This past weekend in Seattle, rumblings that registered as magnitude 1 or 2 marked the first time on record that football fans have triggered seismic activity .
Tremors also can be spurred by drilling for oil , United States Geological Survey seismologists told Life's Little Mysteries last year, by causing rocks to shift into voids left behind by the extracted fluid.
Minor vibrations aside, the question remains: Can humans cause full-blown earthquakes? And, if so, how?
According to John Vidale, director of the Pacific Northwest Seismic Network, there are actually many ways that human activity can trigger earthquakes. "The main one is by filling large dams," he told Life's Little Mysteries. "The weight of the water that builds up behind a dam exerts a huge amount of stress on the land below."
Occasionally the land shifts under all that new weight. In the 1930s, for example, the construction of Hoover Dam in Arizona unleashed a burst of seismic activity in the vicinity that reached magnitude 5 on the Richter scale. And in 1963, a severe earthquake magnitude 7 shook the reservoir behind Koyna Dam in India shortly after its construction, killing 200 people in a nearby town.
Forcefully injecting fluid into the planet's crust also can induce earthquakes. For a three-year period in the 1960s, the government injected wastewater byproducts 12,000 feet deep into rock fractures in the Rocky Mountains in Colorado, Vidale said. "Then suddenly you got a whole slew of earthquakes near Denver, so they had to stop," he said.
Similarly, enhanced geothermal-energy projects have been known to make the ground shake. This process involves pumping pressurized water a mile into the Earth, then sucking up the heated liquid to make steam and drive turbines to generate electricity. But as folks in Basel, Switzerland, found out in 2006, sometimes the injection phase can trigger larger earthquakes, especially if the pumping is centered over an already geologically active area.
Extracting substances from the ground can upset the Earth's crust just as much as piping them into it, according to a 2000 report by a group of Russian seismologists. In the late 1970s and 1980s, natural-gas extraction near Gazli, Uzbekistan, caused a trio of severe earthquakes up to 7.3 in magnitude the biggest seismic events ever recorded in Central Asia.
Skyscrapers also can act as quake-makers. Five years ago, two earthquakes occurred in Taipei, Taiwan, shortly after the construction of what was then the tallest building in the world, Taipei 101. Taiwanese geologists suspected the 700,000-ton skyscraper reopened an ancient fault, triggering the tremors.
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