The largest earthquake to hit North Carolina in a century struck the state shortly after 8 a.m. Sunday. Did scientists see it coming?
No, with good reason — earthquakes aren’t like hurricanes. There is no plausible forecast warning of an impending seismic disaster, and earthquakes “have nothing to do with clouds, bodily aches and pains, or slugs,” according to the U.S. Geological Survey.
“Neither the USGS nor any other scientists have ever predicted a major earthquake. We do not know how, and we do not expect to know how any time in the foreseeable future,” the USGS says. “USGS scientists can only calculate the probability that a significant earthquake will occur in a specific area within a certain number of years.”
They do that, in part, using what’s known as National Seismic Hazard Maps.
A 5.1-magnitude earthquake struck at 8:07 a.m. Sunday with an epicenter in Sparta, North Carolina, near the Virginia border, according to the U.S. Geological Survey.
Unlike Hurricane Isaias — whose every move was tracked by meteorologists as it made landfall as a Category 1 storm last week at Ocean Isle Beach in Eastern North Carolina — this earthquake had no plausible date and time of arrival.
It was, however, felt by people all over the Southeast.
At least 3,000 people recorded feeling the tremors using the USGS “Did You Feel It” tool. But Randy Baldwin, a geophysicist with the National Earthquake Information Center in Colorado, told The News & Observer the number of people who reported to their center was closer to 6,000.
What scientists know
North Carolina is not without earthquakes, but “large, damaging seismic events are infrequent in our state,” according to the N.C. Department of Environmental Quality.
One of the areas most prone to earthquakes in this region stretches along the border of North Carolina, Tennessee and northern Georgia, N.C. DEQ says. It’s known as the Eastern Tennessee seismic zone.
“Scientists are studying this area to determine why so many earthquakes happen here,” according to the state agency.
The USGS, a scientific agency of the federal government, is doing something similar on a nationwide scale with National Seismic Hazard Maps.
Using a combination of factors, those maps “project potential maximum expected ground motions at a site over a particular period of time (say 50 years),” according to the USGS.
They’re mainly used by engineers and construction companies to help design buildings and infrastructure in areas where earthquakes are more frequent. But insurance companies, government officials, land-use managers and members of the public can also use them, USGS says.
The maps designate how many damaging earthquakes are expected in a given region based on their Modified Mercalli Intensity (MMI). USGS defines a “damaging earthquake” with an MMI of Level IV or higher.
According to N.C. DEQ, Level IV is “felt indoors by many” and “feels like a truck has struck the building.” A Level IV earthquake typically has a magnitude of 3.5 or higher — meaning Sunday’s quake in North Carolina would qualify.
The maps are updated every six years, according to USGS.
The 2018 Long-term National Seismic Hazard Map by USGS shows the highest hazard for damaging earthquakes along a portion of the Eastern Tennessee seismic zone as well as the New Madrid seismic zone in western Tennessee and a pocket near the South Carolina coast.
But that doesn’t mean an area with cooler colors like white or blue won’t also experience a damaging earthquake.
“Damaging shaking can and will happen in those areas, too, but less often. In fact, damaging shaking is possible in all 50 states,” USGS says. “The cooler color areas, like grey, are low hazard but not no hazard.”
The map doesn’t show earthquake risks and cannot predict when an earthquake will occur, but it does show the “general earthquake hazard” for a given area and allows the public to compare hazards across the U.S., according to the USGS.
The agency is also careful to distinguish between probabilities, forecasts and predictions.
“Probabilities and forecasts are rather like climate probabilities and weather forecasts, while predictions are more like statements of when, where, and how large, which is not yet possible for earthquakes,” USGS says.
USGS primarily applies the term “forecasts” to aftershocks, which are expected after large earthquakes.
“Most aftershock sequences follow the same pattern, so the probability of an aftershock in a time window following an earthquake can be determined,” USGS says.
Progress on earthquake predictions
“Probabilistic earthquake forecasting,” as Harvard researchers call it, is a tool employed by emergency management officials in California — largely considered a hot zone for seismic activity.
The state typically sees at least two or three earthquakes per year above a 5.5 magnitude, according to the California Department of Conservation.
Officials use a model called the Uniform California Earthquake Rupture Forecast 3 to estimate “the likelihood of earthquake fault ruptures throughout the state,” a Harvard University blog published in April 2019 states.
They also use what’s known as an earthquake early warning system.
Using a smartphone app called MyShake, California residents can be notified of an earthquake “seconds before we can feel them,” according to the California Earthquake Authority. Officials also send out warnings through “existing wireless emergency alerts that sound an alarm on cellphones for flood warnings and missing children (Amber Alerts).”
These alerts are triggered when an earthquake begins, sending notice seconds in advance to people in the surrounding areas.
Even with 10 seconds’ notice, the California Earthquake Authority says the alerts can give people enough time to “drop, cover and hold on” until the shaking passes.