Quakes: Unanswered questions

Persistent earthquakes in Ok­­lahoma continue to spark unanswered questions for researchers.

Oklahoma Geological Survey Director Jeremy Boak said two recent Edmond earthquakes — both measuring more than magnitude 4.0 — occurred in an area where there is little wastewater injection.

“It begins to come back to this question of how widespread is the affected area? Is the injection sort of being spread around so much that it’s capable of affecting a larger distance away than we had anticipated? And is it really this cumulative pulse, not attributable to any one well but attributable to the total amount being injected?” he asked.

OGS officials have seen seismic data indicating how complicated the geological structure is “down in that basement zone,” Boak said.

“There’s a whole layer that had not really been well defined before. I think some people had sort of seen it and, because it’s down below the productive zones, hadn’t really paid much attention to it,” he said.

In looking at the seismic data, OGS officials determined there are two different rock units, Boak said.

“There’s a zone that appears to have relatively good horizontal reflectors in it, that looks like it might be a stack of volcanic rocks ... then beneath that are more traditional, very massive, sort of blocky basement. And that appears to be down where all the earthquakes are happening, on faults down in that part,” he said. “So, there’s an interesting complication, and we’re just in the process. They’re going to give us some of their data — it was a propriety batch of data — but they think the deep part the lawyers will let them part with, because it doesn’t have any oil and gas in it.

“We might get a really interesting data set that might help explain why we have such a diffuse pattern of earthquakes.”

There always are lots of unanswered questions, Boak said.

“We’ve got a lot of people working on this now. There’re exciting developments, but they’re not at that point where we can offer some really exciting conclusion yet,” he said.

More than 900 significant quakes

There were 907 magnitude 3.0 or greater earthquakes in the state in 2015, Boak said. This total included 29 quakes measuring magnitude 4.0 or greater.

“As we review the records, we sometimes discover duplicates or additional, generally smaller earthquakes, and the catalog may be revised to a slightly different number,” he said.

When comparing to the first half of the year, there were about 50 fewer earthquakes in the second half, Boak said. 

There were 14 magnitude 4.0 or greater quakes in the first half, and 15 in the second half of 2015, he said.

“So there’s an interesting question about how significant is that difference? Does it at least reflect a flattening of the pattern? And, is there something that’s actually driving that? We’ve seen some indications that injection has been less, has been declining, and what we don’t know is how much of that is wells that were directed by the Corporation Commission to either shut-in or cut back, or, on the other hand, wells that were shut-in or cut back because of the price drop for oil,” Boak said. “So, we think we’re seeing some side effect. We think we’re seeing injection reducing. We’re not sure whether it’s yet having an effect, but at least we think it’s not continuing to climb.

“I think if you went back and you looked at every six- month interval, from about 2012 onward, you’d see a steady increase every six months. And this is the first case of where that’s not actually happening. I haven’t yet tabulated that to show that that’s the case, but I’m pretty certain it would be true.”

The data could show an important development, he said.

“It could mean that things are changing. It’s, obviously, not going to satisfy people who are upset about it. I think we’d have to see a very substantial decrease in seismic activity for that discontent to go away at all,” Boak said.

Shake less, but more often

At the end of 2015, U.S. Geological Survey records showed fewer magnitude 3.0 or greater earthquakes had occurred than OGS totals.

The USGS and OGS have not really “sat down and worked through exactly what’s driving the differences,” Boak said.

“They’re relatively small, and they’re probably in those smaller range of earthquakes, the smallest earthquakes. Because they cover the whole world, they probably don’t catch all of them, and they may not spend as much time on our local ones as they do on the rest of the world,” he said, referring to the USGS.

Boak said he and USGS officials have been talking about how to collaborate more.

He recently stopped by USGS offices in Golden, Colo.

“We ended up talking for two hours about how to collaborate better, what things can we do,” Boak said. “They were also very interested in knowing — because they’re on the hook to prepare a Seismic Hazard Analysis for the induced seismicity — they’re very eager to hear, ‘what is it we might need to put into that? What should be in there?’”

Boak posed several questions, including, “what happens to a place that is shaken by a lot of smaller earthquakes?”

“Because they do the actual Seismic Hazard Analysis, they’ve got the engineers on board who can answer questions like that. We don’t have any structural engineers here, so we’re not really equipped to answer questions like that, but I think they need to be raised and talked about. So much of earthquake building codes is driven by one big catastrophic earthquake,” he said. “What happens when you shake less, but more often? That’s an area I don’t know anything about, and so I’d love to have somebody write up something that I could read and say, ‘here’s what the experts tell us.’

“All the noise about we had more 3.0s than the rest of the country combined isn’t, to my mind, that really relevant.”

In looking at California earthquakes, the amount of energy released was about 10 times the amount of energy released by earthquakes in Oklahoma, Boak said.

“We have a very high proportion of small earthquakes. A very distinct difference, and while out here in the mid-continent you can feel those 3.0s, in California they hardly feel those because the ground is already so broken up,” he said. “Here, we’re a little stiffer and tougher, and ... people feel it more.”

OGS analysts have communicated they were somewhat astonished by the number of “felt reports” for a magnitude 3.5 quake, but there were no reports when a magnitude 4.0 quake occurred, Boak said.

“So, how you feel it depends very much on where you are, and the soil underneath you ... how deep the earthquake is and a whole lot of other conditions. There’s not a simple conversion of magnitude into felt effect,” he said.