Poster of the 2010 - 2011 Arkansas Earthquake Swarm
A magnitude 4.7 earthquake occurred near the town of Greenbrier in central Arkansas on Sunday night February 27, 2011 at 11:01 PM CST. The mainshock was followed in the next several hours by dozens of small earthquakes, including magnitude 3.8 and 3.4 aftershocks at 11:18 PM and 2:46 AM. This was the largest earthquake to date in the continuing earthquake swarm in the Guy-Greenbrier area known as the Guy earthquake swarm.
The Guy earthquake swarm includes hundreds of small earthquakes that have been occurring since August 2010. The area formed by the swarm is now a distinct 12- to 14-km long northeastern linear trend. The earthquakes since October 2010 have also expanded the area covered by the swarm trend, primarily to the southwest. The linear trend of earthquakes is now about twice the October 2010 length. North central Arkansas has a history of earthquake activity with a swarm of thousands of earthquakes, largest magnitude about 4.7, from 1982 to 1984, 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 to monitor the swarm activity. The AGS and CERI are investigating whether the recent earthquakes are naturally occurring or related to human activities. The earthquake locations plotted on the maps above are from the CERI data catalog.
Earthquake swarms are not unusual east of the Rocky Mountains; although previous instrumentally-recorded swarms have not involved as many small earthquakes as the central Arkansas swarms. Scientists dont know how long to expect swarms to last. Most swarms subside without producing earthquakes large enough to cause significant damage. Most of North America east of the Rocky Mountains has infrequent earthquakes that can strike anywhere at irregular intervals. Earthquakes cannot be reliably predicted.
Earthquakes occur on linear features called faults and most earthquakes are centered miles deep. At well-studied plate boundaries like the San Andreas Fault System in California, seismologists can often determine the specific fault on which an earthquake occurred. East of the Rockies, far from plate boundaries, that is rarely the case and it is much more difficult to link an individual earthquake to an individual, geologically mapped, fault.
Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over an area as much as ten times larger than in the west. 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).
This figure shows the distribution of earthquakes in north-central Arkansas that have occurred since June 2010. Earthquakes on the figure are color-coded, with red circles denoting earthquakes that occurred from June 4th, 2010 to February 14, 2011, and orange circles denoting earthquakes that occurred from February 15, 2011 to March 1, 2011. The space-time pattern shows a systematic shift in seismicity to the southwest starting on February 15, 2011. White triangles show the location of real-time seismic stations in the vicinity of the earthquake swarm that are used by the University of Memphis and USGS in their complimentary monitoring activities.
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