Small quake rattles NC weeks after 5.1 magnitude. How long will aftershocks last?
Nearly three weeks after a 5.1-magnitude earthquake hit North Carolina, the area is still feeling its aftershocks.
A small quake was reported just outside Sparta, where the larger was centered, at 3:11 a.m. Thursday, according to the U.S. Geological Survey. More than 30 people reported feeling the 2.4-magnitude earthquake to the USGS.
It’s the latest of several aftershocks that have followed the 5.1-magnitude quake — which is typical. Most larger quakes are followed by smaller ones.
The 5.1-magnitude was the second-largest in North Carolina since 1900, behind only a 5.2 reported near Skyland in the Asheville area in 1916, McClatchy News previously reported. Its shaking was reportedly felt across the Southeast.
Although the quake caused minimal damage, the Sparta area could feel its effects for a while.
How long do aftershocks last?
They can continue “over a period of weeks, months, or years” near the mainshock, according to the USGS.
“In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue,” the USGS website says.
Aftershocks are typically smaller than the mainshock but “can still be damaging or deadly,” the USGS says.
But earthquakes less than a 2.5 magnitude, such as the one reported Thursday, are “usually not felt,” according to Michigan Tech. There are about 900,000 of them each year.
Additionally, the USGS forecasts low chances of larger quakes following the 5.1 magnitude in Sparta.
Predictions for Sparta
The USGS forecasts the number and strength of aftershocks after a “significant” quake.
But it says “no one” can exactly predict when or where earthquakes, including aftershocks, will occur.
“We calculate this earthquake forecast using a statistical analysis based on past earthquakes. Our forecast changes as time passes due to decline in the frequency of aftershocks, larger aftershocks that may trigger further earthquakes, and changes in forecast modeling based on the data collected for this earthquake sequence,” it says.
Here’s what the USGS predicts following the Sparta mainshock as of Thursday:
In the next year:
▪ A 52% chance of at least one 3.0-magnitude quake or higher.
▪ A 12% chance of at least one 4.0-magnitude quake or higher.
▪ A 1% chance of at least one 5.0-magnitude quake or higher.
▪ A less than 1% chance of anything stronger.
In the next month:
▪ A 24% chance of at least one 3.0-magnitude quake or higher.
▪ A 3% chance of at least one 4.0-magnitude quake or higher.
▪ A less than 1% chance of anything stronger.
The chances of an earthquake with a magnitude stronger than 3.0 within the next week or sooner are 10% or lower.
“The expected rate of earthquakes continues to decline throughout the time windows,” USGS says. “The probabilities in the longer time windows are higher only because the rates are being summed over a longer time period. These longer periods may be useful when planning recovery and rebuilding projects.”
So far, all aftershocks from the quake have been lower than a 3.0 magnitude.
