SHOULD buildings in seemingly stable Memphis be as resistant to earthquakes as those in wobbly California? The construction code proposed by the US Federal Emergency Management Agency (FEMA) says they should, but some experts warn that it would be a waste of money and could leave buildings at greater risk from tornadoes. The dilemma raises questions about the best way to deal with the risks posed by rare but severe quakes.
The proposed measures would raise the price of putting up new buildings by between 2 and 10 per cent. That could add up to $200 million a year to construction costs in Memphis, says Joe Tomasello, a structural engineer with the Reaves Firm in Memphis. But FEMA predicts that on average, building damage caused by earthquakes is likely to cost only around $17 million per year, a figure that would be halved by its proposed measures. “The code has been proposed with almost no consideration of the costs and benefits,” says Seth Stein, a seismologist at Northwestern University in Evanston, Illinois. “We’ve looked at the numbers and they don’t make economic sense.”
The guidelines that FEMA is pushing are part of the International Building Code, which determines the shake resistance needed for local buildings from a national map of seismic hazard zones compiled by the US Geological Survey (USGS). Most areas of the US are deemed to be at low risk of quakes, and are not greatly affected by this part of the code. But the New Madrid Fault Zone, a region along the Mississippi River that suffered several major quakes in 1811 and 1812, has been rated as having a similar earthquake risk to California.
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The area is quiet today compared with California, where frequent quakes occur on a network of faults formed as the Pacific Plate moves north-north-west at 46 millimetres per year relative to North America. The New Madrid zone is far from a continental boundary. Yet the USGS estimates the 1811 and 1812 quakes ranged from magnitude 7 to magnitude 8 on the Richter scale, comparable to the largest recorded quakes in California.
The New Madrid zone has far fewer smaller earthquakes than California, but evidence from local rock layers suggests that the area suffered other big quakes in around 900 and 1450 AD, implying that such quakes happen there around once every 500 years. That gives the city of Memphis, which sits right in the centre of the New Madrid zone, the same overall risk of severe shaking as if it sat on a major Californian fault, according to a FEMA spokesman.
But in a report published last week in Eos (vol 84, p 177), Stein and Tomasello, along with seismologist Andrew Newman of Los Alamos National Laboratory, claim that FEMA’s decision to base building codes on the risk of earthquakes over very long periods of time makes no sense. While both California and New Madrid may endure the same maximal level of shaking over a period of 2500 years or so, they argue that over a more realistic period of 500 years, the worst shaking Memphis is likely to experience will be less severe. The team concludes that buildings in Memphis are between 5 and 10 times less likely to be damaged during their lifetimes than buildings in San Francisco or Los Angeles.
Talking with 91av after returning from tornado-ravaged Jackson, Tennessee, Tomasello warned that as well as wasting millions of dollars, building to withstand earthquakes could increase vulnerability to tornadoes. For example, brick veneers help protect against tornado-force winds, which can hurl objects through lighter walls. Two-storey houses with brick veneers wouldn’t be permitted under the new code, he points out. Instead, builders would opt for more flexible, steel-framed buildings with light walls.
The FEMA spokesman agreed that heavy structures withstand high winds better, but points out that quakes shake all homes in an area, while only a tiny fraction suffer direct hits from a tornado.
