Under the world's greatest cities, deadly plates
Tuesday, February 23, 2010
Megacities are something new on the planet. Earthquakes are something very old. The two are a lethal combination, as seen in the recent tragedy in Port-au-Prince, where more than 200,000 people perished -- a catastrophe that scientists say is certain to be repeated somewhere, and probably soon, with death tolls that once again stagger the mind.
In 1800, there was just one city with more than a million people -- Beijing. Now there are 381 urban areas with at least 1 million inhabitants. Urbanization crossed a threshold last year when, for the first time, more people lived in city settings than rural ones. About 403 million people live in cities that face significant seismic hazard, according to a recent study by seismologist Roger Bilham of the University of Colorado.
The next Big One could strike Tokyo, Istanbul, Tehran, Mexico City, New Delhi, Kathmandu or the two metropolises near California's San Andreas Fault, Los Angeles and San Francisco. Or it could devastate Dhaka, Jakarta, Karachi, Manila, Cairo, Osaka, Lima or Bogota. The list goes on and on.
"You can name about 25 cities that are like Port-au-Prince. They're not going to shake but every 250 years [on average]. But if you can name 25 of them, you're going to have an event like this every 10 years," said David Wald, a seismologist with the U.S. Geological Survey.
In many vulnerable cities, people are effectively stacked on top of one another in buildings designed as if earthquakes don't happen. It is not the tremor that kills people in an earthquake but the buildings, routinely constructed on the cheap, using faulty designs and, in some cities, overseen by corrupt inspectors. The difference between life and death is often a matter of how much sand went into the concrete or how much steel into a supporting column. Earthquakes might be viewed as acts of God, but their lethality is often a function of masonry.
"In recent earthquakes, buildings have acted as weapons of mass destruction," Bilham writes in the journal Nature.
Difficult to predict
For years, earthquake scientists have shouted their warnings about the strong likelihood that a major quake would level an impoverished city and kill hundreds of thousands of people. They have said, for example, that Kathmandu, where masonry structures expand so haphazardly that some eventually cantilever over narrow city streets, is every bit as vulnerable as the surrounding Himalayas are majestic. They have said that a million people could die in a major quake in Tehran.
What's impossible, however, is knowing precisely which of these cities will be the next to crumble. Or when. For all practical purposes, scientists can't predict earthquakes.
The theory of plate tectonics, largely developed since the 1960s, explains why earthquakes happen in general. The major plates of the earth's crust move constantly, creeping along at about the speed of fingernail growth. They rarely move smoothly past one another but are usually locked in place. On a strike-slip fault of the type that ruptured in Haiti, strain builds on the fault line for decades or centuries. The fault in Haiti had not ruptured in 240 years. An earthquake is a sudden, stress-relieving event. The fault is said to "break."
Scientists can map faults and estimate how much strain has accumulated since the last quake. What they can't do is say that a given fault will break tomorrow or next year or 10 years from now. Any calculation of earthquake probabilities has a lot of slop in the numbers.
"The problem is, the slop is huge on a human time scale," said Susan Hough, a seismologist with the U.S. Geological Survey. "We're wired to deal with the immediate. We're not geared to plan and stress about things likely to happen in 30 years."
Some large earthquakes have small precursors, called foreshocks, but others happen without warning. There is one famous case of earthquake prediction, in Haicheng, China, in 1975. A local official sounded the alarm after many foreshocks and reports of snakes emerging from hibernation. But that prediction was more akin to a hunch than a scientific argument. There have been countless, less publicized instances when predicted earthquakes did not materialize.