What Lies Beneath
1:49 pm
Tue March 5, 2013

Was That An Earthquake Or Something Else?

LEXINGTON, Ky. - A November 10th earthquake in Letcher County showed that Kentucky really is "earthquake country".  The Kentucky Geological Survey's Mike Lynch has this report on a new seismic monitoring station in Hazard, Kentucky.

Seth Carpenter and Zhenming Wang, of the KGS Geologic Hazards Section, installing one of the new instruments in Perry County.
Credit Mike Lynch

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A small excavator rolls up just behind the Perry County Public Library with a 9-inch wide augur to drill a five-foot deep hole.           

But no sign post or light pole was being installed here…. Two SEISMIC instruments—for detecting earthquake activity—were being installed at the site near Hazard, Ky. The Kentucky Geological Survey installed the instruments about a month after a 4.3-magnitude earthquake—centered under nearby Letcher County shook the region.

Seth Carpenter, of the Geological Survey, says the November quake spurred the Survey to set up the long-planned seismic station, adding it to the Kentucky Seismic and Strong-motion network operated by the University of Kentucky. Two instruments were installed here-a weak-motion detector and a strong motion instrument.

The Perry County Public Library allowed Carpenter and his supervisor Zhenming Wang to drill a hole through the back wall of the building so the instruments outside can be connected to a computer and other electronic equipment inside. The library also gave KGS access to its network, so Carpenter can monitor the seismic equipment and tweak it from his office at UK. The recordings of ground motion detected by the instruments can be transmitted in seconds to the KGS network for near-real-time viewing on the seismic network’s Web site.

Library Director Elaine Couch thinks it’s a great educational resource for students in the region, since a monitor to display the recordings from the station and an educational display are being set up in the library.

Once the seismic equipment was installed and connected, some shaking was needed to test it…. Without an obliging earthquake at the moment, the KGS researchers had to provide it themselves…. As some library staff gathered around them.

With a strong stomp on the floor, the instruments came to life with signals.  Background noise typically comes from the electronic instruments themselves.           

But there is other “noise” in the region…. Provided by human-caused shaking from mine blasting, which the instruments also record…with a subtly different signal.

Having the instruments in a coal-mining region will help to tell the difference between real earthquakes and mine blasts, which have sometimes gone into USGS records as earthquakes.

In fact, local officials in southeastern Kentucky, news media, and KGS received a lot of calls in January when the ground shook in the area.  Carpenter examined the recordings on the new equipment and quickly determined they had the characteristics of a human-caused blast, not an earthquake.