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Research

Re-Evaluating the Causes and Hazards of South Carolina Earthquakes



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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.

The South Carolina seismic hazard map has a bullseye at the coast near Charleston, illustrating the importance of undersea geology for understanding earthquake causes and seismic hazards.
Above: The South Carolina seismic hazard map has a bullseye at the coast near Charleston, illustrating the importance of undersea geology for understanding earthquake causes and seismic hazards. No tectonic plate boundaries are shown because South Carolina is in the interior of the North American Plate. Map available online at http://earthquake.usgs.gov/earthquakes/byregion/southcarolina-haz.php.

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.

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 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 lines of data collection during John Behrendt’s USGS research cruise in 1981, with his geologic interpretations in the Helena Banks fault zone.
Above: 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 at https://pubs.er.usgs.gov/publication/mf2056. [larger version]

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.”

Related Sound Waves Stories
Tsunami on the Delaware River? Study of Historical Quake and Early East Coast Seismicity
Sept. / Oct. 2013

Related Websites
1886 Charleston earthquake
USGS
Seismic Profiling systems
USGS
Woods Hole Data Library
USGS
Earthquakes Hazards Program
USGS

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in this issue:

Research
Potentially Producible Gas Hydrate Found in Indian Ocean

Re-Evaluating the Causes and Hazards of South Carolina Earthquakes

Fieldwork
Future Fieldwork

Outreach
Duke TIP Summer Programs Visit USGS

Awards
USGS scientists Elected as AGU Fellows

Meetings
Advanced Model Training for Predicting Coastal Storm Impacts

Staff
Visiting Scholars Collaborate on CFD Modeling of Sand and Oil Agglomerates

Publications
Mercury Found in Birds Across Western North America

Cape Cod Susceptible to Potential Effects of Sea-Level Rise

Aug. / Sept. Publications

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