Sorry for the delayed aftershock (heehee!) regarding this news, but I wasn’t close enough to a computer for blog access through most of this weekend. So, which one of you lucky Midwesterners felt the 5.2 and what was it like? Don’t leave out any details.
AP: 5.2 Earthquake Rocks Large Region Of Midwest
“It shook our house where it woke me up,” said David Behm of Philo, 10 miles south of Champaign. “Windows were rattling, and you could hear it. The house was shaking inches. For people in central Illinois, this is a big deal. It’s not like California.”
No, sir, it’s not like California. It’s better. The west coast’s earthquakes have never made it to 8.0 in this nation’s recorded history. Guess which region did in 1811 and 1812 and rung churchbells as far away as Boston. That’s right – New Madrid, which is directly to the southwest of Friday’s earthquake, in another arm of the greater rift complex in northeastern Missouri.
Below is a map (from Structural Features in Illinois by W. J. Nelson, 1995) of the complex structural features underlying the southeastern edge of the Illinois Basin. For you non-geologists, a geologic structure is visible/measurable rock deformation, usually in the form of a fault (break) or a fold (bend). Keep in mind that these structures are at depth, but that quite a bit of modern-day topography follows an existing structural trend.
Friday’s quake occurred on the western edge of the Wabash Valley Fault System which straddles the Illinois-Indiana border immediately to the north of the Rough Creek Graben. Again, the New Madrid Seismic Zone is to the south and southwest. Along with the Clay City anticline, the Wabash Valley Fault system forms the southernmost extent of the La Salle Deformation Belt which extends all the way north to the western suburbs of Chicago. You may consider the flatland of Central Illinois boring, but (cue best Rod Serling imitation) deep beneath lurks the result of eons of geologic turmoil. These structures were active roughly between 450 and 300 million years ago; today’s seismicity is likely reactivation along these faults, given that the depth (~7 km) is right. The more interesting question, though, is not where these earthquakes are happening but why.
Incidentally, in the Fairfield Basin on the northern portion of the map, there should be a structure called the Divide monocline that runs east-west and “connects” the Clay City anticline on the east to the DuQuoin monocline on the west. I know this because I mapped the Divide structure on well logs as part of my undergraduate senior thesis and published my findings in a beautifully-illustrated (mostly), best-selling (not really), 43-page paperback spiritedly entitled A Two-Dimensional Subsurface Study Of The Divide Structure In Southern Illinois.
I hope all of this gives you a better idea of where Friday’s seismic activity occurred, how it relates to New Madrid activity and why geologists map seemingly old and dormant structures tens of kilometers below the earth’s crust. James Hutton and Charles Lyell famously stated the inverse, but I also believe that the past is the key to the present. The only constant about the earth is motion, change, flux, and the more we understand about this the better our predictive capability.
Related: Seismicity of the Lower Wabash Valley: Fact Sheet; Reflection Seismic Profiling of the Wabash Valley Fault System in the Illinois Basin
Hi I am Kim Harpe I have lived on the Wabash fault line for all my life I have felt 4 of the earthquakes from 1968,1987,2002,2008 not counting the 3 aftershocks that I felt after the 2008 quake…I personally think the 1987 quake had more of a rolling affect action than any of the others I felt I was where I could see the outside and as I was looking I could see a rolling affect on the countryside like waves and it was a smoother ride but the one in 1968 and 2008 was more of a blunt force hard action jolt and vibration…The one in 2002 from the Evansville Indiana area was very light…I don’t know why there was a difference in the 1987 quake and the others but there was..And I didn’t feel the after shocks after the 68,87 or 2002 quakes but I did after the 2008 and they seemed as blunt force as the origanal quake..I am located in Lawrence county Illinois…If you would like to trade or talk about this with me please contact me at dream_weaver182@yahoo.com..Thanks..