Growing world urban populations threatened by massive earthquakes

A new study by a University of Colorado at Boulder geological sciences professor suggests one earthquake causing up to 1 million fatalities on Earth each century could occur unless more earthquake-resistant construction materials are implemented.

Professor Roger Bilham’s conclusions are based on a study of the world’s urban population growth in the 21st century, including the number of rapidly expanding “supercities” and their locations close to major fault lines that have caused past temblors.

Bilham will present a paper on the subject at the 2003 Seismological Society Meeting to be held April 30 to May 2 in San Juan, Puerto Rico.

In the pre-1600s few city populations exceeded 1 million people, he said. By 1950, there were 43 “supercities” with populations from 2 million to more than 15 million. “Today, there are nearly 200 supercities on Earth, and the number could double before world populations stabilize.” Roughly 8 million people have died globally as a result of building collapses during earthquakes in the past 1,000 years, he said, although the record is sketchy prior to 1600. “But it is clear that a four-fold increase in the annual death toll from earthquakes between the 17th and 20th centuries is linked to increased urbanization.”

The emergence of “supercities” has increased tenfold since 1700, he said. Of these, more than 40 are located within 120 miles of a major plate boundary or a historically damaging earthquake, including Jakarta, Indonesia, Tehran, Iran, and Mexico City. Mexico City now has a population of roughly 16 million.

“Fifty percent of the world’s supercities now are located near potential future magnitude 7.5 earthquakes,” said Bilham, who is a fellow of the Cooperative Institute for Research in Environmental Sciences based at CU-Boulder.

“A 1997 statistical analysis predicted a future fatality rate of 8,000 deaths per year,” discounting extreme events with fatality counts exceeding 30,000,” said Bilham. “However, such predictions have no utility in extreme events in the 21st century, when urban populations are expected to reach 6 billion worldwide.”

From 1998 to 2002, earthquakes have caused roughly 10,500 fatalities per year. Bilham said a “fractal distribution of fatality count versus the number of fatal earthquakes permits a grim glimpse of possible future earthquake disasters based on past events.”

There is an apparent link between the earthquake fatality count for each earthquake and the number of “fatal earthquakes,” he said. “Fewer people are killed in small earthquakes that happen frequently. Large fatality-count earthquakes occur infrequently,” he said. By averaging these data over a century it is possible to derive a relationship to predict rare, extreme events.”

According to Bilham’s calculations, each year one earthquake event kills 100 people, every two years one event kills 1,000 people, every five years a single event kills 10,000 and for every century, an earthquake kills 300,000.

“But the 300,000 people is probably an underestimate because the size of these huge cites doubled in the last century and are expected to double again in the next century,” he said. “We have never had such a devastating megaquake before, because it simply wasn’t possible. But now we have many more target cities and they are bigger than ever before.”

One of the questions now being asked is whether a 100-year “megaquake” is now possible? Nearly 100,000 people were killed by earthquakes in India in 2000, and recent research by Bilham and his colleagues indicate that at least one and perhaps as many as seven 8.1 magnitude to 8.3 magnitude earthquakes are overdue in the Himalayas facing northern India.

“A great earthquake could occur in a dozen other countries if it occurs near a supercity,” Bilham said.

Bilham believes there is room for optimism, though he characterizes the present era as the age of construction when 3 billion new dwellings presently are being planned for a future doubling of the world population. “We are in a remarkably good position to make these new buildings safe to live in,” he said.

“Earthquakes don’t kill people, but buildings and builders of inferior buildings do.”