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In November 2011, a magnitude-5.7

earthquake rattled Prague, Okla., and 16 other nearby states. It flattened 14 homes and many other buildings, injured two people, and set the record as the state's largest recorded earthquake. And according to a new study in the journal Geology, the event can also claim the title of Largest Earthquake That's Ever Been Induced by Fluid Injection."

In the paper, a team of geologists pinpoints the quake's starting point at less than 200 meters (about 650 feet) from an injection well where wastewater from oil drilling was being pumped into the ground at high pressures. At 5.7 magnitude, the Prague earthquake was about 10 times stronger than the previous record holder: a magnitude-4.8 Rocky Mountain Arsenal earthquake in Colorado in 1967, caused by the U.S. Army injecting a deep well with 148,000 gallons per day of fluid wastes from chemical-weapons testing. So how big can these man-made earthquakes get?

The short answer is that scientists don't really know yet, but it's possible that fluid injection could cause some big ones on very rare occasions. "We don't see any reason that there should be any upper limit for an earthquake that is induced," says Bill Ellsworth, a geophysicist with the U.S. Geological Survey, who wasn't involved in the new study.

As with natural earthquakes, most man-made earthquakes have been small to moderate in size, and most are felt only by seismometers. Larger quakes are orders of magnitude rarer than small quakes. For example, for every 1000 magnitude-1.0 earthquakes that occur, expect to see 100 magnitude-2.0s, 10 magnitude-3.0s, just 1 magnitude-4.0, and so on. And just as with natural earthquakes, the strength of the induced earthquake depends on the size of the nearby fault and the amount of stress acting on it. Some faults just don't have the capacity to cause big earthquakes, whether natural or induced.

How do Humans Trigger Earthquakes?

Faults have two major kinds of stressors: shear stress, which makes two plates slide past each other along the fault line, and normal stress, which pushes the two plates together. Usually the normal stress keeps the fault from moving sideways. But when a fluid is injected into the ground, as in Prague, that can reduce the normal stress and make it easier for the fault to slip sideways. It's as if if you have a tall stack of books on a table, Ellsworth says: If you take half the books away, it's easier to slide the stack across the table.

"Water increases the fluid pressure in pores of rocks, which acts against the pressure across the fault," says Geoffrey Abers, a Columbia University geologist and one of the new study's authors. "By increasing the fluid pressure, you're decreasing the strength of the fault."

A similar mechanism may be behind earthquakes induced by large water reservoirs. In those instances, the artificial lake behind a dam causes water to seep into the pore spaces in the ground. In 1967, India's Koyna Dam caused a 6.5 earthquake that killed 177 people, injured more than 2000, and left 50,000 homeless. Unprecedented seasonal fluctuations in water level behind a dam in Oroville, Calif., are believed to be behind the magnitude-6.1 earthquake that occurred there in 1975.

Extracting a fluid from the ground can also contribute to triggering a quake. "Think about filling a balloon with water and burying it at the beach," Ellsworth says. "If you let the water out, the sand will collapse inward." Similarly, when humans remove large amounts of oil and natural gas from the ground, it can put additional stress on a fault line. "In this case it may be the shear stresses that are being increased, rather than normal stresses," Ellsworth says.

Take the example of the Gazli gas field in Uzbekistan, thought to be located in a seismically inactive area when drilling began in 1962. As drillers removed the natural gas, the pressure in the gas field dropped from 1030 psi in 1962 to 515 psi in 1976, then down to 218 psi in 1985. Meanwhile, three large magnitude-7.0 earthquakes struck: two in 1976 and one in 1984. Each quake had an epicenter within 12 miles of Gazli and caused a surface uplift of some 31 inches. Because the quakes occurred in Soviet-era Uzbekistan, information about the exact locations, magnitudes, and causes are not available. However, a report by the National Research Council concludes that "observations of crustal uplift and the proximity of these large earthquakes to the Gazli gas field in a previously seismically quiet region strongly suggest that they were induced by hydrocarbon extraction." Extraction of oil is believed to have caused at least three big earthquakes in California, with magnitudes of 5.9, 6.1, and 6.5.

Some people worry that hydraulic fracturing, or fracking—wherein high-pressure fluids are used to crack through rock layers to extract oil and natural gas—will lead to an increased risk of earthquakes. However, the National Research Council report points out that there are tens of thousands of hydrofracking wells in existence today, and there has only been one case in which a "felt" tremor was linked to fracking. That was a 2.3 earthquake in Blackpool, England, in 2011, which didn't cause any significant damage. Although scientists have known since the 1920s that humans trigger earthquakes, experts caution that it's not always easy to determine whether a specific event was induced.

Are Human Activities Making Quakes More Common?

Human activities have been linked to increased earthquake frequencies in certain areas. For instance, researchers have shown a strong correlation between the volume of fluid injected into the Rocky Mountain Arsenal well and the frequency of earthquakes in that area.

Geothermal-energy sites can also induce many earthquakes, possibly due to pressure, heat, and volume changes. The Geysers in California is the largest geothermal field in the U.S., generating 725 megawatts of electricity using steam from deep within the earth. Before The Geysers began operating in 1960, seismic activity was low in the area. Now the area experiences hundreds of earthquakes per year. Researchers have found correlations between the volume of steam production and the number of earthquakes in the region. In addition, as the area of the steam wells increased over the years, so did the spatial distribution of earthquakes.

Whether or not human activity is increasing the magnitude of earthquakes, however, is more of a gray area. When it comes to injection wells, evidence suggests that earthquake magnitudes rise along with the volume of injected wastewater, and possibly injection pressure and rate of injection as well, according to a statement from the Department of Interior.

The vast majority of earthquakes caused by The Geysers are considered to be microseismic events—too small for humans to feel. However, researchers from Lawrence Berkeley National Laboratory note that magnitude-4.0 earthquakes, which can cause minor damage, seem to be increasing in frequency.

The new study says that though earthquakes with a magnitude of 5.0 or greater are rare east of the Rockies, scientists have observed an 11-fold increase between 2008 and 2011, compared with 1976 through 2007. But the increase hasn't been tied to human activity. "We do not really know what is causing this increase, but it is remarkable," Abers says. "It is reasonable that at least some may be natural."

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