This is a picture of Meers Fault in southwestern Oklahoma—the only place in the state where tell-tale signs of earthquakes are visible above ground. But, beneath the surface, the Sooner State is riddled with faults—far more than anyone could count, says Andrew Holland, seismologist for the Oklahoma Geological Survey.
Those faults are behind the moderately sized earthquake that struck near Norman, Oklahoma, yesterday morning—somewhere between 4.3 and 4.7 on the Richter scale. Thirty miles North, in Oklahoma City, it rattled my Dad's windows and scared the crap out of his cats.
I always knew earthquakes were possible in the lower Midwest, raised, as I was, on tales of the massive one that shook New Madrid, Missouri, in 1812. But I didn't realize that Oklahoma—and Kansas, and Arkansas—also had active faults. In fact, 2010 has been a banner year for Oklahoma earthquakes. More than 80 have been recorded, and more than 50 of those were felt by people—far above the state average.
I called Holland to find out why there's so much shakin' going on, whether this has anything to do with the infamous New Madrid fault, and how you get a fault in the middle of a tectonic plate, to begin with.
Maggie Koerth-Baker: I think most of us are used to the idea of faults being found at the edges of tectonic plates, but Oklahoma is right in the middle of the North American Plate. Why would you find faults there?
Andrew Holland:Oklahoma has been pulled apart and stretched over previous cycles of plate tectonics. There are spots where the North American Plate has been pulled apart and where old plates fused together. The fault lines still exist in the subsurface. They're buried. But they're under a lot of pressure. You get transferring pressures from the plate boundaries at the Mid-Atlantic Ridge and the Pacific and San Juan plates on the other side. There's these pressures from the edges that get transmitted through the solid rock.
MKB: How are these faults different from what you'd find in, say, California?
AH: The rocks here are older and less deformed on the edges. They're stronger than they are in California, where things are actively deforming. So, here in central U.S., the energy from an earthquake gets radiated much further than in California, because the rock the seismic waves travel through is more solid. It's just like how sound carries faster and louder through metal, than through wood or Styrofoam.
MKB: If these faults are buried in solid rock, how do they move? I can easily picture how a fault at the edge of a tectonic plate builds up and releases energy, but how does that work in Oklahoma?
AH: The sense of motion on our faults in Oklahoma is actually very similar to the motion in the San Andreas. Imagine two bricks sliding past one another. It's harder to imagine, even for me, how you have this slip at depth. But it's basically two rocks sliding past one another.
MKB: The recent earthquakes in Oklahoma aren't being caused by the New Madrid fault, correct? Are they similar faults, though?
AH: It is correct that these earthquakes are occurring on faults unrelated to the New Madrid fault. It's a different fault, but similar in style. There's a bigger demonstrated hazard on the New Madrid fault, but we do have possibility of having a larger earthquake in Oklahoma. There's one fault here that's actually visible from Google Earth, the Meers fault. We can look at the geologic record and see that it was active about 1300 years ago, with a couple episodes equivalent to magnitude 6.5.
MKB: How common are earthquakes in the lower Midwest? It seems like we're always shocked when they happen, but Oklahoma actually has some almost every year, right?
AH: It's more common than people think. In Kansas, my wife's parents carried earthquake insurance on their house in Junction City their whole lives. Arkansas is having earthquake swarms right now, too. It's funny, I go around and older people remember feeling the 1952 'quake. That's the largest we've instrumentally recorded in Oklahoma—somewhere between 5.2 and 5.9. I think people are surprised, though, because it's less common than in California. People have short memories and we have tornadoes every year, so those stand out more.
MKB: This year, and 2009, have been peak years for earthquakes in Oklahoma. Do we know why that increase suddenly happened? Is this weird?
AH: This year is a statistical anomaly. But we've got catalogs all the way back to 1897 and there's basically been earthquakes in all but three counties in Oklahoma. We don't know for certain what triggers peak years. Could just be random chance, the statistics playing out. One thing, that's somewhat applicable here ... if you look at the North Anatolian Fault in Turkey, what they can show is that each rupture adds stress to another section of the fault, which then ruptures, and it just unzips as it goes along. The same thing can happen here. But there's no direct timing. It can be years before the next section goes ahead and ruptures. Anywhere between 10 and 20 years between sections on the Anatolian fault.
MKB: There's a lot of drilling for oil and gas that goes on in Oklahoma, and I've seen some speculation that this activity could be triggering earthquakes and leading to the recent increase in events. Is that possible?
AH: So, that's something I'm actively examining. The research is still in its infancy. I just started here in January. At this point, we don't see any indication that that's the case. But I'm examining it as a real possibility. The jury is still out, I'd say. Until I can prove with good science that it's the case, my assumption is that this is natural seismicity. Earthquakes have happened naturally here in the past. It doesn't have to have an outside cause.
• United States Geological Survey: Oklahoma Earthquake Information
• Oklahoma Geological Survey: Earthquakes Page
• Center for Earthquake Research and Information at the University of Memphis: Map of recent earthquakes in the central U.S.
• CERI: 1812 New Madrid earthquake compendium—scientific reports, eyewitness accounts, newspaper articles and more