19th century quake aftershocks still felt

EVANSTON, Ill., Nov. 4 (UPI) -- A new study suggests aftershocks of Midwestern U.S. earthquakes occurring in 1811 and 1812 are still being felt today.

The joint Northwestern University and University of Missouri-Columbia research, led by Northwestern Professor Seth Stein, suggests most small earthquakes that occur in the central United States are aftershocks of magnitude 7 earthquakes that occurred in the New Madrid seismic zone nearly 200 years ago.

"This sounds strange at first," Stein said. "On the San Andreas fault in California, aftershocks only continue for about 10 years. But in the middle of a continent, they go on much longer."

Co-investigator Professor Mian Liu of the University of Missouri-Columbia added: "Aftershocks happen after a big earthquake because the movement on the fault changed the forces in the earth that act on the fault itself and nearby. Aftershocks go on until the fault recovers …"

The difference, the researchers said, is that the two sides of the San Andreas fault move past each other at about one and a half inches a year -- which is fast on a geologic time scale. The New Madrid faults, however, move more than 100 times more slowly, so it takes hundreds of years to swamp the effects of a big earthquake.

The study appears in the Nov. 5 issue of the journal Nature.