July-Aug-Milkshake-170x65
Jack’s

How Common Are Earthquakes In North Alabama? – A Look At Alabama’s Earthquake History

Posted on: 4:04 pm, March 4, 2013, by , updated on: 05:41pm, March 4, 2013

From time to time we like to post more than just weather updates on the blog, but different science information that we find interesting. Below is a guest post from amateur seismologist Steve Jones at AlabamaQuake (http://alabamaquake.com) on the history of earthquakes in Alabama.

Earthquakes in North Alabama – North and North-Central Alabama Seismicity
Earthquakes are actually fairly common in the eastern half of the United States, with many of these seen in the New Madrid Seismic Zone (NMSZ) which is situated along the Mississippi River, roughly centered at New Madrid, Missouri.

We might see light to moderate shaking in North Alabama if a strong earthquake struck in the NMSZ.  The NMSZ is known for a series of three strong earthquakes (with estimated magnitude between 7 and 8) that struck in 1811-1812.  The United States Geological Survey (USGS) estimates the chance of having an earthquake with a magnitude 6 or larger in the next 50 years is 25 to 40 percent, and the chance of seeing an event similar to one of the 1811–1812 sequence occurring in the next 50 years is about 7 to 10 percent.

But closer to home, we could also be affected by another seismically active area in our region, the Southern Appalachian Seismic Zone (SASZ), which roughly follows the Appalachian Mountains from southwestern Virginia into the northeastern corner and central part of Alabama (see Figure 1).  The USGS says that, “the Southern Appalachian Seismic Zone is one of the most active earthquake zones in the eastern United States.”

"Seismic

In terms of tectonic plate interactions, the NMSZ is known as an intraplate zone, located far away from a tectonic plate boundary.  Earthquakes occur here in the large network of faults, buried beneath hundreds to thousands of feet of mud, sand, and rock, which formed about 500 million years ago when the region was stretched in the northwest-southeast direction.  Additional faults have formed much more recently as crustal rifting/thinning, sediment loading, and downwarp processes have continued in the NMSZ.  Now the region is slowly undergoing east- west shortening, and the reactivation of old, subterranean faults within this rift system is thought to be responsible for the continuing seismicity of the zone, with most earthquakes occurring at depths of 4-12 kilometers.

The SASZ is also an intraplate seismic zone similar to the NMSZ.  The SASZ has released moderate levels of seismic energy for many hundreds of years.  The tectonic setting for SASZ is crustal uplift/compression, as opposed to the rifting/downwarp processes seen in the NMSZ.  No active seismic faults are known to reach the surface in the SASZ, although the area is laced with many ancient faults that developed as the Appalachian Mountains formed several hundred million years ago.  While many surface and buried faults have been located in the region, numerous unknown smaller and more deeply buried faults remain undetected, and these all have an ongoing potential for randomly located small-to-moderate earthquakes.

Northern Alabama Earthquake History
As part of the SASZ, the geology of north and north-central Alabama includes numerous buried and surface faults, and the state has seen many small earthquakes, and some 20 larger events, since shortly before the beginning of the 20th century (see Figure 2). The recorded history shows that the earthquakes have been small-moderate, never reaching above a magnitude of 5.1. This size of earthquake tends to cause local damage only near the quake epicenter, with shaking felt in areas further away.  Major Alabama events include those seen in 1916 in the Birmingham area, and near Fort Payne in 2003.

Alabama’s Largest Earthquake
The largest earthquake in Alabama’s recorded seismic history struck on October 18, 1916, on an unnamed fault located east of Birmingham.  The strongest effects were felt at Easonville, in the Irondale area.  Near the epicenter, chimneys were knocked down by the strong quake, windows were broken, and frame buildings were “badly shaken.”  Shaking by the earthquake was felt by residents in a seven-state area covering some 100,000 square miles.

