Tuesday, November 06, 2012

USGS: Eastern Earthquakes - Rare But Powerful


Map shows earthquakes (circles) greater than magnitude 3.0 since 1974 plotted on the 2008 USGS National Seismic Hazard Map for the central and eastern United States. Warmer colors on this map indicate areas of higher hazard. Larger earthquakes are represented by larger circles.

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Although big earthquakes are rare in the Eastern United States, historic accounts have suggested that when they occur the shock waves are felt much farther away from the epicenter than with earthquakes in places like California.


The Charleston earthquake of 1886, which was estimated to be between a 6.6 and a 7.3 magnitude, was reportedly felt (cite) as far away as Boston, Massachusetts, Milwaukee, Wisconsin, Chicago, Illinois, Cuba and Bermuda.


The 1811-12 New Madrid earthquakes were reportedly felt over much of the Eastern United States, with the USGS reporting that:

  • People were awakened by the shaking in New York City, Washington, D.C., and Charleston, South Carolina.


Although this phenomenon has been talked about for years, direct measurements of a relatively large east coast seismic event have been lacking due to their relative rarity. 


All that changed last year when a 5.8 quake struck central Virginia.


The following shake map from the USGS shows the areas reporting shaking from two recent quakes, a 6.0 in Central California, and the recent 5.8 Virginia quake. By an incredibly large margin, the smaller eastern quake was felt over a much bigger area.




For more on this phenomenon, the USGS has details on a study, that is slated to be published next month in the Bulletin of the Seismological Society of America.


New Evidence Shows Power of East Coast Earthquakes Virginia Earthquake Triggered Landslides at Great Distances

Released: 11/6/2012 8:30:00 AM

Earthquake shaking in the eastern United States can travel much farther and cause damage over larger areas than previously thought.


U.S. Geological Survey scientists found that last year's magnitude 5.8 earthquake in Virginia triggered landslides at distances four times farther—and over an area 20 times larger—than previous research has shown.


"We used landslides as an example and direct physical evidence to see how far-reaching shaking from east coast earthquakes could be," said Randall Jibson, USGS scientist and lead author of this study. "Not every earthquake will trigger landslides, but we can use landslide distributions to estimate characteristics of earthquake energy and how far regional ground shaking could occur."


"Scientists are confirming with empirical data what more than 50 million people in the eastern U.S. experienced firsthand: this was one powerful earthquake," said USGS Director Marcia McNutt. "Calibrating the distance over which landslides occur may also help us reach back into the geologic record to look for evidence of past history of major earthquakes from the Virginia seismic zone."


This study will help inform earthquake hazard and risk assessments as well as emergency preparedness, whether for landslides or other earthquake effects.


This study also supports existing research showing that although earthquakes are less frequent in the East, their damaging effects can extend over a much larger area as compared to the western United States.


The research is being presented today at the Geological Society of America conference, and will be published in the December 2012 issue of the Bulletin of the Seismological Society of America.

(Continue . . . )



People who live on, or near, major fault lines are usually cognizant of the dangers, although not everyone is prepared to meet them. Those of us who live away from these seismic zones rarely give earthquakes much of a thought.  


But this study shows that a large eastern U.S. quake - even one that occurs several hundred miles away - can be felt, and potentially do structural damage.


Just as you don’t have to live on the coastline to be impacted by a hurricane, you don’t have to live on a fault line to be hit by an earthquake. 


Which is why emergency officials urge that while it is important to identify probably threats in your area, you really need to adopt an `All hazards’ approach to preparing.  You really can’t know if the next disaster to strike will be a tornado out of the sky, a shift in the earth’s crust, or a massive winter storm.



For more on how to prepare against `All Hazards', including those you may not automatically assume are a threat where you live, you should visit the following sites:


FEMA http://www.fema.gov/index.shtm

READY.GOV http://www.ready.gov/

AMERICAN RED CROSS http://www.redcross.org/


And lastly, you may wish to revisit some of my preparedness essays, including:


In An Emergency, Who Has Your Back?

An Appropriate Level Of Preparedness

NPM11: Are You Earthquake Prepared?