Still, scientists would like to know whether human activity can trigger a larger event. The National Academy of Sciences is studying the seismic effects of energy drilling and mining and will issue a report next spring.
“This is an area of active research,” said Rowena Lohman, a Cornell University seismologist.
Hydraulic fracturing has been practiced for decades but has grown rapidly in recent years as drillers have learned to combine it with horizontal drilling to tap enormous reserves of natural gas and oil in the United States.
About 5 million gallons of fluid are used to fracture a typical well. That's typically not nearly enough weight and pressure to cause more than a tiny tremor.