Charlotte buildings, dams and bridges face little risk from earthquakes, experts say

The magnitude 5.1 earthquake that rattled Charlotte from its center 100 miles to the north Sunday morning left no apparent local damage. But what keeps high-rises, dams and bridges from tumbling?

Building codes, design standards and frequent inspections — and a region with little history of risk from earthquakes. Sunday’s quake in Sparta, near the Virginia line, was the most powerful to hit North Carolina since 1916.

The state Geological Survey notes that North Carolina has no active fault zones, where fractures between blocks of rock can trigger earthquakes, in contrast to the many active faults that cause large, damaging earthquakes in California. “Earthquakes are more frequent in the western part of our state, but statewide they are relatively small, random and scattered events,” the Survey says.

Moderate risk in Mecklenburg

Mecklenburg County is in a moderate risk zone for earthquakes, based on U.S. Geological Survey data, the county’s 2015 hazard mitigation plan says. The state’s western and southeastern ends are most vulnerable to earthquakes, it says.

The plan shows 16 “significant seismic events” between 1879 and 1976, with the epicenter of most of those earthquakes 100 miles or farther away. Eastern Tennessee, on North Carolina’s western border, is known as an earthquake zone. An 1886 earthquake in Charleston left 60 people dead, buildings damaged and 400 aftershocks over the next three decades, the USGS says.

“Earthquakes of significant magnitude are unlikely occurrences for Mecklenburg County, though the proximity of the area to major faults in several active seismic zones could increase the possibility of feeling some impact of a large, regional earthquake if it were to occur within those zones,” the hazard plan says. “The potential for ground shaking caused by events in these zones is well documented, and modern building codes do take them into account for today’s design and construction standards. Those buildings, infrastructure and assets built long ago and not constructed to these codes are most susceptible to damage during future earthquake events.”

Designers of commercial buildings, including uptown Charlotte’s high-rises, determine the amount of horizontal force — the shaking of an earthquake — the structure should be able to withstand, said Bill Kirk, chief building code consultant for the Office of the State Fire Marshal. Calculations use American Society of Civil Engineers criteria that are based on location, soil types and the building’s size, height and location, among other factors,

“It’s not just a number that you’d look up in a book, it’s a process that the designer goes through,” Kirk said. “The stiffer the building, the more resistance to horizontal motion.”

Earthquakes that do moderate damage strike the inland Carolinas only every few decades, the U.S. Geological Survey says, with small quakes felt about once a year or two. The largest recent earthquake on the East Coast was a magnitude 5.8 event in Mineral, Va., in 2011 that left slight damage.

The USGS reported two magnitude 2.2 aftershocks — below the magnitude 3 level at which earthquakes can be felt — southeast of Sparta on Monday morning.

Duke inspected its dams

Regulators view power plants and the large dams that serve them as particular risks to natural hazards such as earthquakes and flooding because of the damage they could cause if they fail.

Duke Energy said it inspected all 49 of its hydroelectric dams in the Carolinas after Sunday’s earthquake and found no damage. The Federal Energy Regulatory Commission, which regulates Duke’s hydro stations, inspects them annually. Duke does its own inspections at least once a quarter, and whenever ground motion is felt or rain of 2 inches or more falls within 24 hours.

Duke’s nuclear stations, including McGuire on Lake Norman and Catawba on Lake Wylie, are heavily built to survive earthquakes and other natural disasters. Instruments that measure ground movement at those plants did not detect anything Sunday, a Duke spokeswoman said.

This story was originally published August 10, 2020 at 3:04 PM.