While Enid residents feel a number of earthquakes, very few temblors actually have been centered within the city limits.
“Every map I’ve seen ... there is this sort of belt where there don’t seem to be that many earthquakes,” Oklahoma Geological Survey Director Jeremy Boak said. “Woods, Alfalfa and Grant (counties) are dotted with earthquakes, and then you come across and Garfield (County) has some in the southeast corner and some in the northwest corner, and there’s this sort of gap that goes up through there.
“It was referred to as the Enid Gap to me when I first got here, so that’s what I call it.”
The gap may be related to the Nemaha fault, or there could be a shortage of oil plays that are driving wastewater injection — which ultimately drives the earthquakes, Boak said.
“No one’s offered me an explanation for why it’s there,” he said.
There are clusters of earthquakes in two main areas of the state. One cluster is in Grant, Alfalfa and Woods counties, while the other is in Payne, Logan, Oklahoma, Lincoln, Noble and one or two other counties, Boak said.
“It’s all, overall, about 10 or 12 counties that have 90 percent of the earthquakes, I think, something like that,” he said. “The gap that we’re talking about is in between them. It seems to be a zone of less seismicity, and I haven’t sat down with people and said, ‘all right, let’s provide an explanation for this. What do we know about production, about structure, etc., to say why this is happening?’”
In 2015, there were fewer than five magnitude 3.0 or greater quakes centered in Enid, U.S. Geological Survey records indicate. There were 80 magnitude 3.0 or greater earthquakes in Garfield County in all of 2015.
USGS Geophysicist George Choy said there is indeed a zone centered about Enid that separates two large populations of earthquakes.
“Although we have all noticed the gap, I don’t know of any publication that has actually used the name ‘Enid Gap.’ I think the term is appropriate, however,” he said.
Choy noted there are thousands of Class II wastewater injection wells throughout Oklahoma. An Oklahoma Corporation Commission map shows there is injection activity in the gap.
“We don’t know the reason for the gap yet. Local geology may vary considerably over short distances. There may be structural, faulting and geological differences in the basement along both sides of the gap,” he said.
It wasn’t until recently that requirements were stiffened for wells to report injection data, Choy said.
“It (was) not easy to make any correlations with well activity before then. Note that in October 2013, a rash of earthquakes in Love County was attributed by the OGS to disposal well activity. When well activity was reduced, seismic activity was subsequently reduced,” he said.
Choy said questions about the gap highlight the need to instrument the Enid area, in case seismic activity does commence, as well as the need to conduct preemptive geophysical studies on rock properties in the gap.