If you weren’t on the eastern seaboard on August 23, you missed a fairly rare occurrence: an earthquake felt widely up and down the eastern North America. The earthquake itself had an epicenter near Mineral, VA – a location that has seen seismicity in the past. You can find references to earthquakes in the journals of the Jefferson family in Monticello from the 1797 and 1833 – and as recently as 2003, showing that earthquakes are definitely not geologically uncommon. However, this magnitude 5.8 earthquake was one of the larger ones in Virginia since the late 1800′s. What was more surprising was just how widespread the shaking was felt. If you check the USGS website for the earthquake, you can see the “Did You Feel It?” map for the earthquake and sure enough, you can get hundreds of kilometers from the epicenter and still find shaking up to IV or V on the Mercalli scale. In fact, I felt quite a bit of swaying on the 3rd floor of Olin Hall here on the Denison campus for upwards of 30 seconds. It took me a while to figure out that it was, in fact, an earthquake because, to be honest, I wasn’t expecting much seismicity to be felt here in Ohio (the center of the continent is the “stable craton” for nothing).
Earthquakes of this nature are not unheard of on the east coast of North America. Possibly the most famous “passive margin” earthquake was the Charleston, SC earthquake in 1886 that was felt up and down the eastern seaboard. If you visit Charleston, you can see some of the evidence of the earthquake in the older buildings (see below), including places where brick and mortar were replace and earthquake reinforcements were put into the building after the earthquake. Now, that Charleston earthquake was a M7.3, much larger than the Mineral, VA earthquake (remember, the Richter scale is logarithmic, not linear), so the damage was much more significant (not to mention that many buildings in Charleston were brick, which doesn’t stand up well to earthquake shaking). However, there has been some structural damage reported for the Mineral, VA earthquake.
So, even here in Ohio, we can feel how geologically active the continent can be. Sure, it doesn’t happen every day, but you never know when you might find yourself in the middle of a earthquake.