The biggest earthquake in Oklahoma's recorded history was a natural occurrence along a known fault line and likely had nothing to do with oil and gas exploration, geological experts said Monday. All of the tremors Saturday seem to be part of an isolated and inevitable release of seismic energy, G. Randy Keller, director of the Oklahoma Geological Survey, said before another quake shook central Oklahoma on Monday evening.
That quake registered at 4.7 magnitude and was centered about five miles northwest of Prague, in the same area as Saturday's record-setting tremor. The 4.7 quake, the second one registered Monday, came in the midst of heavy thunderstorms when a large portion of the state was under a tornado watch.
These will become the most studied earthquakes ever in the state. A wide array of sensors deployed after a recent surge in smaller earthquakes will provide experts plentiful data on the ground movement.
The magnitude-5.6 earthquake that struck at 10:53 p.m. Saturday northwest of Prague in Lincoln County most likely occurred along the Wilzetta fault line, which stretches across part of central Oklahoma east of Oklahoma City, the OGS and the U.S. Geological Survey reported. A magnitude-4.7 foreshock occurred nearby at 2:12 a.m., along with numerous smaller foreshocks and aftershocks. No major injuries were reported. Several structures sustained cracked walls and damaged brickwork.
Similar to 2010 quake
The quakes were probably the result of two tectonic plates moving past each other laterally, called a “strike-slip” fault line, Keller said. The same movement on a different fault line was responsible for a magnitude-4.7 earthquake centered near Norman on Oct. 13, 2010. “The fact that this stress has been released is a good thing,” Keller said. “It's unlikely to occur there again soon.”
The quake last year and Saturday's tremors were likely unrelated to a series of smaller earthquakes, called swarms, affecting central Oklahoma for the last few years, Keller said. The origins of those quakes are harder to pinpoint. The instruments deployed to study those swarms form the most sophisticated network of seismic sensors in the region.
OGS seismologist Austin Holland, who was already studying the recent seismic activity in Oklahoma, was in the field Monday gathering data about Saturday's quakes. Many people in the region who felt the quake reported hearing a noise, described as anything from a deep rumble to a bang, before or during the strongest Oklahoma quakes. Scientists say that's common for tremors in the region.
Oklahoma and much of the eastern U.S. sit atop hard bedrock that transmits shock waves across a much wider distance than some quakes west of the Rocky Mountains, according to the USGS. Similar earthquakes happen occasionally at unpredictable times and places across the region. Keller has likened it to a metal pole struck by a hammer. The waves move through the Earth's crust in a way that creates a low-frequency sound. People have described it like thunder, a sonic boom or a vehicle crashing into their house. People as far away as Dallas and St. Louis reported feeling Saturday night's earthquake.
Drills likely unrelated
Keller said the chance is remote that oil and gas drilling activities in the area had anything to do with the earthquakes Saturday. The quakes were centered more than two miles below the ground, far below drilling in the area. “It's way below any depth anybody is pumping or injecting or anything else, and it isn't a particularly active area, oil and gas-wise,” Keller said.
Residents in nearby states have blamed hydraulic fracturing and gas injection wells for small earthquakes, and the Arkansas Oil and Gas Commission shut down some injection wells in an area where there were concerns that earthquakes were linked to oil and gas activity. Holland has studied the swarm of smaller quakes in Oklahoma for links to the oil and gas industry, but has said he found little evidence they're related.