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Thursday, November 24, 2011
Pre-Shocks are More Correctly Foreshocks
Following the 5.8 earthquake in Central Virginia in August which damaged the Washington Monument in Washington DC, the National Cathedral there also, and did 58 plus million dollars worth of damage in Louisa, Va, the aftershocks just keep on coming. This quake was also meaningfully felt by those having meetings in skyscrapers in Toronto. There have been in excess of 100 aftershocks since then if you couple those reported by the US Geological Service, along with Virginia Tech, Canada's reporting body, and others. USGS has been slow to detect and to report some of these. To their credit, they are out placing additional listening devices in the area now.
In general, aftershocks are not harmless. Buildings, roads and infrastructure which may be initially damaged by the first quake, may be additionally damaged by aftershocks. In Louisa, a road was not visibly damaged by the initial quake, but collapsed completely during an early aftershock, leaving a sink hole in the road.
It has now been three months since the initial quake, and these smaller magnitude quakes continue. This weekend, we had five alone, ranging around the 2 s but as severe as 2.5 . I have asked a number of times, when we can tell what is truly an aftershock from the original event, and what is, in fact, a harbinger of a severe quake to come, a "pre-shock" ? It seems that the correct word is foreshock.
Apparently, aftershocks can be of severe magnitude, are unpredictable, and can result in serious damage. Foreshocks, which can be harbingers of higher magnitude seismic activity in the near future, cannot be well differentiated from aftershocks. So, we sit wondering which is which. Is an additional 5.8 to 5.9 coming soon, or will the next one be 8.3 ? No one can really tell us with any authority, and perhaps even if they could, they wouldn't anyway.
My own conclusion is that central Virginia is seismically active, and that families need to earthquake proof their homes, and be very attentive to shelving and furniture placement. Placing cribs under windows may be unwise now. Recently, my daughter visited the interior of Luray Caverns, underground in Virginia, which was quite impressive. They lost one small stalagmite during the quake in August. The staff reported that they have not felt or had damage from the aftershocks since. It was well worth the trip. I think that was brave for her to go underground while we are still continuing to have all these aftershocks..