Uncertainty and the Jan. 20 earthquake near Wilmore

Remember that small earthquake that happened the evening of Jan. 20 in Garrard County, Ky., just south of Wilmore? It occurred in a location not known for seismic activity, so Seth Carpenter and Andrew Holcomb of the KGS Geologic Hazards Section have been studying the event, which caught the public’s attention and was reported by several news media.

Their estimate of 2.2 magnitude is slightly smaller than the U.S. Geological Survey’s 2.6 magnitude estimate. The calculated location, based on recordings from several instruments in the Kentucky Seismic and Strong-Motion Network, is also a bit different from the USGS-determined location. The map at right shows the two estimated locations. The oval depicts the area in which Carpenter has 90 percent confidence that the event happened. Based on the location and the characteristics of the recorded waves from the Kentucky network’s instruments, the event was very shallow. Carpenter believes it was shallower than 1.9 miles beneath the surface.

He also believes the Lexington Fault System, shown in red just to the east of the event’s location on the map, is not associated with the earthquake. Though the recordings from the KGS seismic instruments resemble those generated by explosions or mining blasts, Carpenter thinks that it was unlikely the event was human-caused. The recordings of the event do not resemble those of a typical tectonic earthquake, however.

Carpenter added that a possible explanation for the strange characteristics of the seismic waves is the interaction of underground fluids with the fault that slipped to create the earthquake, if fault slippage actually caused the event. “An alternative possibility is that the event was a near-surface failure or collapse,” according to Carpenter. “I’m wondering about karst. In other words, could we have recorded ground motions from a sinkhole collapse, or some other failure at the surface that is not necessarily manmade?” If more seismic activity occurs in the area, Carpenter says he will try to do a more detailed evaluation of its source.

Find out more about earthquakes in Kentucky in this booklet published by KGS.