Earthquakes are rare but not unheard of in North Carolina, a state that lies far from the active tectonic plate boundaries that produce most of the world's seismic activity.
However, a recent series of earthquakes in a small town near the Appalachian Mountains has surprised and puzzled scientists, who said that they reveal the existence of an old fault line that has become active again.
Recently, eight earthquakes in four weeks have shaken West Canton, a town of about 1,200 people in Haywood County.
The earthquake cluster: A rare phenomenon
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The first earthquake in the series occurred on May 23, 2023, at 9:46 p.m. local time.
It was a magnitude 2.8, centered about 2.5 miles north of West Canton, at a depth of 3.1 miles, as per Phys.org.
It was followed by six more earthquakes over the next three weeks, ranging from magnitude 1.8 to 3.2, all within a few miles of the first one.
The latest earthquake occurred on June 16, 2023, at 7:32 a.m. local time. It was a magnitude 2.1, centered about 2.4 miles north of West Canton, at a depth of 3.7 miles.
It was felt by only one person in Clyde, a town about 8 miles away.
According to the U.S. Geological Survey (USGS), this series of earthquakes is known as a cluster, which is defined as "a group of earthquakes that occur close together in space and time with no clear mainshock-aftershock sequence".
Clusters are different from swarms, which are "a sequence of many earthquakes striking in a relatively short period of time within an area several miles across".
Clusters are rare but not unprecedented in North Carolina.
The USGS said that the state has experienced at least three other clusters since 1975, all in the western part of the state near the Blue Ridge Mountains.
The largest cluster occurred in December 1986 near Sparta, where nine earthquakes ranging from magnitude 2.6 to 4.5 occurred over two weeks.
Clusters are also rare but not unprecedented in other parts of the eastern U.S., where earthquakes are generally less frequent and smaller than in the western U.S. The USGS says that some examples of clusters in the eastern U.S. include:
A cluster of six earthquakes ranging from magnitude 3.0 to 4.6 near Mineral, Virginia, in August and September 2011, following the mainshock of magnitude 5.8 that caused widespread damage and was felt by millions of people.
A cluster of seven earthquakes ranging from magnitude 2.0 to 3.9 near Youngstown, Ohio, between December 2011 and March 2012, which were linked to wastewater injection from hydraulic fracturing operations.
A cluster of five earthquakes ranging from magnitude 2.6 to 4.0 near Irving-Dallas, Texas, between January and February 2015, which were also linked to wastewater injection from hydraulic fracturing operations.
The cluster of earthquakes near West Canton has revealed the existence of an old fault line that runs through the area, according to the USGS.
A fault is a fracture or zone of fractures in the Earth's crust along which rocks have moved relative to each other.
The USGS said that the fault near West Canton is part of an ancient system of faults that formed hundreds of millions of years ago when the Appalachian Mountains were created by the collision of tectonic plates.
These faults have been inactive for a long time, but they can occasionally become active again when stress builds up in spots where the rock is weak.
It is not clear what triggered the recent activation of the fault near West Canton, but it could be related to natural variations in the stress field or to human activities such as mining or groundwater withdrawal.
Moreover, the USGS said that it is also not clear how long the cluster will last or whether it will lead to larger or more frequent earthquakes in the future.
However, it says that there is no evidence that clusters increase or decrease the likelihood of larger earthquakes on nearby faults.
Currently, the USGS is monitoring the cluster closely and providing updates on its website and social media platforms.
