here," said Rob Harper, a spokesman for the Washington Emergency Management Division. "They're continuing to look at data across the whole spectrum of issues. I don't think anybody figures we can stand pat."
Harper said the Cascadia Region Earthquake Workgroup has taken a lead role in assessing the seismic risk and recommending upgrades in building standards as well as preparedness guidelines. A 2005 CREW report specifically addresses the 9.0 megaquake scenario, and state officials try to keep up to date on the latest findings.
Tim Walsh, chief hazards geologist at the Washington Department of Natural Resources, is familiar with Heaton's line of research, although he hasn't yet seen what was being presented at the Monterey meeting. "He thinks that [long-period earthquakes] are more severe than has been typically thought," Walsh told me.
Walsh acknowledged that uncertainties remain. "It's a quandary, to be sure, because our building codes - and building codes around the world - have never been tested on these long-period motions," he said.
"I've talked to structural engineers about this issue, and in general they're concerned that we don't have enough information," Walsh said. However, he added, most of those engineers "are confident that there's enough redundancy" in the area's building practices to provide an extra margin of safety for the megaquake scenario.
Can the earthquake in Italy provide insights for dealing with future megaquakes? Because of the way that the magnitude scale is structured, a 9.0 quake is much more than 1,000 times as energetic as Italy's 6.3 quake - which makes this week's tragedy a "very different kind of situation from what we're talking about," Heaton said.
Nevertheless, there is a lesson to be learned from the Italians.
"I feel quite confident that they wouldn't have had the same tragic outcome if people had been in modern buildings with a good building code," Heaton said. "It's really not that big an earthquake. It was the inadequate reinforcing of masonry structures that primarily was the problem there. ... It's a good example showing that it's worth paying attention to your buildings ahead of time."
For still more about monster earthquakes, check out our earthquake preparedness guide, our list of nine killer quakes and our interactive fact file about seismic science.
