Hilcorp Energy Co. has stopped stimulating two horizontal shale wells in western Pennsylvania as state regulators investigate what caused a small earthquake on Monday near the company’s operations, a Department of Environmental Protection (DEP) spokesperson said Wednesday.

The state’s seismic monitoring network detected an earthquake in Lawrence County’s Mahoning Township early Monday, said DEP spokeswoman Melanie Williams. According to the U.S. Geological Survey, the earthquake was a 1.9-magnitude. Williams said no link has been made between the tremor and Hilcorp’s operations, but she said the agency, in conjunction with the Pennsylvania Department of Conservation and Natural Resources (DCNR), is continuing to investigate the cause.

The earthquake’s epicenter was in Mahoning, about 60 miles northwest of Pittsburgh, where Hilcorp has a four-well pad being operated by subsidiary North Beaver NC Development, Williams said. Two of the wells on that pad were fractured (fracked) on March 30 and have since been completed. Williams said the other two wells were being stimulated and were almost complete when the earthquake was recorded.

At about noon EDT on Monday, she said, Hilcorp “stopped fracking operations and demobilized the same day from that location.” Hilcorp was not available for comment on Wednesday.

The incident comes about two years after a 3.0-magnitude earthquake was recorded in Poland Township, OH, six miles west of Mahoning (see Shale Daily, March 11, 2014). After a month of investigation, the Ohio Department of Natural Resources (ODNR) concluded that sand and water injected into one of six wells on another Hilcorp pad increased pressure on an unknown micro-fault in the area, triggering the earthquake and a series of smaller ones in the days after.

ODNR and Hilcorp halted stimulation at the Poland site during the investigation. Privately owned Hilcorp is one of the most active operators in Northeast Ohio and Northwest Pennsylvania, where it has both Marcellus and Utica shale wells. It has been issued more than 150 unconventional drilling permits in Lawrence County alone, according to state records.

Last year, the DEP and the DCNR stepped up efforts to monitor for naturally occurring and human-induced earthquakes to improve the safety of oil and gas permitting decisions and geologic mapping (see Shale Daily, Sept. 29, 2015). The agencies contributed about $531,000 to maintain a real-time network of 30 seismic monitoring stations in conjunction with Pennsylvania State University, which had already been monitoring seismic activity in the state for years.

Lawrence County borders Mahoning County, OH, where the Poland earthquake was recorded two years ago. In 2011, about 12 miles to the west of Mahoning in Youngstown, OH, a 4.0-magnitude earthquake rattled the city. That tremor was later linked to an underground oil and gas wastewater injection well in the city (see Shale Daily, Jan. 4, 2012).

After ODNR determined that Hilcorp’s operations in Poland caused the 3.0-magnitude earthquake, it prompted regulators to issue an unprecedented set of permitting conditions for fracking horizontal wells near fault lines or areas of previous seismic activity (see Shale Daily, April 11, 2014).