Re-Evaluating the Causes and Hazards of South Carolina Earthquakes

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Unlike many other parts of the world where earthquakes occur along boundaries between tectonic plates, South Carolina earthquakes result from the reactivation of ancient geologic structures associated with much older tectonic events such as the building of the Appalachian Mountains and the rifting that opened the Atlantic Ocean.

This article is part of the August-September 2016 issue of the Sound Waves newsletter.

2014 Seismic Hazard Map

2014 Seismic Hazard Map of state of South Carolina.

South Carolina has earthquakes and the hazards that go with them, such as seismic shaking and ground failure. The 1886 Charleston earthquake caused extensive damage in the city and throughout the Southeast, and was felt as far away as Ontario, Bermuda, and Cuba. Unlike many other parts of the world where earthquakes occur along boundaries between tectonic plates—the Pacific “Ring of Fire” for example—South Carolina earthquakes result from the reactivation of ancient geologic structures associated with much older tectonic events, such as the building of the Appalachian Mountains and the rifting that opened the Atlantic Ocean.

In the 1980s, USGS geologist John Behrendt published important work on South Carolina earthquakes, based in part on offshore seismic reflection data collected by the USGS Woods Hole office in 1979 and 1981. Today, in light of additional information, University of South Carolina professor James Knapp is revisiting Behrendt’s interpretation, especially the nature of the offshore Helena Banks fault zone. To make a good case for the new intrerpretation, Knapp needed to examine the original seismic data.

Line drawing of a map shows the paths a ship followed near the coast while collecting data, and fault lines are labeled.

The lines of data collection during John Behrendt’s USGS research cruise in 1981, with his geologic interpretations in the Helena Banks fault zone. Map available online.

Knapp consulted his colleague Daniel Lizarralde at Woods Hole Oceanographic Institution, who had downloaded the digital data for the 1981 field activities from the USGS Woods Hole Coastal and Marine Science Center Data Library website, and knew about additional data at the Data Library that had not been posted online. In February, Knapp contacted Woods Hole Data Librarian Linda McCarthy and requested high quality digital images of the 1979 data and the associated navigation data. McCarthy digitized 11 seismic reflection data records from the 1979 expedition and sent them, with metadata (“data about data”), to Knapp in March.

The data library’s contribution will assist Knapp’s project to create an improved understanding of the geometry and origin of offshore faults, which could help to set seismic building codes in 21st century South Carolina, and guide decisions about offshore resource extraction and other planning issues. The new interpretation will also be a model for understanding other “passive” continental margins that are more seismically active than you might expect. As Knapp said later, “Preservation of these legacy marine geophysical data provides scientists a tremendous opportunity to re-evaluate the geology of the subsurface.”

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Sound Waves Newsletter - August-September 2016

USGS helps the Government of India discover large, highly enriched, producible accumulations of natural gas hydrate in the Bay of Bengal, USGS re-evaluates the causes and hazards of South Carolina earthquakes, and more in this August-September issue of Sound Waves.