North and North/Central Alabama Seismic Faults and Earthquake EpicentersImage courtesy of the Geological Survey of Alabama http://gsa.state.al.us/gsa/geologichazards/Quakes_AL.htm

North and North/Central Alabama Seismic Faults and Earthquake Epicenters
Image courtesy of the Geological Survey of Alabama
http://gsa.state.al.us/gsa/geologichazards/Quakes_AL.htm

North Alabama Earthquakes
On April 29, 2003, an earthquake with magnitude of 4.9 (some reports say 4.6) struck in DeKalb County, a few miles northeast of Fort Payne.  The focal depth of the quake was determined to be approximately 15 km (9.3 miles).  It caused some minor landslides in the area, and caused a shutdown of the underground water supply for the town of Valley Head.  Shaking from the earthquake caused some minor building damage in Fort Payne, including broken windows, minor cracks in masonry, and broken or collapsed chimneys.  The quake effects were felt across several Southeastern USA states.

Huntsville – Guntersville Area Earthquakes
An earthquake with an estimated 3.8 – 4.0 magnitude occurred just north of Huntsville on August 12, 1959.  Felt over a small area of southern Tennessee and northern Alabama, it shook bricks from chimneys at Hazel Green, and damaged another chimney and a newly constructed concrete block building at Meridianville.  Buildings were shaken violently at New Sharon, with canned goods being knocked from shelves and frightened residents fleeing from their homes to the outdoors.  The shaking cracked plaster walls and knocked grocery goods from store shelves in Huntsville.

In more recent years, the historical record shows that several small quakes have been seen in the Huntsville area.  These include a pair of events, measured at M2.2 and M2.5, in 2011; a magnitude 3.9 quake that occurred a few miles east of Huntsville in 2001; and a M3.0 quake seen east of Huntsville in 1984.

In April 1957, a small earthquake occurred east of Guntersville.  Though it was felt from northern Alabama to Georgia and Tennessee, and resulted in some cracking of walls and movement of some small objects from shelves and tables, it caused little substantial property damage.  According to local reports, there was “minor damage to several chimneys; one report of cement steps cracked in two; and several small cracks in walls. Table-top items tumbled to the floor.”

Recent Earthquakes
In the past few days, two magnitude 2.7 earthquakes occurred in Walker County, Alabama, near the city of Jasper, at similar focal depths in the range of 4-5 km (2-3 mi).  The seismogram of one of these quakes, the M2.7 event registered on March 01, is shown as recorded at the AlabamaQuake seismic station at a distance of some 122 km.

AlabamaQuake SeismogramEarthquake, Walker County, Alabama, Magnitude 2.7, March 01, 2013

AlabamaQuake Seismogram
Earthquake, Walker County, Alabama, Magnitude 2.7, March 01, 2013

It does not appear that any particular significance can be immediately drawn from the occurrence of these two events, even in such close location and time proximity, given the rich history of such small quakes in this area.  In Figure 4 below, the three most recent Walker County quake epicenters have been superimposed onto a close-up portion of the larger North and North-Central Alabama Faults and Epicenters map seen above.  Note the appearance of these three most recent quakes near identified buried faults traversing the area, and the proximity of these new quakes to numerous other events seen in the area.

Location of Three Most Recent Walker County, Alabama EarthquakesBase map image courtesy of the Geological Survey of Alabama http://gsa.state.al.us/gsa/geologichazards/Quakes_AL.htm

Location of Three Most Recent Walker County, Alabama Earthquakes
Base map image courtesy of the Geological Survey of Alabama
http://gsa.state.al.us/gsa/geologichazards/Quakes_AL.htm

Summary
North Alabama is not considered to be in a high-risk area for significant damages caused by earthquakes.  However, our proximity to the New Madrid Seismic Zone calls for a need to be aware of the potential for local impacts due to a strong earthquake occurring in that regionally-nearby area.  And the Southern Appalachian Seismic Zone which extends across much of our area is primarily responsible for the low-level seismicity and periodic small earthquakes, with occasional minor damages, seen in North and North-Central Alabama.

For more information, contact AlabamaQuake at alabamaquake@gmail.com.