Killer Quakes on Rise With Cities on Fault Lines: Roger Bilham
Is the world falling apart? Surely it isn’t, though this year’s frequent earthquakes and the occasional volcano have left an exceptional path of ruin and misery, raising concerns that something very unusual is occurring.
Three strong earthquakes hit the Solomon Islands in early January, followed only weeks later by the 7.0 quake that leveled part of Haiti, leaving more than 1 million people homeless and killing more than 200,000. Korea and Japan were rocked. Then Chile was hit by an 8.8 earthquake, one of the biggest ever measured. More seismic events followed in Japan, Mexico, Sumatra and most recently western China.
Violent temblors across the globe during the past decade killed about 650,000 people -- more than any decade in history.
What could explain this apparent burst of seismic energy? And is worse to come?
The Earth, in fact, is behaving quite normally. A “major” earthquake is one measuring greater than magnitude 7.0. They release enough energy to produce considerable shaking. On average the world has been shaken by 16 major earthquakes annually since 1900. Six major quakes have occurred in the past four months. In other words, if the trend continues for the rest of the year, we will be only slightly above the average. In some years, such as 1986 and 1989, we have recorded only six major quakes while 32 of them occurred in 1943.
People may also believe this year’s rash of quakes is unusually severe, as indeed they are. That’s because they have all shaken large populations. Seismologists call these “significant” earthquakes. They range in size from those with trivial magnitudes of 4.0 that wake a million people at night in Illinois or Oklahoma, to those that break records for their savage destruction of cities, or for the long reach of their tsunami.
This year’s combined death toll from these natural disasters is almost 250,000 people, and we are only in April. Quakes that killed as many people as the one in Haiti are rare. A quake in China in 1556 killed 834,000 while another one there in 1976 left 255,000 dead. The Sumatra earthquake and tsunami of 2004 killed 228,000 people.
The difference between a major earthquake and a significant one is whether it occurs near a population center. Seismic events that people feel are newsworthy, those that shake fish or cows are not. Those that collapse cities are especially destructive in lives and rebuilding costs.
Regrettably, there is little doubt the number of significant earthquakes will rise. This is because the world’s major earthquakes repeat over and over again at plate boundaries, where a large portion of the planet’s population lives. The plates are driven rather smoothly by radioactively produced heat deep in the Earth’s mantle, but when these plates grind past each other the result is spasmodic and violent.
The time between a major earthquake and its successor on your local neighborhood plate boundary varies from several decades to a few hundred years. But the world’s population has increased by a factor of 10 in the past 200 years. Most people now live in cities, and more than half the world’s largest cities are located on plate boundaries.
This is the big problem. Earthquakes that occurred 200 years ago shook villages that are now vast urban agglomerations. Take Los Angeles, for example. The Big One, a 7.9 earthquake that occurred in 1857 shook a rural population of about a thousand village dwellers. Its recurrence (pretty soon) will shake a population of 8 million, and half again as many in adjoining areas.
Not only have populations grown, the vulnerability of their dwellings has also increased. Concrete and steel multistory blocks have replaced wooden single-story constructions, and the quality of these new structures in developing nations is often appallingly poor.
Do earthquakes signal to each other, encouraging their neighbors to join in? Yes, sequences of earthquakes at plate boundaries do indeed occur. The 9.1 Indonesian earthquake of 2004 was the start of a great unzipping of the entire plate boundary from Myanmar to Bali and beyond that is still ongoing. A similar progression of major earthquakes was seen in northern Turkey between 1939 and 1999. The next one, if it follows the established pattern, will occur alarmingly close to Istanbul, where about 13 million people live.
In the past 500 years, the worst earthquakes have killed more than 12 percent of an urban population. Until 1950 no population center exceeded 8 million. In 2007 there were 17 cities with at least this many people, and more are added to the list each year as cities grow.
Fire and Ice
Never before has it been possible to kill 1 million people in a single earthquake, but cities are now big enough to make this possible. Prominent among them are Los Angeles and Tokyo, which are well aware of their seismic future and have made preparations, whereas Istanbul and Tehran, where building codes are known to be unevenly applied, are much more vulnerable.
What about volcanoes, and why is Iceland now splitting apart? Previous eruptions there occurred in 1600 and 1820, so today’s eruption 190 years later is almost on schedule. But whereas Iceland’s volcanic eruption was not triggered by any nearby major earthquake, the eruption of one or more dormant volcanoes in the months and years following major earthquakes is quite common. It happened after the 1960 Chilean earthquake and will no doubt occur after this year’s disaster. Scientists attribute many eruptions to the violent shaking of volcanic subsurface magma chambers and the subsequent opening of conduits to the surface.
So the statistics of earthquakes and volcanoes shows the Earth to be going about her business as usual. The only difference is that its occupants have become recently aware of how busy the Earth actually is.
(Roger Bilham is a seismologist at the University of Colorado. The opinions expressed are his own.)
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