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Nearly 60 Small to Moderate Earthquakes Strike Arkansas and Are Widely Felt



Nearly 60 Small to Moderate Earthquakes Strike Arkansas and Are Widely Felt

Nearly 60 small and moderate earthquakes struck Arkansas since Feb. 15, 2011, the most recent a magnitude 4.3 earthquake this morning 37 miles away from Little Rock. Many of the earthquakes are large enough to be felt.

“These earthquake swarms are not that unusual for the region,” said Harley Benz, scientist in charge at the U.S. Geological Survey National Earthquake Information Center. “Central Arkansas has a history of earthquake activity with a swarm of thousands of earthquakes smaller than magnitude 4.5 in the early 1980s and another swarm in 2001, known as the Enola earthquake swarms.”

Citizens are encouraged to report any experience feeling the earthquake(s) at the USGS Did You Feel It? website. The earthquake swarm that began Tuesday may continue. This area is slightly south of and most likely related to similar ongoing activity involving hundreds of small earthquakes near Guy, Ark., from August 2010 to present.

Scientists do not 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 caused any reason to expect a future earthquake large enough to cause significant damage in central Arkansas.

USGS scientists have been working with their partners at the Center for Earthquake Research and Information (CERI) at the University of Memphis and the Arkansas Geological Survey (AGS) to carefully monitor this situation. They have deployed a local network of stations – an array – that measure seismicity in the Greenbrier-Enola area to augment regional seismic stations. 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; other Central or Eastern U.S. swarms, however, have not involved as many small earthquakes as the central Arkansas swarms. 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 to reliably make short-term predictions.

USGS Newsroom



More information

Parameter Value Description
Magnitude mb The magnitude for the event.
Longitude ° East Decimal degrees longitude. Negative values for western longitudes.
Latitude ° North Decimal degrees latitude. Negative values for southern latitudes.
Depth km Depth of the event in kilometers.
Place Textual description of named geographic region near to the event. This may be a city name, or a Flinn-Engdahl Region name.
Time 1970-01-01 00:00:00 Time when the event occurred. UTC/GMT
Updated 1970-01-01 00:00:00 Time when the event was most recently updated. UTC/GMT
Timezone offset Timezone offset from UTC in minutes at the event epicenter.
Felt The total number of felt reports
CDI The maximum reported intensity for the event.
MMI The maximum estimated instrumental intensity for the event.
Alert Level The alert level from the PAGER earthquake impact scale. Green, Yellow, Orange or Red.
Review Status Indicates whether the event has been reviewed by a human.
Tsunami This flag is set to "1" for large events in oceanic regions and "0" otherwise. The existence or value of this flag does not indicate if a tsunami actually did or will exist.
SIG A number describing how significant the event is. Larger numbers indicate a more significant event.
Network The ID of a data contributor. Identifies the network considered to be the preferred source of information for this event.
Sources A comma-separated list of network contributors.
Number of Stations Used The total number of Number of seismic stations which reported P- and S-arrival times for this earthquake.
Horizontal Distance Horizontal distance from the epicenter to the nearest station (in degrees).
Root Mean Square sec The root-mean-square (RMS) travel time residual, in sec, using all weights.
Azimuthal Gap The largest azimuthal gap between azimuthally adjacent stations (in degrees).
Magnitude Type The method or algorithm used to calculate the preferred magnitude for the event.
Event Type Type of seismic event.
Event ID Id of event.
Event Code An identifying code assigned by, and unique from, the corresponding source for the event.
Event IDS A comma-separated list of event ids that are associated to an event.

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