Cascadia's Fault Read Online Free PDF Page B

waves. Who knew that soil had its own frequency and would quiver like a bowl of jelly? I tried to imagine evenly spaced ripples rolling through dirt like waves on the ocean.
    It turns out the speed of vibration is determined by the type of soil (what it’s made of) and how hard or loosely packed it is. Weichert explained that structures of a certain height (six to seventeen stories tall) can also have a natural vibration period of roughly two seconds—which amplify the shockwaves yet again. The image of a seventeen-story tuning fork came to mind.
    The seemingly random damage pattern Robb Douglas had told me about on the phone from Mexico City back in September began to make sense. Before leaving on this shoot I had studied his news footage from the disaster zone: one condo block in ruins while another right beside it, maybe only a few stories taller or shorter, was left standing.
    To my surprise, the Mexico City building code already included specific and more stringent regulations for anything constructed on the lakebed because there had been damaging quakes there in the past and local officials thought they knew what to expect. But Ron DeVall, a civil engineer in his midthirties who sat across from Weichert, pointed out that this bizarre, unlucky, two-second rhythm had never been seen before.
    â€œThey knew that the effect of the soil was pronounced in these areas and their code did account for it,” confirmed DeVall. “On firm ground they were talking about basic acceleration of 4 to 5 percent of gravity.” In other words, to survive the kinds of shaking that had hit Mexico City in 1957 and 1979, any new project constructed on bedrock since that time was in theory designed to withstand a lateral jolt against its
foundations equal to 4 or 5 percent of gravity—meaning 4 or 5 percent of the building’s own weight. That much lateral reinforcement was a fundamental requirement of the local code.
    Anything constructed on the lakebed zone of the city was supposed to be even stronger, able to withstand twice as much lateral force—up to 10 percent of gravity. The shockwaves of September 19, unfortunately, had been much more powerful than that. “In actual fact,” said DeVall, “some of the instruments were measuring 20 percent gravity. So they were getting a very large shake, much higher than they had anticipated.”
    DeVall compared the crumbled towers to a child on a swing. “If you can time the rhythm of your pushing, you can just drive that swing higher and higher and higher. In essence, that’s what happens to a building sitting on the lakebed. The soil and the base rock were almost in resonance and the vibration was amplified to the building. The forces generated were much higher than what the building codes predicted. And the effect on the buildings was dramatic. It produced a much wider range of major damage than the previous earthquakes did.”
    Two other factors added to the toll of damage and destruction as well. Most of the Mexico City seismic regulations were written and passed into law as a result of the 1957 event and did not apply retroactively to thousands of city blocks constructed before that. Tragically, this included many schools and hospitals built decades earlier. “Seeing a lot of hospitals and schools damaged, which are traditionally thought to be areas of refuge in a disaster,” said DeVall, “to find them gone is, to my mind, a serious problem.”
    If there was any encouraging news from Mexico, it was that the damage was not evenly spread across the entire urban area, according to Robert Lo, a civil engineer who specializes in soil conditions. Lo even joked that on the day they had arrived in Mexico, roughly three weeks after the temblor, they hadn’t seen any significant damage. “We thought we were in the wrong city,” he smiled.
    It soon became obvious that the majority of the urban area had been
built on solid