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The intervals between eruptions along the big faults are measured in centuries, whereas the secondary cracks ``may only slip in a big earthquake every 1,000 to 5,000 years,'' notes seismologist Wayne Thatcher of the U.S. Geological Survey. ``Yet there are so damn many of them that they pose a seismic hazard equivalent to the Big One we've all been so focused on.'' Seismologists also point out that quakes could endanger places where citizens have rarely thought about them: Seattle, for instance, which sits close to a fault under the Pacific that seismologists now conclude has triggered major quakes in the past.
The destructive behavior of each quake is subject to countless variables: the direction of fracturing, the composition of underlying soils, whether the motion occurs close to the surface (as in Kobe) or deeper underground (Northridge), even the time of day or night. The severity of a quake as gauged by energy released is also no measure of its destructiveness. A small quake in the center of a city can kill 1,000 people for every life lost to a monster tremor in a thinly populated place--like the death toll if any (there doesn't seem to be an exact count) in New Madrid, Missouri, in 1811-12, when it was rocked by one of the most severe series of earthquakes ever to strike the U.S. The Kobe quake was only slightly bigger than the Northridge tremor but more disastrous in part because its full force appeared to hit densely populated parts of the city. The 6.7 Northridge quake slammed 80% of its kinetic energy into the sparsely populated Santa Susana mountains rather than the buildings of downtown Los Angeles.
Given all these vagaries, how can cities plan to withstand earthquakes? One cardinal rule probably ought to be, Do not build on filled land. Such areas are subject to a phenomenon called liquefaction. Quake vibrations rupture the surface, allowing water-saturated soil to rise up and turn what seemed to be solid ground into something like a quaking bowl of Jell-O. In both Kobe and the Marina district of San Francisco, site of the worst damage from the 1989 Loma Prieta quake, liquefaction proved disastrous; the same could happen in the Oakland area across San Francisco Bay. Warns Ross Stein, Geological Survey physicist in Menlo Park, California: ``Kobe is almost a dress rehearsal for an earthquake on the Hayward fault in the East Bay.''
Progress has been made in designing earthquake-resistant--but not earthquake-proof--buildings. U.S. building codes are written to save lives, not prevent all damage. Modern structures are designed to sway and maybe even crack in a quake, but not to break apart and crush their inhabitants under falling debris. Says Bill Iwan, director of the earthquake engineering research laboratory at Caltech: ``With buildings, if you walk out after a quake, the designers did their job.''
Skyscrapers built to sway with a buckling earth and low-rise buildings that sit on rubber pads that act like shock absorbers, a common feature of hospital design, have proved their worth. In Kobe it appears that few, if any, buildings constructed after 1980, when a stricter code was enacted, were destroyed. And the widespread wreckage of wooden houses in Kobe is no clue to what might happen elsewhere; wooden houses in Northridge, built to a very different pattern, stood up well.