Earthquakes are a reality in Alabama, a fact that surprises many who assume seismic activity is limited to the western United States. While the state is not situated along a major tectonic plate boundary, these seismic events occur with a measurable frequency. The earthquakes experienced in Alabama are classified as intraplate, meaning they happen within the interior of the North American tectonic plate, far from its edges. This article explores the history, geological mechanisms, and current distribution of these seismic events.
Historical Overview of Alabama’s Noteworthy Seismic Events
The largest known earthquake to originate within the state was the Irondale event on October 18, 1916. Centered in north-central Alabama, this earthquake is estimated to have reached a magnitude of 5.1, making it the strongest on record. The tremors caused minor damage, primarily limited to Shelby and Jefferson counties, including fallen chimneys and cracked windows.
On October 24, 1997, a magnitude 4.9 earthquake struck in Escambia County. Shaking was felt across multiple states, and ground effects included large cracks in the sand along a creek. More recently, the Fort Payne earthquake in DeKalb County registered a magnitude 4.9 on April 29, 2003. That event was felt in 13 states and caused minor structural damage, such as broken windows, and muddied the underground water supply for a nearby town.
Alabama is also susceptible to major seismic events originating in nearby regions. The powerful 1886 Charleston, South Carolina earthquake (estimated magnitude 7.3) was felt strongly throughout the southeastern United States, including Alabama. The New Madrid Seismic Zone, which produced massive earthquakes in 1811–1812, continues to pose a threat, as a large event there could cause significant damage across northern Alabama.
The Geological Causes of Intraplate Earthquakes
Earthquakes occur in Alabama due to intraplate seismicity, despite the state’s location far from the edge of the North American plate. The entire continent is under constant, low-level compression from the movement of the surrounding tectonic plates. This enormous stress builds up over millions of years and is not easily released within the plate’s stable interior.
This accumulated stress often seeks out weak points in the Earth’s crust instead of fracturing new rock. These weak points are ancient fault lines that formed hundreds of millions of years ago during past continental collisions, such as the formation of the Appalachian Mountains. These deep, buried structures, sometimes referred to as basement faults, can be reactivated when compressional forces become too great.
When the stress exceeds the strength of the rock along one of these ancient faults, the fault slips, generating an intraplate earthquake. The energy released by these mid-continent earthquakes travels much farther than similar-sized quakes in the western United States. This is because the crustal rock in the eastern U.S. is older, colder, and more rigid, meaning a moderate earthquake in Alabama can be felt over a much wider geographic area.
Current Seismic Activity and Geographic Distribution
Modern seismic activity in Alabama is characterized by frequent, low-magnitude tremors that rarely cause damage. Most current seismic events are associated with the Southern Appalachian Seismic Zone (SASZ), also known as the East Tennessee Seismic Zone (ETSZ). This zone of moderate seismic risk extends southwestward from Virginia into the northeastern and central parts of Alabama, following the Appalachian mountain chain.
The majority of earthquakes recorded have a magnitude of 3.0 or lower. These smaller events are often not felt by residents and are only detectable by sensitive seismic monitoring equipment. A typical year may see several dozen very small earthquakes, with the largest event rarely exceeding magnitude 2.5.
Another area of influence is the Bahamas Fracture Seismic Zone (BFSZ), which extends through southern Alabama. While less active than the SASZ, the BFSZ was responsible for the 1997 Escambia County earthquake and continues to produce occasional seismic events. Clusters of small earthquakes have also been observed in areas like Greene County. The specific cause of these localized swarms is sometimes unknown or being investigated for possible links to deep, ancient fault movements.