1968 Illinois Earthquake - Geology

Geology

The quake struck on Saturday, November 9, 1968 at 11:02 a.m. The quake's epicenter was slightly northwest of Broughton in Hamilton County, and close to the Illinois–Indiana border, about 120 miles (193 km) east of St. Louis, Missouri. Surrounding the epicenter were several small towns built on flat glacial lake plains and low hills. Scientists described the rupture as "strong". During the quake, surface wave and body wave magnitudes were measured at 5.2 and 5.54 respectively. The magnitude of the quake reached 5.4 on the Richter scale. The earthquake occurred at a depth of 25 km (16 mi).

A fault plane solution for the earthquake confirmed two nodal planes (one is always a fault plane, the other an auxiliary plane) striking north–south and dipping approximately 45 degrees to the east and to the west. This faulting suggests dip slip reverse motion and a horizontal east–west axis of confining stress. At the time of the earthquake, no faults were known in the immediate epicentral region (see below), but the motion corresponded to movement along the Wabash Valley Fault System roughly 10 miles (16 km) east of the region. The rupture also partly occurred on the New Madrid Fault, responsible for the great New Madrid earthquakes in 1812. The New Madrid tremors were the most powerful earthquakes to hit the contiguous United States.

Various theories were put forward for the cause of the rupture. Donald Roll, director of seismology at Loyola University Chicago, proposed that the quake was caused by massive amounts of silt being deposited by rivers, generating a "seesaw" effect on the plates beneath. "The weight of the silt depressed one end of the block and tipped up the other," he said. However, scientists eventually realized the cause was a then-unknown fault, the Cottage Grove Fault, a small tear in the Earth's rock in the Southern Illinois Basin near the city of Harrisburg, Illinois.

The fault, which is aligned east–west, is connected to the north–south trending Wabash Valley Fault System at its eastern end. Seismographic mapping completed by geologists revealed monoclines, anticlines, and synclines, all of which suggest deformation during the Paleozoic era, when strike-slip faulting took place nearby. The fault runs along an ancient Precambrian terrane boundary. It was active mainly in the Late Pennsylvanian and Early Permian epochs around 300 million years ago.