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
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.
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.
