35 Recent advances in our understanding of the New Madrid seismic zone
Thorough understanding of the New Madrid seismic zone will require integration of observations relating to surface features, the crust and the upper mantle. All contributions are welcomed.
Wednesday, 11 March 2026
8:30 AM - 12:00 PM
RCC, 104
Northern Alabama Earthquake Ring
Paper number: 35-6
Program: Technical Sessions
Time: 10:00 AM – 10:15 AM
Authors
Abstract
Instrumentally-recorded earthquakes in northern Alabama are distributed in a ring pattern 230 km in diameter (earthquakes are absent in its center). The width of the ring is ~70 km and the center is roughly 33.9o, -86.8o. The 136 earthquakes composing this ring have depths up to 24 km, and the largest is magnitude 4. The M4.8 Fort Payne, AL earthquake was near the NE edge of the ring, at the SW end of the eastern Tennessee seismic zone.
There is no expression of the ring in the Paleozoic bedrock. The ring overlaps the change from 1.5 Ga Granite-Rhyolite basement on the west to 1.1 Ga Grenville basement on the east. The presence of basement fractures in unknown because no basement core samples have been collected in this part of Alabama. However, segments of the ring follow arcuate features in the Bouguer gravity anomaly map. Furthermore, a large (110 km x 60 km), regionally strong (48 mgals) gravity anomaly is contained in the NW quadrant of the ring. This strongly positive anomaly is associated with regionally high S-wave velocities (3.8 km/s) at depths of > 15 km, and a 3D S-wave seismic tomography model of the associated high velocity region resembles a large laccolith with a feeder pipe. High S-wave velocities within the gravity anomaly are consistent with ultramafic composition.
A large ultramafic intrusion within the earthquake ring may have resulted from penetration into the mantle by a large bolide impact such that the earthquake ring follows the impact crater margin. If so, it is a very large crater, as the Vredefort crater (250 km diameter) is Earth’s largest reported impact crater. The Chicxulub crater diameter is 180 km. The position of the ultramafic intrusion in the NW quadrant suggests the bolide approached from the SE. Although the earthquake ring is circular, experiments show that crater shape remains circular for bolide approach angles >15o. This impact would have struck in post-Greenville/pre-Paleozoic time, and thus would be a candidate for triggering the Varangian glaciation.
Geological Society of America Abstracts with Programs. Vol. 58, No. 1
doi: 10.1130/abs/2026SE-411607
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