Another round of moderate earthquakes rattled an area of northwestern Oklahoma on Wednesday.
Two temblors shook the ground about 10:30 p.m. with magnitudes of 4.7 and 4.8. Several smaller quakes also were recorded.
At least 22 earthquakes, registering at magnitude-2.5 or more, were reported across the state over a 13-hour period Wednesday night into Thursday morning, most of which took place in Woods County, the US Geological Survey (USGS) said.
No injuries were reported, according to police in Fairview, Okla., where the quakes took place, about 100 miles outside of Oklahoma City. However, several residents did tell one local television station their homes incurred some damage.
Dale Grant, a USGS geophysicist, told The Associated Press the two larger quakes were likely felt in up to eight surrounding states.
According to the Oklahoma Geological Survey (OGS), the magnitude-4.8 quake was the fourth strongest on record in Oklahoma, a state that has seen a rash of earthquakes since it began allowing hydro fracturing.
In April, the OGS said most of the state's recent earthquakes were caused by the injection of wastewater from oil and natural gas drilling operations Earth, which has become increasingly common in recent years.
State seismologist Richard D. Andrews and Dr. Austen Holland said at the time the earthquakes "are very unlikely to represent a naturally occurring process” with data indicating the number of earthquakes were probably linked to drilling.
In 2013, Oklahoma experienced 109 earthquakes at more than magnitude-3. By 2014, that number increased fivefold to 585, while in 2015 the number of earthquakes further increased, the OGS said.
Geologists said before 2009, the state historically averaged 1.5 earthquakes of magnitude-3 or greater each year, and is now experiencing 2.5 earthquakes of the same magnitudes each day.
The current average rate of earthquakes is approximately 600 times historical averages, according to state government.
“While we understand that Oklahoma has historically experienced some level of seismicity, we know that the recent rise in earthquakes cannot be entirely attributed to natural causes,” the state says. “Seismologists have documented the relationship between wastewater disposal and triggered seismic activity. The Oklahoma Geological Survey has determined that the majority of recent earthquakes in central and north-central Oklahoma are very likely triggered by the injection of produced water in disposal wells.”
A spokesperson for Oklahoma’s oil and gas regulatory body told The Christian Science Monitor last year that officials are now conducting earthquake risk assessments for future disposal wells and is also restricting wells from operating in certain areas of the state.
The Oklahoma Oil and Gas Association, however, disputes the state's conclusions. “There may be a link between earthquakes and disposal wells, but we... still don’t know enough about how wastewater injection impacts Oklahoma’s underground faults,” said OKOGA President Chad Warmington, Reuters reported.
Last year, a report released by a group of seismologists, researchers, and oil and gas industry representatives overwhelming connected hydro fracturing to the surge in earthquakes.
The group said there are nearly 151,000 wells operating in the United States, while in central and eastern portion of the country earthquakes have dramatically increased since 2008, when fracking became more common.
The region averaged 21 earthquakes of magnitude-3 and larger each year from 1973 to 2008. But between 2009 and 2013 that number rose nearly 100 earthquakes per year of the same magnitude.
Variations in state drilling laws and geography make federal regulation difficult, according to one person involved in the report.
“A one-size-fits-all approach would not be an effective tool for state regulators,” the report said.