Detailed analysis of regional and teleseismic waveform data from the June 18, 2002, Evansville, Indiana earthquake indicates that the earthquake occurred at a depth of about 18 km (±2 km). The source mechanism determined from regional waveform analysis shows predominantly strikeslip faulting along near-vertical nodal planes with a near horizontal P axis (plunge= 10° and trend= 252°). The orientation of the NNE striking nodal plane (strike= 28°, dip=82) of the source mechanism is consistent with the overall trend of the high-angle, normal faults of the Wabash Valley fault system, in particular, the compound Caborn fault near the epicenter (see, Figure 7, above). Hence, the nodal plane striking NNE is the more likely fault plane and this implies that the fault motion is right-lateral strike-slip.

On June 18, 2002, a magnitude 5.0 earthquake occurred in southern Indiana, followed by a 1.2 magnitude aftershock on June 25, 2002. Because the region of occurrence, the Wabash Valley Seismic Zone, is seismically active, Dr. Won-Young Kim, a seismologist with the Lamont-Doherty Earth Observatory at Columbia University, conducted research to determine the potential hazard of future earthquakes to this region. His findings suggest that an ancient fault line dating back to the Precambrian era of geological history (from 4.6 billion to 570 million years ago) has become reactivated and was the likely cause of the June 2002 earthquakes. Kim is presenting his findings at the Seismological Society of America in May, and publishing in the Bulletin of the Seismological Society of America.

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 w ith 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 are 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.

"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.

The Earth Institute at Columbia University is the world's leading academic center for the integrated study of Earth, its environment, and society. The Earth Institute builds upon excellence in the core disciplines -earth sciences, biological sciences, engineering sciences, social sciences and health sciences -and stresses cross-disciplinary approaches to complex problems. Through its research training and global partnerships, it mobilizes science and technology to advance sustainable development, while placing special emphasis on the needs of the world's poor.

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