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Threat of Earthquakes Occurring in Central United States Still Alive

Threat of Earthquakes Occurring in Central United States Still Alive

PASADENA, Calif. — Earthquake activity in the New Madrid Seismic Zone in the central United States does not seem to be slowing down.  In a new study published in the journal “Science,” seismologists Morgan Page and Susan Hough of the U.S. Geological Survey investigate whether current quakes in the region could be aftershocks of large earthquakes that occurred 200 years earlier.

Using extensive computer modeling of aftershock behavior, they show that the dearth of moderate (Magnitude 6) earthquakes following the series of large earthquakes in 1811-1812, combined with the high rates of small earthquakes today, is not consistent with the long-lived aftershock hypothesis.

A debate has swirled in recent years, fueled in part by past studies suggesting that continuing New Madrid seismic activity could be the tail end of a long-lived aftershock sequence following the 1811-1812 earthquakes.   If modern activity is an aftershock sequence, the argument goes, then there is no evidence that stress is currently building in the zone. Instead, Page and Hough conclude that the current level of activity must be the signature of active, ongoing processes that continue to generate stress in the region –stress that we expect will eventually be released in future large earthquakes.  In other words, the New Madrid Seismic Zone is not dead.

The New Madrid Seismic Zone in the central United States produced 4 large earthquakes with magnitudes upwards of 7 over the winter of 1811-1812.  Over the last two centuries, small quakes have continued to occur in the zone at a higher rate than elsewhere in the central United States.  Geologic evidence also shows that large earthquake sequences occurred there in about 1450 A.D. and 900 A.D.

The paper, “The New Madrid Seismic Zone: Not Dead Yet,” is available online. Additional information about the New Madrid Seismic Zone and its history is available from the USGS online.

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Recent earthquakes in the New Madrid Seismic Zone (CEUS-SSC catalog, 1990-2008). (Larger image)
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A timeline of earthquakes in the New Madrid Seismic Zone (top) differs significantly from a typical aftershock sequence (bottom). A new study shows that earthquakes occurring today in the region are not aftershocks of the 1811-1812 earthquakes. Rather, they are evidence that stress is continuing to accumulate. Data source: CEUS-SSC catalog. (Larger image)

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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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