Are there more earthquakes than there used to be?

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Jan. 14 2010 3:33 PM

Are There More Big Earthquakes Than There Used To Be?

How human activity affects the tectonic plates.

Read more about the Haiti earthquake in Slate.

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Earthquake rescue efforts in Haiti

A 7.0-magnitude earthquake struck Haiti on Tuesday, three days after a 6.5-magnitude quake rocked the extreme north of California. In total, there have been 270 earthquakes worldwide in the last week. Can humans affect the frequency of earthquakes, just like we've affected the global climate?

Not significantly. While certain activities, like mining, oil extraction, and dam-building can trigger an earthquake by changing the weight on tectonic plates or lubricating the joints between them, there is no evidence that humans have caused a quake of greater than 5.3 in magnitude. Major earthquakes like the one in Haiti are natural and unstoppable events. If the Earth's plates are going to shift and cause one of these disasters, there is nothing we can do.

The frequency of major earthquakes has remained fairly constant throughout recorded history. Since 1900, there have been approximately 18 earthquakes of 7.0 or greater magnitude per year. (One usually crosses the 8.0 barrier.) There are no data suggesting an upward trend in that rate. Seismologists are detecting more and more smaller quakes, but that phenomenon can be attributed to the quality and quantity of detection devices: Since 1931, the number of seismological stations worldwide has increased from 350 to more than 8,000.

Because earthquakes occur deep beneath the Earth's surface, most of the human activity capable of causing earthquakes involves digging or drilling. For example, oil companies sometimes inject water into their wells to force out the black gold under pressure. The water, pumped to a depth of thousands of feet, can decrease the friction holding two plates in place, causing a slip and a minor tremor.

The largest earthquake known to be triggered by people occurred four decades ago in Colorado. In 1962, the U.S. Army drilled a 12,000-foot-deep well to dispose of the waste from a chemical and conventional weapons factory in Commerce City. Over four years, the reservoir was filled with 165 million gallons of contaminated liquids. The Army stopped pumping when it realized that the process had caused a series of small-to-moderate tremors. One year later, a 5.3-magnitude quake caused more than $1 million in damage to the Denver metropolitan area.

As water accumulated behind Hoover Dam in the late 1930s, hundreds of small earthquakes shook the area. Some suspect that the 7.5-magnitude Koyna earthquake in India in 1967 was caused by a nearby reservoir, but most seismologists reject this theory. Many scientists now fear that the Three Gorges Dam in China poses a serious seismic risk. If the dam causes an earthquake that fractures the retaining wall, flooding would likely affect the millions of people living downstream.

Nuclear detonations are extremely unlikely to cause major earthquakes, contrary to Hollywood depictions, but subterranean tests can register small amounts of seismic activity. A 1968 nuclear test with the code name "Faultless" did manage to open up a new fault in the Nevada desert. The incident moved seismographic needles, but that was mostly attributable to the nuclear explosion itself rather than to any resulting tectonic activity.

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Brian Palmer is Slate's chief explainer. He also writes How and Why and Ecologic for the Washington Post. Email him at explainerbrian@gmail.com. Follow him on Twitter.