HOUSTON (Reuters) – An earthquake with a preliminary magnitude of 4.2 on Friday struck Stroud, Oklahoma, about 20 miles (32.2 km) from the key oil storage hub in Cushing, Oklahoma, the Oklahoma Geological Survey (OGS) said.

There were no reports of damage from the quake, Oklahoma Corporation Commission (OCC) spokesman Matt Skinner said by phone. The OCC and OGS were investigating Friday’s seismic activity, the OCC said in a statement.

A series of aftershocks shook the area in the hours following the earthquake, which occurred at 8:47 a.m. CDT (1347 GMT), including two quakes measuring at preliminary magnitudes of 3.5 and 3.7, according to the U.S. Geological Survey.

Early data from the USGS showed six seismic events in the area on Friday morning, the OCC said.

Oil companies operating the tanks and pipelines around the Cushing hub have reported no damage to their facilities. Phillips 66, Enterprise Products Partners, TransCanada Corp, Tallgrass Energy Partners LP, Enbridge Inc and Magellan Midstream Partners LP in separate statements said operations were not impacted by the quake.

The Cushing storage hub is the delivery hub for U.S. crude oil futures and houses some 57.6 million barrels of oil. It is sometimes referred to as the “Pipeline Crossroads of the World.”

Oklahoma has experienced a rash of earthquakes in the past few years following a surge of oil and gas production in the state. The quakes have been attributed to the injection of saltwater, a normal byproduct of oil and gas production, into disposal wells.

Friday’s earthquakes occurred within a part of the state identified as an “Area of Interest,” the OCC said, referring to a part of the Arbuckle formation where the agency has taken measures to reduce the disposal of wastewater into injection wells.

The agency said it is reviewing the operations of eight disposal wells within 10 miles of the preliminary epicenters of Friday’s quakes.

Steps taken by regulators to curb wastewater injection have diminished the frequency of large earthquakes in Oklahoma in the past year. Friday’s quake was only the third magnitude 4.0 or greater quake reported this year, versus 15 in 2016 and 27 in 2015.

Several larger quakes have occurred previously near the Cushing storage hub. In November 2016, a magnitude 5.0 earthquake impacted residents and businesses about two miles (3.2 km) west of Cushing.