AN UPDATE THE VIRGINIA QUAKE: USGS DATASeptember 9, 2011
The USGS released its official data for the Virginia quake and I thought it bore updating here. You can find the data here:
As you look at this data, it is important to note that you should click the tabs at the top of the page to see all the USGS's most recent thinking about it.
There's a lot of speculation out there, including some by myself, that the Virginia quake could have been artificial. Arguing somewhat in favor of that hypothesis, but by no means nearly conclusively, is the fact that the USGS is reporting the quake to have occurred at a depth of 3.7 miles.
However, if one goes to the "Summary" tab at the top of the link above, one reads the following important information:
"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.
"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." (http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/se082311a.html#summary, emphasis added)
What this means is that there is no really good geological explanation for the magnitude of the Virginia quake except the possibility that a new fault or fracture may have been created, like cracking a rock under stress. But what produced the stress to begin with? In the past, prior to similar earthquake and even the tsunami in Indonesia, witnesses describe seeing either large electrical storms prior to the event, or strange "globes of light" and other EM-type effects. It remains, now, to be seen if witnesses in Virginia described seeing similar things. Either way, to my mind, the USGS data leaves open both the possibility that it was a natural, though unusual, event, and an event possibly triggered by artificial means.