The Oklahoma Geological Survey said continued analysis of Wednesday's earthquake data shows the source likely was natural and occurred on a previously unknown fault line.
The survey also reduced its rating of the earthquake to magnitude 4.7 from the latest Wednesday estimate of 5.1, research seismologist Austin Holland said. Officials had anticipated the reduction based on the stark difference between its rating and the magnitude 4.3 rating given by the U.S. Geological Survey.
Holland said dealing with numerous calls from government officials, the media and state residents also limited the amount of time seismologists had to review the data Wednesday.
The Oklahoma survey and the U.S. survey use different equipment at different locations to measure different data sets, resulting in numbers that can vary from each other and don't mean the same thing, Holland said. The measurement the U.S. survey used for its rating, moment magnitude (Mw), records the low-frequency energy released in the earthquake. The Oklahoma survey rating (mbLg) measures the amplitude of surface waves that people can feel.
"They both provide relative measures of the earthquake," Holland said. "They're just making different assumptions about the earth."
Holland said the depth of Wednesday's quake, about eight miles, points to a natural source.
"It's deep enough that I would be highly suspect that (the) oil and gas (industry) had anything to do with this earthquake," Holland said. "That being said, I'm continuing to examine this possibility. It will continue to be a part of my research for the foreseeable future."
The quake likely was along a "strike-slip" fault line unknown to scientists before Wednesday, Holland said. Tectonic plates underground move past each other laterally along strike-slip fault lines. The well-known San Andreas fault in California is a strike-slip fault.
Holland said there are countless faults underneath Oklahoma that scientists are not aware of until an earthquake occurs.
"We know it's not a major fault, or we would already know about it," Holland said. "Because we can't see into the earth, it's really hard to identify these things. ... We can use these (earthquake) recordings to learn something new about the subsurface."
No major damage or serious injuries were reported after Wednesday's quake.
Inspectors checked bridges Thursday within a 50-mile radius of the epicenter of the quake as a precaution, but experts said it was unlikely a quake of this size would cause major damage.