1916 Irondale earthquake
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| Date | 18 October 1916 |
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| Origin time | 22:04 UTC |
| Magnitude | 5.1 ML |
| Epicenter | 33°32′N 86°41′W / 33.53°N 86.69°WCoordinates: 33°32′N 86°41′W / 33.53°N 86.69°W |
| Areas affected | Alabama United States |
| Max. intensity | VII (Very strong) |
| Casualties | no fatalities |
The 1916 Irondale earthquake struck north–central Alabama on October 18. The greatest earthquake in state history, it registered an estimated magnitude of 5.1 on the Richter magnitude scale and resulted in extensive minor damage. Damage, limited to Shelby and Jefferson counties, reached its maximum severity near the epicenter in the city of Irondale, including cracked windows, fallen chimneys, and dried-up wells. While there were no fatalities, the earthquake spawned widespread panic, sending alarmed workers from tall buildings.
The earthquake originated in the Eastern Tennessee Seismic Zone, a fault noted for earthquakes of moderate magnitude. Faulting in the area is strike-slip-oriented probably because of the Alabama-New York Lineament, which runs adjacent to the fault. Several scientists believe that the small earthquakes from the zone highlight the reactivation of deep, ancient faults. Alabama has seen roughly 20 earthquakes since the beginning of the 20th century. The earthquakes have been moderate, never reaching above magnitude 5.1, and tend to cause damage only near their epicenters while reaching areas much further away. Major events include those in 1916, 1997, and 2003. Despite the lack of powerful seismic events in Alabama, earthquakes from nearby fault zones, including the New Madrid Seismic Zone, pose a serious threat to infrastructure. Even an earthquake similar in size to the 1916 Irondale event today could damage thousands of buildings and cause up to $1 billion in damage.
Seismicity in the south–central United States is moderately powerful. The energy from a mid-continent earthquake extends over as much as ten times area of a similarly sized earthquake in the western United States. A magnitude 4.0 earthquake west of the Rocky Mountains reaches up to 60 miles (97 km) from its epicenter, while a magnitude 5.5 earthquake in the eastern US can be felt 300 miles (483 km) from its epicenter. Science failed to explain the phenomena until 1978, when geologists discovered a massive fault line running from the middle of the continent to its eastern coast. More recent magnetic surveys show that the structure is a 500-million year-old strike-slip fault, which starts under the Eastern Tennessee Seismic Zone and runs to the eastern coast. This corresponds to the strike-slip faulting exhibited in the Eastern Tennessee Zone, and explains the presence of mid-continent earthquakes in the United States. Still, it is not clear if the fault is definitely related to mid-continental seismicity.
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