SAN FRANCISCO — SAN FRANCISCO -- There's "a whole lotta shakin' goin' on," as the song says -- and some of it may be our fault.
Humans have inadvertently triggered a small number of earthquakes in recent decades, most of them harmless, by drilling for oil and building reservoirs in geologically touchy spots, scientists say.
These "induced earthquakes" -- possibly of as great a magnitude as 5.5 -- may help explain how subterranean fluids influence and perhaps spark natural earthquakes, researchers said at the American Geophysical Union meeting that ended Friday in San Francisco.
True, induced quakes tend to be the lesser of the seismic world: One of the stronger ones managed "to knock a toilet-bowl cover off a toilet," said R. B. Horner of the Geological Survey of Canada.
He was referring to one of a series of induced shakes in northeastern British Columbia during the past decade. Those quakes -- as large as magnitude 4.3 -- began occurring in the 1980s, at about the same times and locations as oil and gas operations. Among other things, drillers injected water into the ground to force fuels to the surface.
Mr. Horner and other experts acknowledge that they haven't found rock-hard proof that humans triggered a specific quake. But they certainly have grounds for suspicion, especially because "no earthquakes were located in this area before 1984," he said.
The shaker was one of 12 quakes of magnitude 3.0 and higher in the area in the last two decades, said Scott D. Davis of the University of Memphis.
Scientists first blamed quakes on humans a few decades ago, when a curious swarm of quakes occurred in Colorado.
These were later blamed on a defense plant that had injected toxic wastewater into the ground.
The worst Colorado quake measured about 5.5 magnitude and caused about $1 million in damage, Mr. Davis said.
Why might mining cause quakes? For one thing, oil and gas mining changes the distribution of underground fluids and gases. In turn, this eases the pressure on some faults and makes them shift, perhaps violently enough to cause a minor quake. Pressure can also be altered by injecting water into the ground to restore pressure needed to force fuels to the surface.