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The tremors appeared to build in July, reached a crescendo of several hundred per day then stopped at 2 p.m. Aug. 2, hours before the sinkhole was found early Aug. 3 on the property of Texas Brine Co. of Houston, officials said.
Later that same evening, the sinkhole prompted parish officials to call for a mandatory evacuation of 150 households in the Bayou Corne and Grand Bayou areas. It remains in effect.
Steve Horton, research scientist with the University of Memphis Center for Earthquake Research and Information, said Friday that about a week ago, one or two sharp tremors began reoccurring every day or so.
Horton sits on a scientific advisory committee that the Louisiana Department of Natural Resources formed to study the sinkhole, the tremors and natural gas releases that are also occurring in area bayous.
Horton, who is working with the U.S. Geological Survey to study the tremors, said the returning tremors are in the same vicinity as the earlier ones — a shallow area underground on the northwestern edge of the Napoleonville Dome.
The sinkhole and the abandoned Texas Brine salt cavern suspected of causing the sinkhole are also in that area.
The 1-by-3-mile dome is a solid salt deposit pushed up from an ancient seabed and used for decades for brine production, oil and gas exploration and hydrocarbon storage. The cavern was hollowed out of the dome throughout nearly three decades of solution mining to make brine for various industries.
Horton said seismic monitoring has narrowed down the source of the tremors to an area larger than — but containing — the sinkhole and the Texas Brine cavern.
While Horton said there is no cause for concern from the tremors, he said one question is whether they might be occurring in some sort of pattern, given how they built up, stopped and have now restarted.
“We were wondering if there might be a cycle to this and so we may or may not be starting a cycle,” Horton sai.d “We’ll just have to wait and see.”
“That is why we’re monitoring it at this point.”
Other developments surrounding the sinkhole have occurred in recent days, officials said.
- On Thursday, a contract administrator working for Texas Brine issued 153 checks for the weekly $875 payments to evacuated households and 135 checks for retroactive payments for the two-week period before weekly checks began.
- Another Texas Brine contractor continued work Sunday on changing out one drilling rig for another that is better able to handle gas pockets encountered during the drilling of the well, which is aimed at investigating whether the company’s salt cavern is the source of the sinkhole.
He said a state trooper was awakened by a tremor about 2:30 a.m. Friday. The trooper was sleeping in cabins near the sinkhole response command post in Bayou Corne along La. 70 South.
Horton said the most recent tremors are probably about a 2 on the Richter scale, which measures the energy of earthquakes.
What might be most vexing about the return of these sharp tremors is that they come in addition to long-period, lower frequency tremors that began Aug. 6. These long-period tremors come from northwest of the sinkhole and not in the salt dome, Horton said.
On a seismometer, the sharp tremors are recorded as having one back-and-forth oscillation in less than a second — multiple oscillations are bunched together like scribble-scratch on a graphical depiction — and the tremors last no more than a minute.
In contrast, on a seismometer, the long-period tremors are recorded as having one back-and-forth oscillation over 20 to 40 seconds — the wave pattern is easily distinguishable on a graph — and the tremors last for three to four minutes.
Horton said he has never seen such a combination of tremors before from the same area at the same time.
“Yes, it does lead to a lot of questions and that’s really where we are,” he said. “We have a lot of questions as to what’s going on and not a lot answers.”
He said the long-period tremors could be related to something connected with natural gas, but he cautioned that was speculation.
“There is something going on with these very long-period signals, and we don’t know what it is … I haven’t figured it out yet,” Horton said.
Recent discoveries of natural gas in a shallow aquifer and in the caprock above the Napoleonville Dome prompted DNR’s Office of Conservation Commissioner James Welsh on Friday to order operators working on the dome to look for gas below their operations. The operators must analyze any gas found and vent or flare it off.
