States most at risk for earthquake damage
The damage could be anywhere from a crack in a building to a full collapse, although most of the modeled hazard would be slight, said Mark Petersen, lead author of a new study detailing the model. The model shows shaking from a moderate sized earthquake, magnitude 5.
The 2023 model, the biggest update in five years, is the most comprehensive analysis of active faults, past earthquake activity and geological factors that can affect seismic activity.
“It’s been almost 120 years (1906) since we’ve had a large earthquake make a direct hit on an urban area,” Greg Beroza, a professor of geophysics at Stanford University who was not involved in creating the model, wrote in an email. “The US has grown so much and so much has changed in the world since then, that the next such earthquake is likely to bring some unanticipated consequences.”
Earthquakes are expected to cost the United States $14.7 billion each year due to groundshaking-related damage to buildings, according to USGS and the Federal Emergency Management Agency (FEMA). FEMA advises people to have an earthquake plan in place and follow safe practices, like dropping during the event and evacuating after.
Earthquakes are caused by a sudden slip of a fault or fracture in Earth’s crust. Tectonic plates are constantly slowly moving, but their rough edges often contain faults and get snagged. As the plates get caught, stress builds. When the stress grows too big, it can overpower the frictional forces holding the plates together. When the plates are released, the pent up energy is released and sends seismic waves in all directions — shaking Earth’s surface.
Locations on the boundaries of these tectonic plates showed the greatest risks, unsurprisingly, the model showed. Alaska and California, the most seismically active zones in the nation, have more than a 95 percent chance of feeling shakes from at least a slight earthquake in the next 100 years.
Earthquakes are less common in the central and eastern United States, but they still occur and pose risks.
“The biggest change in risk appears to be in the Eastern Seaboard,” Beroza wrote in an email. “The earthquake hazard is increased, though still moderate there compared to places like California; but, the concentration of people and built infrastructure there is very high, so that’s a lot of exposure.”
Because the faults are hard to see, scientists look for other evidence of a past earthquake. When an earthquake shakes the ground, it causes soils, sand and water to vent to the surface - a process known as liquefaction. Scientists look for these large sandy deposits on the surface, known as sand blows.
In the central United States, the most prominent hotspot is the New Madrid seismic zone in the Mississippi Valley - a small sliver that has a 75 to 95 percent of experience earthquake-related shake. The seismic zone branches into southeastern Missouri (near the town of New Madrid), northeast Arkansas, southwestern Kentucky and northwest Tennessee.
A standout location on the east coast is Charleston, S.C., in the Coastal Plain, which is one of the most seismically active areas in the eastern United States. In the Coastal Plain, underlying rocks are faulted or fractured from the break up of tectonic plates - weakening that area of that plate. In 1886, a magnitude 7.0 earthquake hit Charleston killing 60 people and damaging buildings, making it the most damaging quake in the eastern United States.
In more recent memory, Mineral, Va., experienced a magnitude 5.8 earthquake in 2011 that shook the East Coast and damaged the Washington Monument in D.C. The earthquake occurred in the Central Virginia Seismic Zone, laced with faults likely formed from plates colliding hundreds of millions of years ago. Adding new data about the seismic zone since the last model, the update bumped the location’s chance of feeling earthquake shaking to 25 to 50 percent.
Quantifying earthquake hazard in this way, Beroza said, is foundational to the country’s efforts to mitigate risk. “We’re constantly learning more, so it’s important to capture that improved understanding in hazard characterization,” he said. He said this update is a “very welcome development.”
There are still many faults in the eastern United States that are not included in model, Petersen said. The biggest reason is they don’t have evidence of movement on those faults because they’ve been eroded or covered up. But he thinks the general hotspots on the East Coast will probably persist even with new data.
“We try to make the best model we can that can be used in building codes, insurance rates, planning by FEMA and by states,” said Petersen. “We want to produce more resilient communities so that when an earthquake does strike, we can quickly recover.”