Scientists and regulators agree that earthquakes like the 5.6-magnitude tremor that struck Oklahoma on Saturday, and thousands of smaller ones in recent years, are triggered by the disposal of millions of tons of wastewater that is pumped to the surface, and then injected back into the ground, during oil and gas production. Theshock last week tied a record set in 2011 in Prague, Okla., for the strongest such tremor in the state’s history.
State regulators have ordered well operators to stop wastewater injections in a 725-square-mile ellipse around the quake’s epicenter. But they conceded that trying to prevent more quakes was an inexact science. And in Oklahoma, where oil and gas are dominant economic and political forces, any effort to regulate the industry produces an entirely different set of shocks.
Dr. Todd Halihan is a geologist and a specialist in hydraulic fracturing, known as fracking, at Oklahoma State University in Stillwater, about 30 minutes by car south of the epicenter of Saturday’s quake. He answered some questions about the quake, and the risks the state still faces.
We don’t know yet. There were several injection wells nearby, so it could be similar to the Prague event. The earthquake was about the same size as Prague. This time, there’s going to be a lot more seismological data available and a lot more understanding in terms of what’s going on in terms of injection well activity, so we hope we’ll be able to pinpoint what happened.
In the last year, Oklahoma has been more aggressive about controlling underground disposal of oil wastes, and the number of earthquakes that people could feel has been slowly declining. Yet you just had one as big as any in the state’s recorded history.
We had started ratcheting down waste disposal in some places. But the scientific literature says you can actually have larger events as you go downward — and some of the literature actually says that might be causing larger ones. So we’ve been nervous about it. Over all, things are going in the right direction; we’re not increasing total earthquake activity any more. But we’re still high on the curve relative to what the base level was. Oklahoma is supposed to have about two earthquakes a year above magnitude 2.5, and in the last few years we’ve had thousands. I tell people we’ve had thousands of years of earthquake activity in a short period of time.
So even though the number of quakes is in decline, scientists aren’t ready to write off the risk of a genuinely damaging one?
Yet there wasn’t a tremendous amount of damage and no lives were lost. Is the risk here being exaggerated?
Oklahoma got very lucky in that the epicenter of this quake was functionally in the middle of nowhere. And it happened in the early morning. If you’d stuck it underneath a town at a different time of day, when people were on the sidewalks, you would have had a much greater problem. As for magnitude, you can have the same-size number as a California earthquake, but you get a lot more shaking here. The intensity at the surface and the response of your house is almost a Richter magnitude greater — and an increase of one point on the Richter scale is the equivalent of releasing 10 times as much earthquake energy.
Why is that?
It’s the rock properties and the depth of the earthquakes. In places like California and Alaska, earthquakes are deep. A wave in California can be pretty tiny by the time it gets to the surface, so a lot of energy is lost. But here, the energy comes right to the surface. These quakes are shallow, and they’re right under people. People may think these are really mild earthquakes, but at this level you’re shaking pretty hard. In the supermarkets, instead of a couple things falling off the shelves, you have everything in the middle of the aisles.
Eighteen months ago the state seemed almost powerless to address the earthquake threat. This time, regulators were able, within hours, to order disposal-well operators to shut down dozens of wells in the quake zone. Is that enough to deal with the problem?
There’s been a huge change in terms of our ability to react. But we need to be proactive instead of reactive. This same process is happening across the country. We’re looking at underground carbon dioxide storage to fight climate change, and we’re sinking geothermal wells. We’re generating earthquakes, but we still haven’t done the large-scale studies we need to do to understand how to manage them. We can keep monitoring seismicity and mess with injection volumes, but unless we get boreholes at depth and really study what’s happening, this problem is going to continue.