EARTHQUAKES:

Two tremors hit east Texas drilling area

Two strong but not damaging earthquakes shook east Texas on Monday, in an area where scientists are researching whether oil and gas activities cause earthquakes.

The U.S. Geological Survey reported two earthquakes Monday near Timpson, Texas. The first was magnitude 4.1 and the second was magnitude 4.3. The Associated Press reported that the Shelby County Sheriff's Office said there were no reports of damage.

Timpson is near the Haynesville Shale natural gas play. State records show that Shelby County has more than 20 injection wells. They've injected as much as 4.4 million barrels (185 million gallons) of drilling wastewater per month in recent years. But the amount started falling steeply in December 2012.

Wastewater injection has been linked to numerous earthquakes in the middle of the country during the past few years as shale drilling has intensified across the country.

After an earthquake in May 2012, seismologists from the University of Texas, Austin, and Stephen F. Austin State University started studying whether it and other rumblings could be linked to drilling and injection. At magnitude 4.8, the 2012 quake was the largest quake ever recorded in east Texas.

Their findings, reported earlier this year at a conference of the Seismological Society of America, were that "it is possible the Timpson earthquakes were induced by injection."

But the scientists also noted that a smaller earthquake associated with a fault system was reported in 1981 about 15 miles away.

Timpson also had a magnitude-4.1 earthquake on Jan. 25 of this year. There has been at least nine other quakes in the area since the beginning of 2011. Scientists have also tied drilling and injection to small quakes around Dallas. The Texas Railroad Commission, which regulates oil and gas production, is working on a large-scale revision of its rules but opted not to look at man-made earthquakes.

Scientists have known for years that deep-underground injection of industrial wastewater, whether from oil and gas or other activities, can cause earthquakes.

USGS has reported a "remarkable" rash of quakes taking place across the middle of the country that are "almost certainly man-made" and likely linked to injection of drilling wastewater (EnergyWire, March 29, 2012).

The strongest was a damaging magnitude-5.7 quake east of Oklahoma City in November 2011. It injured two and destroyed 14 homes. Scientists have also linked injection to quakes in Arkansas, Colorado and Ohio.

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