Cascadia's Fault - Jerry Thompson [142]
That was the problem Tom Heaton, who heads the Earthquake Engineering Research Laboratory at Caltech, and Jing Yang, one of his doctoral students, decided to tackle next. Heaton and Yang built a computer model to simulate the effects of a magnitude 9.2 Cascadia rupture on downtown Seattle. They began with data from the 2004 Sumatran quake and Japan’s magnitude 8.3 Tokachi-Oki rupture in 2003, along with geological data about various soil and rock conditions in and around Seattle.
They tested a series of hypothetical steel-frame buildings from six to twenty stories tall with older “brittle” and newer “perfect” welds at the critical joints. They ran the model several times, factoring in different possible distances from Seattle to the locked zone of Cascadia’s fault, just in case the real rupture does extend farther inland toward the city. Their digital temblor made the ground shake for four minutes, the dominant shock being the low-frequency kind that caused so much grief and damage in Mexico City in 1985. The deep sedimentary soils in some areas of Puget Sound predictably amplified the waves, just as the dried-up lakebed did in Mexico City. In the new Caltech simulations, the soils increased the duration of shaking as well.
Heaton and Yang presented their results at the annual meeting of the Seismological Society of America in Monterey in April 2009 and journalists immediately wanted to know the bottom line. A glance at the poster Jing Yang had prepared for the meeting pretty much told the story: “Our simulations show that Seattle high-rise buildings with brittle welds have a significant potential for collapse.”
When Yang started work on her numerical model, she was forced to simulate steel-frame buildings rather than large concrete structures simply because it was easier for a computer to predict how steel would bend and eventually fail. The fracturing of concrete was much more difficult to model. It was certainly plausible, according to Heaton, that older concrete buildings could be at even greater risk because they were probably even more brittle. But there was no reliable way to re-create that kind of failure in a computer.
Another reason to study steel was that the Northridge jolt of 1994 had shown scientists that brittle welds in older steel-frame buildings had failed more often than anyone had expected. Amendments to the building code made in the wake of Northridge have changed the way structural joints are welded, presumably giving newer buildings extra strength. And Yang’s model did seem to confirm that newer towers would be stronger—but only up to a point.
If the rupture of Cascadia’s fault happens to extend down below the west coast beaches to some point underneath the Olympic Mountains—closer to Seattle—the shaking would be much worse. And when that scenario was run in the Caltech simulation, all the high-rise buildings in Yang and Heaton’s experimental model collapsed. Even those with “perfect welds.”
Some of the science writers saw parallels to Mexico City and wanted to know more. “All the crummy little buildings that existed in Mexico City were completely undamaged,” Heaton explained, offhandedly, to one reporter, “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.”
Low-rise, low-tech buildings simply did not vibrate or resonate at the same frequency as the big shockwaves generated by a subduction zone. High-rise buildings, on the other hand—even relatively new ones—constructed on thick sedimentary soils, vibrated more than any engineer or any building code had anticipated.