When the rocks roll

kw: local events, earthquakes

It is just four hours since I felt my chair shake under me. I work on the ground floor, and from the Philly area the earthquake motion was rather subtle, or so I thought. Then I heard co-workers' voices as some of them left the building. From the third floor the motion was much more disturbing. Within a few minutes we had reports that the epicenter was between Richmond and Charlottesville, VA, and I knew it was rather a big one.

Big for the East Coast, anyway. An earthquake like this one would be considered small in California or anywhere around the Pacific Rim. But there is no active tectonic margin at this end of the continent. The faults that are still able to rupture are old and "sticky". However, we do need to remember that some of the largest earthquakes ever to hit North America were centered in Missouri.

A 5.8 or 5.9 is big enough to do damage near its center, even shift some buildings on their foundations a little. When I took earthquake geology oh, so many moons ago, we were told that the shaking near the epicenter of a 6.0 or 6.1 is as intense as it can get, and that larger and larger earthquakes just spread around the energy over larger areas. So as time goes on, I expect we'll hear at least a few stories of substantial damage from central Virginia.

And all this last few days my main worry has been Hurricane Irene!