Update 11/6/2011 16:20 UTC: I’ve added a summary explanation of the Wilzetta fault, distilled from the Shale Shaker article listed below, since this appears to be people’s primary interest. Read on for the whole description.
Last night Oklahoma’s fairly active seismic zone unleashed a magnitude 4.7 earthquake shortly after 2am. The quake struck about halfway between Oklahoma City and Tulsa. People felt it in places as far north as Wichita, Kansas, and as far south as Dallas, Texas. The quake was followed within 20 minutes by a more modest aftershock, and the mild (M~3.5) aftershocks have continued into this morning at intervals of about two hours.
Update 11/6/2011 06:35 UTC: two and a half hours ago (~20 hours after the M4.7) a much larger quake rippled across the state, centered in the same area as the quake that struck in the wee hours. This 5.6 was the largest earthquake recorded in Oklahoma’s history, and has actually caused some damage around the state, on top of undoubtedly tossing objects from walls and shelves. The geologic and tectonic context described below applies to this earthquake as well as the earlier one. This quake was nearly the size of the one that struck the east coast in August, so keep the comparison in mind as you take in the news. In my last blog post I attempted to clarify why these earthquakes happen and why people from so many states feel them.
The Did-You-Feel-It map from the USGS map as it appeared 12 hours after the earthquake. Responses are still coming in, so check back at the USGS site, or fill one out yourself! Mind the exaggerating outliers... sparse population and sparser reporting leads to over-excited survey respondents dominating some zip codes.
The Did-You-Feel-It report for this evening's 5.6, which followed the 4.7 from the night before by about 20 hours. The quake was quite strong near the epicenter, resulting in some damage, but it was felt over many states, including many major cities.
Surprising as the thundering rattle may have been to people who are more likely to suspect violent lightning storms or tornadoes, it was far from unprecedented. Oklahoma is no stranger to earthquakes.
A geologist from the Oklahoma Geological Survey suggests that last night’s earthquakes occurred on the known Wilzetta fault, and seismograms indicate is was the result of right-lateral strike-slip faulting, just like along–although completely unrelated to–the San Andreas. A search for the Wilzetta fault returns relatively little information, but I did find a single article [pdf] in the Oklahoma City Geological Society’s “Shale Shaker” newsletter that mentions it briefly. Sure enough, the mapped Wilzetta fault (lower right corner of figure 20 on page 39) occupies the precise area where these quakes occurred, and the quakes’ focal mechanisms are consistent with the geometry of the fault line.
To summarize the Shale Shaker article, the Wilzetta fault is one of the easternmost structures associated with the Nemaha uplift, a ridge of ancient bedrock that underwent a phase of compressional deformation before Pennsylvanian time (overlying rocks of Pennsylvanian age, 320-300 million years old, are undeformed). Thus the Wilzetta fault was active over 320 million years ago, but has probably sustained these very modest earthquakes sporadically and infrequently throughout its existence. Currently the Nemaha uplift and associated faults and folds are primarily of concern to the oil industry, since the bulges and cracks in the bedrock are sites at which oil collects. The steeply dipping (meaning nearly vertical) fault has been mildly reactivated, and the ongoing sequence of earthquakes is not entirely unexpected for the foreshock-aftershock sequence of a 5.6.
Figure 20 from "The Nemaha Trend - A System of Compressional Thrust-Fold Strike Slip Features in Kansas and Oklahoma". Click image for the article.
Maps of historic seismicity in Oklahoma reveals a very active belt stretching northeast from the vicinity of the capitol city, of which these earthquakes appear to be a part. This same area–around Oklahoma City–has experienced plenty of earthquakes of similar magnitudes recently. Here are the notable examples. The quakes have been so common recently that the OGS has a special FAQ page for its concerned citizens. The USGS also has an info page detailing Oklahoma’s rich earthquake history.
These quakes were smallish, and the “big one” occurred in the wee hours, so there’s unfortunately unlikely to be much video of them happening. I haven’t found any yet. Hope this has given you the resources to fulfill your curiosity!
If you’d like to read more, Chris Rowan at Highly Allochthonous has a nice summary of details and resources related to this earthquake, and Maggie Koerth-Baker has conducted a very informative interview with OGS geologist Austin Holland.
Big quakes in the past week
The Trembling Earth 
At approximately 10:50 pm 11-05-11 a strong “tremor” was felt in Charlie, TX. (near to Wichita Falls, TX) What was that? An after shock or another quake? Please respond.
I’m sure by now you’ve figured out the news. Stay safe there.
I live in Tulsa, about and 1.5 hours from the epicenter, I felt the 2am quake, and we just had the strongest in OK history tonight, and it was pretty crazy experience to have so many in 24 hours, I think a total of 9 quakes and aftershocks total.
Aftershocks of modest magnitudes may continue for a few days. Clearly the 5.6 this evening illustrates that you never know exactly what to expect! Best you can do is be prepared!
Thanks Austin, for clarification. I just posted a link to your blog to NSU Geography’s Facebook Page. I was trying to find out more about this fault (I’m California Transplant), but as you said, an internet search doesn’t turn up much.
