Ancient Fault Lines May Have Become Re-activated

Through analysis of high-quality broadband waveform data from the June 18 earthquake, Kim determined that the earthquake's epicenter occurred at a depth of 18±2 km (11.2 miles) below ground level, deeper than most earthquakes in stable continental regions. By combining this location with the June 25 aftershock, which occurred at 20 km depth, Kim suggests that the earthquakes can be attributed to a steeply dipping fault, known as the Caborn Fault, associated with a rift system once responsible for the breakup of an ancient supercontinent.

"Old continental crust contains a billion-year record of past tectonic activity. This area was once as seismically active as the Gulf of California is today," said Won-Young Kim. "The reactivation of this fault may be due to the forces that are moving the North American Plate over the Earth's mantle. The depth of this earthquake suggests that these forces are quite large, even though they far away from present plate boundaries."

The June 2002 earthquake is one of the largest seismic events instrumentally recorded for the Wabash Valley Seismic Zone, which extends to southeastern Illinois, southwestern Indiana and parts of western Kentucky. This zone is considered a source of strong earthquakes with geological evidence of prehistoric earthquakes of up to magnitude 7.5. The Wabash Valley Fault System, a fault system within the Seismic Zone, is probably the best documented fault system in the eastern United States due to past petroleum exploration in the area, yet seismologically it is poorly understood. It is known that many of the Wabash Valley faults extend into rocks from the Precambrian era, to at least 7 km depth.

Kim's research is the first to directly correlate an earthquake with one of the known faults in the Wabash Valley Fault System. His findings suggest that the strike-slip faulting on this Caborn fault was happening on a near vertical fault plane at 18 km depth, indicating that ancient buried faults associated with a possible Precambrian rift system are being reactivated by contemporary compressive stress.

"We don't yet understand how faults are reactivated, but it appears that some pre-existing faults are more likely to break than others. The study of this sequence should help us to determine the likelihood of future occurrences. More research on these anomalous quakes is required," said Kim.

The Lamont-Doherty Earth Observatory, a member of the Earth Institute at Columbia University, is one of the world's leading research centers examining the planet from its core to its atmosphere, across every continent and every ocean. From global climate change to earthquakes, volcanoes, environmental hazards and more, Observatory scientists provide the basic knowledge of Earth systems to inform future health and habitability of our planet.

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