Saturday, January 16, 2010

"What is most remarkable is how effective the codes have been"

Below are two exerpts from "Annals of the Former World," by John McPhee, from the "Assembling California" chapter, the scariest thing I've read in a long time:

California building codes that involve seismic requirements were first written in 1933. They covered school buildings and nothing else. San Francisco did not extend such codes to other structures until the late nineteen-forties, when they appeared in the laws of virtually all communities around the bays and of many around the state. While most buildings are still 'pre-code,' what is most remarkable is how effective the codes have been. In the 1989 Loma Prieta earthquake, 62 people died. In an earthquake of similar magnitude in Armenia in 1988, 55,000 died. In Mexico City in 1985, 10,000 died. In the Iranian earthquake of 1990, 50,000 died. The difference may lie partly in luck, in site, in relative intensity, but largely it lies in building codes, and the required or suggested strengthening of existing structures. Certain vulnerabilities notwithstanding, California seems to know what it is up against, and what to try to do about it. Never mind that in October, 1989, 21,000 homes and commercial buildings were cracked, crumpled, or destroyed, and nature's invoice for a few moments of shaking was $6 billion...

...After the earthquake on the Hayward Fault in 1868, geologists clearly saw the dangers varied with the geologic map, and they wrote in a State Earthquake Investigation Commission Report, "The portion of the city which suffered most was...on made ground." In one minute in 1906, made ground in San Francisco sank as much as three feet. Where landfill touched natural terrain, cable-car rails bent down. Maps printed and distributed well before 1989---stippled and cross-hatched where geologists saw the greatest violence to come---singled out not only the Nimitz Freeway in Oakland but also, in San Francisco, the Marina District, the Embarcadero, and the Laocoonic freeways near Second and Stillman. Generally speaking, shaking declines with distance from the hypocenter, but where landfill lies on loose sediment the shaking can amplify, as if it were an explosion set off from afar with a plunger and a wire. If a lot of water is present in the sediment and the fill, they can be changed in an instant into gray quicksand---the effect known as liquefaction. Compared with what happens on bedrock, the damage can be something like a hundredfold, as it was on the lakefill of Mexico City in 1985, even though the hypocenter was far to the west, under the Pacific shore...

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