Thanks! As far as I can summarize the Shale Shaker article–and please, if someone is more familiar with the geology/tectonics, correct me–the Wilzetta fault is part of the Nemaha uplift, a Paleozoic compressional belt, which has been mildly reactivated recently. I may add this detail to my description above for more thorough background description. It appears that this fault line has piqued people’s curiosity!
I have been googling for about 1/2 hour before I found this article. Thanks. Austin for the clarification. The Wilzetta is now a part of my OK vocabulary as much as New Madrid.
No geologist here, but it appears stresses are being redistributed. Do you think these are smaller faults responding to stress from a larger system? So far 19 quakes. The severity (relative, no panic here) and increase in number don’t fit the normal aftershock expectations. These seem to be of relatively short duration; does that indicate smaller systems moving?
In the first 4.6 there was a distinct lessening in intensity about 1/2 way through the event…I’m located in Skiatook some ways away. But last night the intensity grew markedly from start towards finish.
So anyway, I’ve watched the USGS sight for years now and noticed the uptick some time ago…acquired earthquake insurance 18 mos ago and it was cheap then. Might still be.
So, far now at least I’m thinking these are small fault systems redistributing stress over the last couple of years. Right, wrong, I dunno. Could it be there is a larger system building stress and these smaller faults are responding? I guess what I’m trying to say, and it may be the $64,000 question, is this long time stress energy that’s being redistributed across a number of small faults or is a larger system building stress and that is being evidenced by the movement of small faults? Or…am I totally wet?
This is sort of the million-dollar question for earthquake scientists as well; one of the holy grails of earthquake science is understanding the state of stress in the crust and how it evolves.
It’s important to consider scale, though. Every earthquake represents a redistribution of stresses, but not all of them are particularly consequential. Although the 5.6 may seem large, because earthquakes of this magnitude are rare here (although you’ll note from the USGS history that there were plenty going on even as far back at the 1950s), it’s important to keep in mind what a relatively small amount of stress change such an earthquake is actually the cause–or result–of! As far as tectonic forces go, the stress needed to generate a 5.6 are minimal, and likewise, the stress change that a 5.6 induces are relatively minor. Obviously they’re large enough to spawn a flurry of local aftershocks, but these are much smaller still!
The bottom line is that any stress change that’s going on is not large enough to be of much real consequence, in the sense that it’s not like a new plate boundary is opening up in Oklahoma. On the other hand, there could continue to be earthquakes associated with this sequence (there almost certainly will continue to be 3s and 4s for a few days, perhaps months), but it’s unlikely that this sequence alone could INDUCE a larger one that wasn’t going to happen anyway, and it’s impossible for us to tell at this point whether or not a larger one would happen, although statistically the chances are very slim.
For the first quake, 4.6, that I described I meant to say a lessening in intensity followed by a strengthening…a bimodal type of thing. We had a goodly shaking that lessened about 40% through the quake then it reintensified for the remainder. The last “1/2” stonger than the first “1/2”.
As to the variable shaking intensity, this is not at all unexpected from a moderate earthquake like the ones yesterday. The primary difference in shaking intensity during an earthquake is the different types of seismic waves, compressional versus transverse, and at the surface, these types of waves are able to “flap” the ground freely, so they become even stronger (are transformed into surface waves). These different wave types travel at different speeds and so arrive at different times during the earthquake. There may also be local effects, where the seismic waves bounce around a basin or are reflected off of bedrock “walls” below. There are also “source effects”, basically meaning that the duration of shaking you experienced was related to the actual progression of seismic waves generated by the rupturing fault. Ruptures are messy things, and they accelerate and decelerate, intensifying the waves they radiate. This is most likely what you experienced.
Imagine a toy boat, or a rubber ducky sitting in a pool, and someone jumping in. There’s a distinct “source” of waves, that uniquely depends on the size of the person and the way they jumped in, but the waves reflected off the sides of the pool will have their own specific patterns depending on the shape of the pool. The toy boat will bob up and down in a surprisingly variable fashion as these intersecting and conflicting waves come at it from all directions. That’s like people and houses in an earthquake.
Thanks for your clarification of the Wilzetta fault.
Thanks you for your site info. It was very helpful to me. I was needing some knowledge to combat the fear. Thanks again.
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Thanks for the information on this site. I never thought I would be getting coverage for earthquakes in Oklahoma.
My name is Amy Winter….I live 4 miles east of Meeker, 8 miles west of Prague, and maybe 5-7 miles south of Sparks…we were hit hard out here. The 5.6 was unbelievable for us…being so close to the epicenter. We thought the whole house was coming down….and we lost power in the middle of it. It threw most everything out the cabinets on the north wall, our chimney crumbled, and the fireplace area in our house will have to come down too. We have some cracks in the sheetrock..and in the brick of our home. We were traumatized by this and hope it never happens again. The movement inside the house was hard to describe…and I think most people don’t believe us. It was violent jerking side to side….and up and down all at once. I am amazed that my house staying held together.