Earthquakes In Eastern US
Comparing Richter scale values of earthquakes is demonstrated in the following chart that can be downloaded;
East of the Mississippi River in the US, earthquakes noticed by inhabitants are relatively rare compared to constant ground shaking action along the US west coast, Alaska and other earthquake prone regions of our volatile planet.
However, the unusually strong tremor (8-23-11) centered in southern Virginia is a "shocking" reminder that, no matter how low we think the risk might be, there is always at least some risk that the ground might move underneath us just about anywhere. Initial reports show the quake was 5.9 on the Richter Scale, more than strong enough to cause significant damage to buildings that were not designed with even the thought of earthquake (seismic) forces.
Much to my surprise, I felt the quake here in Belmar, New Jersey, about 250 miles away (northeast) of the epicenter. First floor of the two-story building moved back and forth for a good 5 to 7 seconds at least.......quite the strange sensation for those of us who have never really felt the ground shake a building like that before.
Interesting is that, per the standard USGS earthquake maps (see ASCE 7-05 and the building code; IBC 2009), seismic force for design of buildings in and around Richmond, Virginia is very low. This quake may result in changes to the seismic map.
Before looking at the seismic map, it is worth noting that forces applied to a building during an earthquake are caused by inertial effects as the ground moves. For building design, we calculate seismic loads using standard formulas based on research and actual measured results from past earthquakes.
Areas around southeastern corner of Missouri and mid-coast of South Carolina are designated high risk earthquake areas, based on known major earthquakes that occurred since colonial times.
For comparison purposes, we can look at "response acceleration" factors (used for building design) from the standard seismic maps.
Using the "0.2 second" factors for "Site Class B", we see major differences in design values, which can be taken as representing relative risks;
- 200 along narrow fault line corridors; 270 peak near LA
Missouri, southeast corner
- 300 over a large area, extending into southern Illinois, northwest Arkansas and western edge of Tennessee
South Carolina, mid coast
- 200 over small area; 258 peak
New York City, New York 35
Washington DC 15 to 20
Northwest of Richmond, Virginia 15 to 20
See the USGS site for much more information about earthquakes.