Oil & gas drilling linked to earthquakes

New York: The recent spike of earthquakes in parts of Oklahoma in the US coincides with dramatic increase in the disposal of salty wastewater into the Arbuckle formation - a 7,000-foot-deep, sedimentary formation under Oklahoma - a study says. The researchers showed that the primary source of the quake-triggering wastewater is not so-called "flow back water" generated after hydraulic fracturing operations.

Rather, the culprit is "produced water" - brackish water that naturally coexists with oil and gas within the Earth. Companies separate produced water from extracted oil and gas and typically re-inject it into deeper disposal wells. Before 2008, Oklahoma experienced one or two magnitude four earthquakes per decade, but in 2014 alone, the state experienced 24 such seismic events.

The Earth's crust contains many pre-existing faults, some of which are geologically active today. Shear stress builds up slowly on these faults over the course of geologic time, until it finally overcomes the frictional strength that keeps the two sides of a fault clamped together. When this happens, the fault slips and energy is released as an earthquake, the researchers said. Even if companies opt to use producing formations to store wastewater, however, the quakes would not cease immediately.