Sharp tremors have been recorded four to five times since Oct. 16 as coming from the northwest corner of the Napoleonville Dome in northern Assumption Parish, parish officials and earthquake researchers reported Friday.
That general area of the 1- by-3-mile salt dome contains a failed Texas Brine Co. cavern believed to have caused a 5.5-acre sinkhole located just off the edge of the dome. The sinkhole’s discovery Aug. 3 between Bayou Corne and Grand Bayou immediately prompted an evacuation order affecting 150 homes in the Bayou Corne community.
The largest and best-recorded of the small tremors occurred at 9:06 p.m. Tuesday and two more came back-to-back at 10:11 p.m. Wednesday, said Stephen Horton, researcher with the University of Memphis Center for Earthquake Research and Information.
“It’s as big as we’ve had recently,” Horton said about the Tuesday tremor.
Horton, who is working with the U.S. Geological Survey to track tremors in the Bayou Corne area, said researchers are trying to determine the recent tremors’ cause.
He said the tremor on Tuesday was shallow, roughly 1,600 feet underground, though he noted researchers have not pinned down the precise depth. The peak of the salt dome lies about 700 feet underground and its bulk extends thousands of feet deep.
The tremors on Wednesday were smaller and their location has been less well determined, though they are in the same area, he said.
The salt dome pushed up over geologic history from deep salt beds left by ancient seas and has been the focus of oil and gas exploration, solution mining and gas storage for decades.
Inside but near the western edge of the salt dome, the failed Texas Brine cavern is located between 3,400 and 5,600 feet underground. It is also about 200 feet southeast of the sinkhole. The Tuesday tremor was just southeast of the cylindrically shaped cavern.
Horton said the recent tremors are similar to the shallow, sharp tremors that seismometers recorded in the dome’s northwest corner in the weeks since the sinkhole was found.
Horton said the current tremors are not a reason for concern nor do they appear to be an indication that they are leading to any major geological developments, such as the formation of another sinkhole.
“There was time when there were one or two per day over a period of a couple of weeks. That may be what we’re seeing, another period of that,” Horton said.
“It doesn’t mean it’s leading to something big or a bigger earthquake. It may be part of a cycle, as these things progress.”
Tremors of varying kinds have been reported in the area since at least early June.
Texas Brine officials have maintained regional seismic activity caused their cavern to fail and USGS officials have said their consensus is that seismic activity in the area was a consequence of the cavern failure.
Horton said Friday that he is not aware of a consensus regarding the cause of these sharp tremors during any period in the past several months.
Scientists working with DNR and Louisiana Office of Conservation officials have said the cavern failure stemmed from a breach in its lower side wall.
That breach allowed 3.3 million cubic yards of sediment, as well as naturally occurring crude oil and natural gas, from outside the dome to flood inside the cavern.
Horton noted that the tremors in the days before the sinkhole’s formation increased to hundreds per day but stopped about 2 p.m. Aug. 2.
Officials believe the sinkhole formed sometime overnight between Aug. 2 and Aug. 3 and was found in the early morning of Aug. 3.
John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness, said Friday that the group of scientists working with DNR on the sinkhole investigation is looking at whether Texas Brine’s removal of crude and natural gas from the failed cavern may be connected to the tremors.
Boudreaux cautioned there is no indication of a connection so far, but the group is still looking at the possibility.
Under orders from the state Office of Conservation to abate environmental and safety concerns from the sinkhole, Texas Brine began last week to pump brine into the cavern to push out the crude. On Monday, company officials announced that they believed they had removed most of the crude.
Sonny Cranch, spokesman for Texas Brine, referred questions about the tremors to Horton and USGS but said the Houston company is continuing to monitor the cavern and sinkhole.
Horton added that previously noted long-period tremors radiating from the vicinity of the sinkhole have continued.
He said researchers believe those tremors are consistent with some kind of fluid flow underground.