The USGS is providing scenarios to estimate the chance of larger earthquakes as part of an ongoing swarm in the region near Elgin and Lugoff, South Carolina.
The swarm began on December 27, 2021, with a magnitude 3.3 earthquake. The largest earthquake to occur so far has been a magnitude 3.6 event on June 29, 2022.
An earthquake “swarm” refers to a prolonged sequence of earthquakes that lacks any clear primary event or mainshock, in contrast to an aftershock sequence where a large mainshock is followed by a decaying sequence of (mostly) smaller earthquakes. Swarms can keep the earthquake rate elevated for a few days to many months. It is generally not possible to predict how long an ongoing swarm will last until it has run its course and it’s impossible to predict the size of the largest earthquake in the sequence.
This area has a history of occasional small, scattered earthquakes, but none of particularly large magnitude. The largest earthquake within 50 miles (80 km) was in 1913 in Union County, when a magnitude 5.5 earthquake struck about 90 km northwest of the recent earthquakes. That quake caused damage to brick and stone buildings, destroyed chimneys, and displaced furniture in homes. The most damaging earthquake in South Carolina history was the magnitude 7.0 1886 Charleston earthquake, located about 87 miles (140 km) to the southeast of this current swarm.
During an earthquake swarm, the rate of earthquakes is increased, and the probability of larger earthquakes goes up accordingly. This swarm has produced a fairly constant trickle of earthquakes since December 2021, with 0 to 15 earthquakes larger than M2.0 occurring each month. Many smaller earthquakes – some of them still large enough to be felt – have also been recorded in the area. The rate of small earthquakes allows us to estimate the probability of larger earthquakes.
The USGS provides scenarios that are based on the assumption that the rate of smaller earthquakes remains roughly the same over the next month.
The following three scenarios describe possibilities of what could happen over a one-month timeframe (as of August 22, 2022).
Only one of these scenarios will occur within a particular month.
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Scenario One (Most likely, about 95% chance): Earthquakes continue but with none larger than magnitude 4 within the next month.
- The most likely scenario is that the swarm continues as it has over the past months, confined to the region already affected by the swarm. The rate of earthquakes in the swarm is likely to remain the same, if not decrease slightly, over the next 30 days. Smaller magnitude earthquakes will likely be felt by people close to the epicenters. The swarm could also stop completely during this time.
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Scenario Two (Less likely, about 5% chance): A larger earthquake (magnitude 4 – 5)
- A less likely scenario would be a somewhat larger earthquake in the magnitude 4 range. Such an earthquake would be felt over a larger area but would not cause significant damage. An earthquake of this size would be followed by aftershocks that would temporarily increase the number of smaller earthquakes per day.
3. Scenario Three (Least likely, less than 1% chance): A much larger earthquake (magnitude 5 or higher)
- A much less likely scenario, compared with the previous two scenarios, is that the ongoing swarm could trigger an earthquake significantly larger than the M3.6 that occurred on June 29. While this is a very small probability, such an earthquake could have significant impacts on communities nearby and would be followed by aftershocks that would increase the number of smaller earthquakes per day.
