Too Many People Living on Fault Lines - Our
packed planet equals deadly quakes
The death and devastation caused by major earthquakes around the world can only worsen in the years to come, as growing urban development and unprecedented population growth compound the lethal effects of natural seismic hazards, experts said yesterday.
Big earthquakes strike with the regularity of an alarm bell -- with about 18 earthquakes of magnitude 7.0 or greater every year on average, and four or five above the very dangerous 7.6 level. So the recent rash of destructive temblors in Turkey, Greece and now Taiwan does not signify any increase in quake activity.
What has changed, however, is that more and more people are living near faults. With the global population estimated to pass the 6 billion mark this year, there are fewer unpopulated places for quakes to strike. With ever more people to accommodate, there is more multistory construction in vulnerable fault zones as well.
As a result, destructive earthquakes such as those of the last several weeks ``are the wave of the future,'' said seismic expert Kerry Sieh of the California Institute of Technology. ``There are 40 cities of a million or more people within 100 kilometers of a major plate boundary, and all those are good candidates for a large event. Our exposure to the hazard is increasing.''
Moreover, some experts suggest that in recent decades the world has actually experienced a lull in the most severe earthquakes -- those of magnitude 8.0 or greater. If so, even more destructive temblors are to be expected when the lull eventually ends.
A predawn quake in Taiwan was caused by the inexorable crush of two major tectonic plates that squeeze the island from the east and west at the relatively rapid rate of several centimeters a year, building up seismic energy like the tension in a coiled spring.
The disaster was the most recent in a series of damaging urban earthquakes in just over a decade.
Devastating tremors killed at least 16,000 people during a 7.4 earthquake in Turkey in August. At least 122 people died during a 5.8 temblor in Athens, Greece, several weeks later. More than 6,400 people died in a 1995 quake in Kobe, Japan. The 1994 Northridge earthquake in Los Angeles and the 1987 Loma Prieta temblor near San Francisco were among the most costly natural disasters in U.S. history.
The destruction caused by any single earthquake is unpredictable.
Yesterday's Taiwanese quake, at 7.6, was roughly twice as powerful as the 7.4 quake that racked Turkey last month. But the death toll may be only one-tenth as high, in large part because construction codes in Taiwan were more strictly enforced than in Turkey, several experts said.
Other factors can also make a huge difference.
Had the epicenter of the Northridge earthquake been located a few miles south, more directly under the downtown area, or had it occurred during the day, the death toll might have reached the thousands, rather than the dozens, with damages totaling $100 billion or more, several seismic hazard experts say.
In the same vein, government officials in Taiwan said yesterday's temblor could have been even more deadly had its timing and location been slightly different.
``We roll the dice every time,'' said earthquake hazard analyst Charles Kircher.
But as urban boundaries expand to encompass growing populations, those dice are being weighted for disaster. ``We get closer to known faults and put more people on top of the faults,'' Kircher said.
``There has been a fourfold increase in the world's population since the 1906 San Francisco quake, and, if you look at the numbers, most of the million people who have died this century in earthquakes have died in poorly built urban areas.''
While better construction can clearly save many lives, some experts worry that quake-specific engineering solutions will foster the belief that it is safe to build in areas with large quake hazards -- thereby making the long-term hazard worse.
``We think technology can make us totally safe, independent of the planet we live on,'' said Dennis Mileti, a sociologist who directs the University of Colorado's Natural Hazards Center in Boulder and lead author of a recent government analysis of natural disaster risks. ``The problem with that approach is that there is always an event in nature that exceeds what we designed for.''
(Robert Lee Hotz, Los Angeles Times - Wednesday,September 22, 1999 )
Earthquake odds and ends -
Earthquakes of magnitude 7.0 or greater have remained fairly constant throughout this century and according to records have actually seemed to decrease in recent years. In the last 20 years more earthquakes are noticed yearly because of the increase in the number of seismograph stations in the world and improved global communications. This increase has helped seismological centers to locate many small earthquakes which were undetected in earlier years. The National Earthquake Information Center now locates about 12,000 to 14,000 earthquakes worldwide each year, or 35 a day on average. They kill about 10,000 around the world each year. Major earthquakes above seven on the Richter scale hit on average 18-20 times a year since 1900. The year of the least large quakes was 1986 with 6. The year of the most large quakes was 1943 with 41. There have been 13 so far this year as of November 12,1999. About 1,200 Moderate quakes occur annually worldwide. Through August of this year (1999), fewer than 600 such quakes had been recorded
Frequency of Earthquakes Worldwide Annual Average : 1 that is 8.0 or higher 18 that are 7 - 7.9 120 that are 6 - 6.9 800 that are 5 - 5.9 6200 that are 4 - 4.9 49,000 that are 3 - 3.9 and 9000 a DAY that are 1 - 3. ( Source: Infoplease, National Earthquake Information Center, U.S. Geological Survey.
for netscape chart
Deep quakes usually have no aftershocks, and, if any at all, they're very small.
Deep quakes are harder to measure at close range.
Within about six miles of a fault, there is a huge pulse of movement rather than a long series of wiggles. Though it's brief, that pulse can be devastating. One of the first recordings taken close to a big quake was at Loma Prieta. Later, seismograms from the Northridge and Kobe earthquakes showed the ground near the fault moving at more than three feet per second.
Quakes turn out to be more capricious than scientists had thought. They can leap from one fault to another, vaulting three-mile gaps and ignoring geographical barriers that were once thought to confine them and limit their size. Even more astonishing, their influence can extend hundreds of miles: The 1992 Landers quake in the Mojave Desert triggered small tremors all the way from the eastern flank of the Sierra Nevada to Yellowstone National Park, some 800 miles away.
In the United States, the places most likely to be struck are in Alaska, along the Aleutian fault; Southern California, along the San Andreas fault; on the New Madrid fault, which lies along the Missouri-Illinois border; on the coasts of Northern California, Oregon and Washington, along the so-called "Cascadia Subduction"; and along the Intermountain seismic belt near Southern Nevada, Utah, Wyoming and Montana.
