Understanding the Magnitude 5.8 Virginia earthquake (August 23, 2011)

Earthquake overview :  An earthquake in central Virginia was felt across much of the East Coast on Tuesday August 23 2011, causing light damage and forcing hundreds of thousands of people to evacuate buildings in New York, Washington and other cities.
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Read also : Understanding the Magnitude 5.8 Virginia earthquake (August 23, 2011)

Tectonic Summary
The Virginia earthquake of 2011 August 23 occurred as reverse faulting on a north or northeast-striking plane within a previously recognized seismic zone, the "Central Virginia Seismic Zone." The Central Virginia Seismic Zone has produced small and moderate earthquakes since at least the 18th century.
The previous largest historical shock from the Central Virginia Seismic Zone occurred in 1875. The 1875 shock occurred before the invention of effective seismographs, but the felt area of the shock suggests that it had a magnitude of about 4.8.
The 1875 earthquake shook bricks from chimneys, broke plaster and windows, and overturned furniture at several locations.
A magnitude 4.5 earthquake on 2003, December 9, also produced minor damage.

Seismic hazard map, courtesy USGS

Previous seismicity in the Central Virginia Seismic Zone has not been causally associated with mapped geologic faults. Previous, smaller, instrumentally recorded earthquakes from the Central Virginia Seismic Zone have had shallow focal depths (average depth about 8 km). They have had diverse focal mechanisms and have occurred over an area with length and width of about 120 km, rather than being aligned in a pattern that might suggest that they occurred on a single causative fault.
Individual earthquakes within the Central Virginia Seismic Zone occur as the result of slip on faults that are much smaller than the overall dimensions of the zone. The dimensions of the individual fault that produced the 2011 August 23 earthquake will not be known until longer-term studies are done, but other earthquakes of similar magnitude typically involve slippage along fault segments that are 5 - 15 km long.

Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., 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 a similar magnitude earthquake 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) from where it occurred, and it infrequently causes 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 causes damage as far away as 40 km (25 mi).

USGS Felt Reports
Widespread damage occurred from central Virginia to southern Maryland including the Washington D.C. area. Minor damage reported in parts of Delaware, southeastern Pennsylvania and southern New Jersey. Felt (VII) at Boston, Bumpass, Kent Store, Louisa, Mineral, Rhoadsville and Summerduck. Felt strongly in much of central Virginia and southern Maryland. Felt throughout the eastern US from central Georgia to central Maine and west to Detroit, Michigan and Chicago, Illinois. Felt in many parts of southeastern Canada from Montreal to Windsor.

I Have Felt It map courtesy USGS

Remark of earthquake-report.com ; Based on the I have Felt It reports that we have received (also several hundred), our max. values even in Central Virginia were VI.
We believe that the VII MMI, described by USGS as "moderate damage for resistant buildings" and "Moderate to Heavy damaged for vulnerable buildings" for central Virginia to southern Maryland is a little bit exaggerated unless for the direct epicenter area (Mineral, Louise, Cuckoo, etc). Of course, the values are not given by USGS but are based on the 132,553! I Have Felt it  Reports that they have received.
This remark is based on our experience in the field and the kind of damage we are experiencing for a certain MMI elsewhere.

Additional information material :

  • 15 slide Powerpoint presentation (zipped ppt/animation 6.90 MB / pdf 2.71 MB)
    Please note, animation (while referenced) is not embedded in the powerpoint

Animations and Visualizations

  • USArray Wave Propagation (mpeg-4 2.50 MB)
  • 24-hr Seismogram from closest station, CBN Fredericksburg, VA (08/23/11 571 KB) (08/24/11 885 KB)
  • 24-hr Seismogram from USArray station, KMSC Blacksburg, SC (08/23/11 867 KB) (08/24/11 917 KB)

Text and images courtesy USGS