November 5, 2011

Oklahoma earthquake – not as uncommon as you think

Posted by Austin Elliott

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  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.