[NEWS] Why Bigger Quake Sows Less Damage

Why Bigger Quake Sows Less Damage

Scientists Say Recent Temblors Are Unrelated; Underwater Topography May Explain Where Tsunami Wreaked Havoc
By GAUTAM NAIK

Three massive earthquakes have struck in less than two months, raising the questions: Are they related, and are we living in a time of more and bigger temblors?

The Chile earthquake, 8.8 on the Richter scale, was by far the largest. But a little noticed 7.0 quake struck near Japan's Ryukyu islands just a few hours earlier, triggering its own tsunami warnings.

The Jan. 12 temblor in Haiti was also 7.0, about 500 times less powerful than the Chile quake, though it appears to have killed many more people. That prompts yet another question: Why did a much larger quake cause much less destruction?

Scientists say the three recent earthquakes probably aren't related, mainly because they occurred at such great distances from one another.

To be sure, the back-to-back quakes in Japan and Chile both occurred along the notorious "ring of fire," a 25,000-mile zone of constant seismic and volcanic activity encircling the Pacific Ocean that accounts for 95% of the world's quakes. But the Chile and Japan quakes are separated by about 10,000 miles, so, seismologists say, they are probably unconnected.

Earthquakes are caused by friction between tectonic plates, which are essentially shards of the earth's crust. They slip-slide past each other, very slowly but inexorably. Sometimes they get stuck, then jerk forward again, producing a quake.

The Haiti quake was caused by different regional plates than the Chile and Ryukyu temblors.


The Chile earthquake occurred at the boundary between the Nazca and South American tectonic plates, according to the United States Geological Survey. The two plates are converging at a rate of 80 millimeters a year, with the Nazca plate moving down and eastward under the South American plate.

The last big earthquake near this point on the faultline occurred in 1835—when Charles Darwin was sailing nearby—and had an estimated 8.5 magnitude. Since then, the plates at this location have been trying to move past each other, but have been locked in place. Over the ensuing 175 years, the stresses and strains gradually built up.

Despite the spate of recent temblors, the frequency of quakes has not increased. Scores of quakes are recorded every day, most too tiny or too far from populated areas to cause significant damage. Since 1964, when a consistent global catalog of earthquakes was created, the annual number of earthquakes has been roughly constant. An average of 17 quakes of magnitude 7.0 or higher occur each year.

"These are geological processes whose reoccurrence rates occur over centuries, but we tend to look it only as a snapshot—over a few decades," says Harley Benz, scientist in charge of the United States Geological Survey's National Earthquake Information Center in Golden, Colo.

Quake magnitudes are calculated on a logarithmic scale. Thus, a magnitude 8 quake releases 33 times the energy as a magnitude 7 quake, and compared with a magnitude 7 event, a magnitude 9 quake releases 1,089—33 multiplied by 33—times as much energy. The quake that hit Chile was about 500 times more potent than the one that hit Haiti.

What makes modern quakes particularly devastating are megacities located near seismically active zones. "Earthquakes don't kill people, buildings kill people," says David Wald of the National Earthquake Information Center of the USGS.

That is especially true in poorer countries where many buildings are shoddily designed and constructed from weak materials, or they simply fail to meet basic building standards. That was the case in Haiti, where the death toll is estimated to exceed 220,000—much greater than what is thought to have occurred in Chile, even though the Haiti quake was far less powerful. Chile enforces strict building codes, partly because it has endured several major temblors, including the highest magnitude quake on record, 9.5, in 1960.

Chile's large cities were also farther away from the epicenter than was Port-au-Prince, and the Chile epicenter was about three times deeper: about 22 miles compared with about eight miles.

But the danger hasn't abated in Chile. Some 100 aftershocks were recorded in Chile of magnitude 5 or larger, according to the NEIC. The largest had a 6.9 magnitude. The Chile temblor's aftershock zone—the length of the affected faultline—stretches for 375 miles, far longer than the 37-mile aftershock zone in Haiti.

Some scientists are now trying to forecast where major aftershocks might occur. Ross Stein, a geophysicist at the USGS, studies a new area of earthquake science known as "Coulomb stress transfer." When a large quake strikes, it changes the stress field and sometimes puts extra pressure on the next segment of the fault line. In recent years, scientists have computed the transfer of stress along a fault line to predict regional earthquakes after a big temblor.


Dr. Ross's calculations suggest that two zones outside the main earthquake zone—one stretching 60 miles above its northern tip, the other stretching 60 miles below its southern tip—may be vulnerable to big aftershocks.

His analysis also suggests that the huge 9.5 magnitude quake of 1960 that occurred on the same fault line may have transferred some stress and helped bring about Saturday's quake.

The region has long been known for intense seismic activity. Charles Darwin, who studied geology, was on the Beagle in 1835 when an 8.5 magnitude temblor struck near the location of Saturday's quake.

When he went ashore, he noticed that the coastline had been forced up permanently by the quake. He speculated that the distant Andes could similarly have been created by a series of quakes over millennia—an observation that contributed to his insight that the earth was extremely old, a key component of his theory of evolution.
"It was one of the most spectacular insights" in geology, says Dr. Stein.
The Chile quake caused a massive uplift of the sea floor, triggering tsunami waves that race across the ocean at up to 550 miles per hour—the speed of a jet plane. When the waves approach a coast, they slow down to about 20 to 30 miles an hour. They are now at their most dangerous: As all their energy gets compressed into much less depth, the waves can dramatically increase in height.

The Chilean tsunami spared Hawaii, Japan, and almost every other location across the Pacific, but it did devastate some coastal towns in Chile, such as Pellehue, population 1,000. Why that town was hit while most other areas were spared is as yet unknown but may be due to the topography under water. Experts say irregular coastlines and those with sudden drop-offs and changes in water depth are the most likely to generate large, destructive tsunami waves.

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