Scientists are studying whether two recent earthquakes in east Texas might have been caused by underground injection of oil and gas drilling waste.

"It's possible they were natural," said Cliff Frohlich, a senior research scientist at the University of Texas' Institute for Geophysics. "It's possible they were man-made."

The quakes were centered a little more than 25 miles northeast of Nacogdoches, near the Louisiana border, which is in the midst of the Haynesville Shale. Frohlich said there are waste disposal injection wells in the area.

Frohlich is studying the quake in partnership with a scientist from the geology department of Stephen F. Austin State University in Nacogdoches.

The first magnitude-3.9 earthquake was May 10. Then a second, much stronger magnitude-4.3 quake hit the same area Thursday.

Frohlich said he was studying the seismological reports on the first earthquake when the second one hit. He was not close enough to feel either.

Frohlich was part of a team of researchers who studied a series of small quakes that struck near Dallas in 2008 and 2009 and found that they were related to drilling waste disposal under the Dallas-Fort Worth International Airport. The largest quake measured magnitude 3.3.

State oil and gas officials in Arkansas and Ohio shut down disposal wells after they were tied to earthquakes that took place last year. The largest quake in the "swarm" of seismic activity in Arkansas was magnitude 4.7. The Ohio quakes culminated in a magnitude-4.0 earthquake near a disposal well in Youngstown in early January of this year.

U.S. Geological Survey scientists and academic seismologists have found a correlation between drilling waste disposal wells and larger earthquakes last year in Oklahoma and Colorado. The magnitude-5.3 Colorado quake occurred near the New Mexico border. The Oklahoma quake was larger, at magnitude 5.6 and closer to a metropolitan area. It injured two and destroyed 14 homes.

USGS says the Oklahoma and Colorado quakes are part of a "remarkable" rash of quakes taking place across the middle of the country that are "almost certainly man-made" (EnergyWire, March 29).

Oklahoma and Colorado officials have questioned the USGS findings and have not taken action against the disposal wells.