For decades, scientists have known that activities like mining, drilling and building dams can actually create earthquakes. As early as the 1960s, observers noted that deep-earth gold mining changed the stresses in rocks and caused earthquakes. Above, miners drill into the rock at the Sub Nigel East Gold Mine in Johannesburg in 1961, more than 6,000 feet below ground.
Small earthquakes in Ohio and Arkansas associated with hydraulic fracturing for gas have taken many people by surprise. Gas industry executives say there's no hard evidence that their activities are causing these quakes. But some scientists say it's certainly possible; in fact, people have been causing quakes for years.
In the 1960s, geologists realized that gold mines in South Africa had created small earthquakes. Caverns dug into the earth thousands of feet below the surface collapsed. The "pancake" effect caused quakes, in one case a magnitude 5.2 temblor.