Historic Earthquakes: Tectonic Summary

Magnitude 4.6 ALABAMA
2003 April 29 08:59:39 UTC

TECTONIC SETTING AND SEISMICITY CONTEXT: EASTERN TENNESSEE SEISMIC ZONE

EARTHQUAKES
Eastern Tennessee Seismic Zone The Eastern Tennessee seismic zone, which extends from south west Virginia to north east Alabama, is one of the most active earthquake areas in the Southeast. Although the zone has not had a large earthquake in historic times, a few earthquakes have caused slight damage. The largest recorded earthquake in this seismic zone was a magnitude 4.6 that occurred in 1973 near Knoxville. Sensitive seismographs have recorded hundreds of earthquakes too small to be felt in this seismic zone. Small, non-damaging, felt earthquakes occur about once a year.

Earthquakes in the central and eastern U.S., although less frequent, are typically felt over a much broader region than the western U.S. East of the Rockies, an earthquake can be felt in an area as much as ten times greater than a similar magnitude earthquake on the west coast. For example, a magnitude 4.0 eastern U.S. earthquake typically can be felt at many locations as far as 100 km (60 mi) from where it occurred, and it might or might not cause damage near its source. A magnitude 5.5 eastern US earthquake usually can be felt as far as 500 km (300 mi) in most directions and can cause damage out to 40 km (25 mi).

FAULTS
At plate boundaries, earthquakes can commonly be related to specific faults or fault systems. In contrast, in the eastern Tennessee seismic zone the relation between faults and earthquakes is more enigmatic. The Eastern U.S. is far from the plate boundaries, the nearest of which are in the center of the Atlantic Ocean and in the Caribbean Sea. No active faults are known to reach the surface in the region, although the area is laced with ancient faults that developed as the Appalachian Mountains formed several hundred million years ago. The larger faults, particularly those that have been exposed at the Earth's surface by erosion, are likely to have been mapped by geologists. Unknown but probably numerous smaller or more deeply buried faults remain undetected. Even those faults that are mapped at the surface are poorly located at earthquake depths. Accordingly, few, if any, earthquakes in the eastern Tennessee seismic zone can be linked to known faults, and it is difficult to determine if a specific fault could still slip and cause an earthquake. As in most other areas east of the Rockies, the best guide to earthquake hazards in the seismic zone is the earthquakes themselves.

Epicentral Map PAST EARTHQUAKES IN ALABAMA
The documented history of small earthquakes in Alabama spans about 100 years and includes about half-a-dozen small- to moderate-sized damaging events. The largest recent earthquake recorded in the State was a magnitude 4.9, which occurred south of the Eastern Tennessee seismic zone near Atmore, Alabama, on October 24, 1997. For an in-depth summary of historical earthquake activity in the State, see the Earthquake History of Alabama.

SEISMIC MONITORING IN ALABAMA
The U.S. Geological Survey works in cooperation with the Center for Earthquake Research and Information (CERI) at the University of Memphis to monitor seismicity in the Alabama region. In response to the 1997 magnitude 4.9 Atmore, Alabama earthquake, the U.S. Geological Survey in cooperation with the State Survey of Alabama installed a seismic monitoring station near Birmingham, Alabama in 2001. This station is a key part of an upgraded seismic monitoring network being implemented by the USGS as part of its Advanced National Seismic System (ANSS). Additional stations are planned for the southeastern U.S., pending the allocation of Federal funds.

IMPACT OF THE QUAKE
Today's earthquake produced very limited damage. Cracked foundations and bricks fallen from chimneys were reported at Fort Payne. The water system at Valley Head also reported muddy water. Some schools in the region are closed as a precaution.

PUBLIC RESPONSE TO THE TREMOR
Thousands of web surfers have recorded their observations of today's earthquake on a USGS internet site designed to compile public observations into an integrated shaking map for the epicentral region. This "Did You Feel It" web site not only provides a concise summary of the distribution of perceived shaking, but it also provides researchers with data they need to supplement limited seismic recordings. The public is encouraged to visit the "Did You Feel It" site and record their own observations.