If Nobody Felt It, Did It Occur?

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[Thanks to ImpactWeather’s in-house geologist Fred Schmude for many of the technical details of this YWB posting.]

My good friend and Oklahoma City meteorologist on KOCO-TV, Sarah Libby, asked her facebook friends if they felt yesterday’s 4.3-magintude earthquake centered near Norman, OK. Some did and some didn’t. What’s up with that?

Any number of things could be up with that. First, a 4.3 magnitude quake is at the lower end of the Richter Scale’s “Light” category (“noticeable shaking of indoor items; rattling noises. Significant damage unlikely”) which means if you were sleeping, or in a vehicle, or perhaps jogging or working out you might not have felt it at all. Second, and more importantly, a 4.3 earthquake will have different effects in different regions of the world. It depends on depth and sediment.

All earthquakes are not created equal. Some are centered deep in the mantle, some occur in softer ground, some occur in solid granite. Yesterday’s quake was centered five kilometers below the surface (considered “shallow” if less than 10km), so much of the energy was attenuated before it reached the streets and floors of central Oklahoma.

Oklahoma resembles other Midwestern states in that it has Paleozoic sedimentary rocks folded up against the ancient Appalachian mountain belt, however the mountain belt runs east-west. The small colored areas in the south and the deeply folded area in the southeast are, from west to east, the Wichita, Arbuckle and Ouachita Mountains. These represent a western extension of the Appalachians that also appears in Texas. The westward sweep of gray to blue represents sedimentary rocks of Pennsylvanian (299 to 318 million years ago) to Permian age (251 to 299 million years ago), most of them laid down in shallow seas. In the northeast is part of the uplifted Ozark Plateau, which preserves older rocks of Mississippian (318-359 million years ago) down to Devonian age (359-416 million years ago). The strip of green in southernmost Oklahoma represents Cretaceous-age (65-145 million years ago) rocks from a later incursion of the sea. And in the western panhandle are still younger layers of rock debris that were shed from the rising Rockies in Tertiary times (1.5 to 65 million years ago). These have been eroded in more recent times to reveal deep-seated older rocks in the farthest west end of the state in the High Plains.

But there’s more. Depending on the composition of the ground, energy will travel and dissipate in different ways. Love waves and Rayleigh waves will disperse the energy in a “ripple.” If the ground is solid enough, much like a ripple in a pond (ask any water skier and they will tell you water does not compress and it hurts when you land on it!), the crests and troughs of these energy waves are most dramatic near the center and then dissipate with distance. Considering a single point along a Rayleigh wave, the jolt will occur, then be gone; a second or third jolt may follow. Ground that is more loose will disperse the energy in a fashion similar to a bowl of Jell-O — little jiggly vibrations going every which way that will last much longer than a single jolt. This would be the difference between, for instance, a 4.3 earthquake experienced in Norman and a 4.3 earthquake experienced in the silty sediment of Houston. Though not pure granite, the ground in Oklahoma is mostly solid sedimentary rock.

A Love Wave transmits energy away from the center. Image: Wikipedia.

Oklahoma seismologists are estimating yesterday’s quake to be about a 5.1, but the USGS is (for now) maintaining its measurement of 4.3. The 5.1 measurement would make the quake the second strongest in Oklahoma since records began; the 4.3 measurement is just a little bit out of the ordinary.

It’s possible that yesterday’s Norman quake could be considered a foreshock. There’s some very old subsurface faulting from previous mountain-building processes in the region that lead to these sorts of quakes. Since this was a fairly good-sized and deep jolt, it’s likely there could be small aftershocks over the next week or two until equilibrium can be re-established in the crust.

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