Tuesday, September 22, 2020

Magnitude 5.1 earthquake strikes North Carolina, felt hundreds of miles away

Aug 09. 2020
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By The Washington Post · Matthew Cappucci · NATIONAL, SCIENCE-ENVIRONMENT

A preliminary magnitude 5.1 earthquake occurred near Sparta in northwestern North Carolina at 8:07 a.m. Sunday, near the Virginia border. The quake rattled the Carolina Piedmont, but was reportedly felt as far away as Atlanta, part of Tennessee, Washington, D.C., and Ohio.

It appears to be the strongest North Carolina quake in more than a century, since the 1916 quake that struck near Asheville in Buncombe County. That quake was also likely a magnitude 5.1 or slightly stronger.

It is the strongest tremor in the Mid-Atlantic since the 5.8-magnitude event on Aug. 23, 2011, in Mineral, Va.

There were some reports of minor damage in the vicinity of Sparta, including of items tossed off shelves at a local grocery market and belongings shaken off their perches atop shelves and shattered. Some power outages were also reported in Sparta.

Social media revealed a photo of one home partially damaged. There were no immediate reports of injuries.

Sunday's quake occurred at a shallow depth estimated at 2.3 miles (3.7 kilometers) deep, according to the U.S. Geological Survey (which has been revising this number), amplifying the effects felt at the ground. Earthquake surface waves - the kind that produce shaking we feel on the ground - travel farther on the East Coast than in the West. That's because crust east of the Rockies is less fragmented and more consolidated.

A magnitude 2.6 foreshock was registered at 1:57 a.m. near the epicenter of Sunday morning's larger tremor.

Numerous aftershocks, many of which will be felt in the immediate vicinity of the 5.1 magnitude quake, are likely for upwards of a week to come as the earth continues to settle, shift, and release stress.

It is highly unlikely, but not impossible, that the 5.1 quake is itself a foreshock of a larger earthquake to come. That was the case in Ridgecrest, Calif., when a magnitude 6.4 quake on July 4, 2019, was followed by a 7.1 magnitude quake the next day.

At first glance, Sunday's earthquake appears to have originated near the Eastern Tennessee seismic zone, a strip of seismic activity that stretches from the high terrain of eastern Tennessee northeast up the Appalachians into the Carolinas. At least five magnitude 5 or greater quakes have occurred in that pocket since the late 1600s, including near Sparta.

While rumors did briefly circulate on social media, Sunday's quake did not appear to be associated with hydraulic fracturing, or "fracking."

Initial data suggests Sunday's quake was from an oblique reverse slip event. Reverse faults are fractures in the earth's crust that occur at an angle. Ordinarily, one block of crust is displaced vertically above the other. The higher fault, known as the "hanging wall," is forced up the lower "foot wall." Reverse faults feature the buildup of compressional stress. The Rocky Mountains and Himalayas were both formed as a result of reverse fault processes.

Thrust faults are a type of reverse fault where the interface between two blocks of crust is closer to horizontal than it is vertical. That in essence causes one plate to slip beneath another.

Sometimes, the blocks slip laterally alongside one another while one moves beneath the other. That appears to be the case with Sunday morning's event, making it an "oblique reverse" slip.



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