Magnitude 4.1 - ARKANSAS

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2011 February 18 08:13:35 UTC

Earthquake Details

  • This event has been reviewed by a seismologist.
Location35.271°N, 92.377°W
Depth6.3 km (3.9 miles)
  • 5 km (3 miles) NNE (13°) from Greenbrier, AR
  • 7 km (4 miles) SE (140°) from Twin Groves, AR
  • 7 km (4 miles) SSW (213°) from Guy, AR
  • 60 km (37 miles) N (356°) from Little Rock, AR
  • 419 km (260 miles) SSW (208°) from St. Louis, MO
Location Uncertaintyhorizontal +/- 0.5 km (0.3 miles); depth +/- 0.4 km (0.2 miles)
ParametersNST= 16, Nph= 20, Dmin=3 km, Rmss=0.2 sec, Gp= 76°,
M-type=centroid moment magnitude (Mw), Version=D
Event IDnm1182
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Earthquake Summary

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Tectonic Summary


This earthquake is part of a swarm of earthquakes that began on Tuesday February 15, 2011 and is continuing. This area is slightly south of and most likely related to similar activity (known as the Guy earthquake swarm) of hundreds of small earthquakes near Guy, Arkansas from August 2010 to present. Central Arkansas has a history of earthquake activity with a swarm of thousands of earthquakes smaller than magnitude 4.5 to 4.7 in the early 1980s and another swarm in 2001 (known as the Enola earthquake swarms). The Center for Earthquake Research and Information (CERI) at the University of Memphis and the Arkansas Geological Survey (AGS) have deployed a local seismic array in the Greenbrier-Enola, Arkansas, area to augment regional seismic stations to carefully monitor this situation. USGS scientists have been working with their AGS and CERI colleagues. The CERI and AGS array and personnel are the best source of the most current information about the new earthquake swarm. The AGS and CERI are investigating whether the earthquakes occur naturally or are related to human activities.

Earthquake swarms are common east of the Rocky Mountains; although none of the others have involved so many small earthquakes as the central Arkansas swarms. Scientists don't know why swarms start, why they stop, or how long to expect them to last. The possibility of a larger earthquake cannot be discounted but none of the other swarms have given us any reason to expect an earthquake large enough to cause significant damage in central Arkansas in the near future. Most of North America east of the Rocky Mountains has infrequent earthquakes that can strike anywhere at irregular intervals. The causes of earthquakes are not understood well enough for us to predict earthquakes reliably.

Earthquakes occur on faults. Most earthquakes occur miles deep. At well-studied plate boundaries like the San Andreas Fault System in California, often seismologists can determine the specific fault on which an earthquake occurred. East of the Rockies, far from plate boundaries, that is rarely the case. Most of the known faults are deep, and probably there are other faults that have not been discovered. It is hard to link an individual earthquake to an individual fault. In most areas, the best guide to earthquake hazards is the earthquakes themselves.

Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than if the earthquake had occurred on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) and more from where it occurred, and it can cause slight damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes it causes damage as far away as 40 km (25 mi).