Studies of Nepal Quake Raise Concern About Skyscrapers
Version 0 of 1. In April, as email alerts ricocheted with the news that a large earthquake had shaken Nepal near its capital, Kathmandu, experts somberly expected a toll comparable to the roughly 85,000 who died in 2005 when a smaller earthquake struck a more sparsely populated area in the Kashmir region. Over the following days, the number of deaths in Nepal from the magnitude 7.8 earthquake rose to more than 9,000. That figure “is actually a small number given the density of the population in the Kathmandu area and the vulnerability of the buildings,” said Jean-Philippe Avouac, a professor of geology at the California Institute of Technology and the University of Cambridge in England, and an author of two scientific papers published Thursday about the Nepal earthquake. “I think we understand why that is the case.” The relatively low death toll is owed neither to blind luck nor the sturdiness of Nepalese construction. Rather, the 40 seconds of rumbling from the earthquake turned out far different than expected, based on what was known from earlier earthquakes and the geology of the region. Instead of quick oscillations, one second or faster, which shake apart smaller buildings, the ground shook back and forth once every four or five seconds, a slower rate that is more dangerous to tall structures. That could explain why a 200-foot-tall watchtower, which had survived a stronger earthquake in 1934, fell to the ground while many rickety but shorter buildings remained standing. “The two papers provide a good first-cut characterization of the earthquake — exactly how the fault moved — and the shaking that it generated,” said Susan E. Hough, a seismologist with the United States Geological Survey who was not involved with the research. The findings, however, raise some concern that a similar earthquake could pose unexpected danger to cities with skyscrapers. In a paper appearing in Nature Geoscience, a team led by Dr. Avouac used seismological records and radar imagery to describe the rupture in Nepal. India is colliding into the rest of Asia at a pace of up to two inches a year, pushing up the Himalaya Mountains. But the two tectonic plates have been stuck, not sliding smoothly. “Over the last 20 years, at the surface, the fault was completely locked,” Dr. Avouac said. When the strain becomes too great, the fault breaks, setting off an earthquake. On April 25, the earthquake started northwest of Kathmandu and spread east at 1.7 miles per second, rupturing 87 miles of the fault. “This earthquake is actually a relatively small earthquake,” Dr. Avouac said. Stress remains in other parts of the fault, particularly in western Nepal, where the fault has been locked since a magnitude 8.5 earthquake struck in 1505. “I’m worried about that part,” he said. In the second paper, published in the journal Science, Dr. Avouac and his colleagues, using data from GPS stations and a seismometer near Kathmandu, showed that the 12-mile-wide and 87-mile-long patch where the fault slipped broke smoothly, generating long-period waves and less of the higher frequencies that would have occurred had the ground fractured more jerkily. The rupture also did not reach the surface. Dr. Hough said the new findings are “clearly part of the story,” but added that other factors also played a role. The shape of the Kathmandu valley, partly filled with sediments, tends to amplify shaking with a period of one second, Dr. Hough said, similar to the characteristic frequency of a ringing bell. That was observed during the aftershocks, but for the magnitude 7.8 earthquake, the shaking was so strong that the behavior of the sediments changed. “We talk about a bowl of Jell-O as an analogy for how it shakes,” Dr. Hough said. “But it’s actually more like a big sandbox.” For the strong shaking, “it doesn’t transmit the energy the same way it does for a small earthquake,” she said. “It’s basically behaving like a different material.” What the scientists do not know is whether long-period shaking is common for earthquakes along the Himalayas, or whether similar faults elsewhere, usually quiet, could generate similar earthquakes. “There’s one in the Los Angeles area, for example,” Dr. Hough said. “Five-second energy like this earthquake released would be especially damaging to 50-story buildings.” |