LONG-TERM RESERVOIR INDUCED SEISMICITY AT LAKE MONTICELLO, SOUTH CAROLINA

We will present evidence that reservoir-induced seismicity continues until the present (January 2024) at Monticello Reservoir, South Carolina; >45 years after first impoundment. Monticello Reservoir is perhaps the clearest example of seismic activity induced by an artificial reservoir. A sequence of over 4000 microearthquakes occurred starting upon first impoundment of the reservoir in December 1977 and continued until at least the early 1990’s. Renewed seismic activity (700+ events) also occurred between 1996 and 1999. Unfortunately, the local seismic network around Monticello Reservoir was deactivated in December 2006 and only one seismic station (JSC - Jenkinsville, SC) of that network is currently operational. Scattered seismicity was recorded by regional seismic networks after that time (6 events between 2008 and 2021) but two significant earthquake swarms occurred in October-November 2021 and again in November-December 2023. Here we make use of seismic waveform matching techniques to overcome the limitations of a sparse seismic network to detect and potentially locate smaller earthquakes that have occurred since April 13, 2009, i.e., when digital seismic data from JSC is publicly available. First, we relocate the events best recorded on the regional seismic network and find they appear to occur beneath Monticello Reservoir itself. Next, we use the similarity of the seismic waveforms at JSC and other regional stations to spatially associate smaller events with their better located brethren. Finally, we search for additional smaller events using data from JSC that can be associated with the locatable earthquakes. We find that there are many more microearthquakes (i.e., hundreds) occurring beneath the reservoir than are reflected in the USGS earthquake catalog. We also find that the most recent earthquake swarm started on November 11, 2023, not on November 17 as listed in the USGS earthquake catalog. This is ~2 days after a relatively rapid drop of the lake level of 0.5-1.0 meters, suggesting that the most recent earthquake swarm was triggered by this drop in reservoir level. These results suggest that induced seismicity in the southeastern USA can continue for many decades after the initial disturbance.