Only a few days into January 2020, the Associated Press reported the third earthquake near Columbia, SC in the short period of two months. And since earthquakes aren’t something we ordinarily concern ourselves with in South Carolina, we decided to take a closer look at our history of earthquakes. Aside from the massive quake that struck South Carolina in 1886, we discovered a brow-raising pair of earthquakes in 1969 and 1971. One of them was strong enough to move furniture in the upstate. And the theories about why the two earthquakes occurred are even more astounding.
In spite of the absence of any recent sizeable quakes in the Palmetto State, there are a number of faults that affect the state. One of them, the Brevard fault, closely follows the border between North and South Carolina.
The above world map shows the epicenter locations of earthquakes taking place between 1963 and 1998. South Carolina's upstate is smeared with black dots indicating seismic activity during the period.
The two earthquakes mentioned above occurred along the Brevard fault in the vicinity of Lake Keowee, although reports of the earthquakes were felt all the way into Georgia.
Lake Keowee was formed by Duke Energy in a multi-year project that created the Keowee reservoir (a.k.a. Lake Keowee). Lake Keowee is 26 miles long and three miles wide. When filled to capacity, the average depth is approximately 54 feet.
The lake has quietly become a quiet slice of serenity for those living on and near the lake, as well as those who travel to many access points to enjoy a day on or in the water. But it hasn't always been this quiet.
In 1969 and again in 1971, after periods of rapid filling of the newly formed reservoir, two large earthquakes occurred in the immediate vicinity of Lake Keowee.
The first, felt in 1969, occurred immediately after filling the lake to Stage 2. The quake measured an intensity of V on the Mercalli scale. Then, in 1971, after another period of rapid water rising, a more intense earthquake struck — this time near Seneca, located on the shores of Lake Keowee. The Seneca quake of 1971 measured a VI on the Mercalli scale.
While it may seem to be somewhat of a stretch to blame the weight of the water on the occurrence of the two Lake Keowee earthquakes of 1969 and 1971, seismic activity associated with reservoirs is widely known and accepted among seismologists.
The Seismology Research Center devotes an entire page on their informative website to the correlation between dams and earthquakes. The Center cites large reservoir-triggered earthquakes in China and India that measured 6.2 and 6.7 on the Richter scale.
The Center also details that earthquakes are an associated risk when a new reservoir is constructed. So it was a perfectly normal — or at least, expected — occurrence for Lake Keowee to spur earthquakes. But why does it happen?
The Seismology Research Center cites two likely reasons: First, rapid filling of the reservoir caused a change in stress on the ground underneath the water. Secondly, the increased pore pressure from the water effectively decreases the strength of the rock underneath the new reservoir.
Seismologists are beginning to frown upon using terms like reservoir-induced (or triggered) earthquakes. And some seemingly shy away from assessing blame for an earthquake on a particular reservoir.
Instead, the new approach is to argue that an earthquake occurring at the site of a dam or new reservoir is simply the result of normal tectonic strain (in this case, strain upon the Brevard fault) that would have occurred naturally over a long period of time and was simply accelerated by the stresses caused by the new reservoir's water.
What do you think? And were you in the nearby vicinity when the newly formed Lake Keowee’s water levels were rising to capacity and these earthquakes occurred? And finally, do you believe South Carolina is long overdue for another major earthquake? Weigh in with your thoughts in our comments section.
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