Wouldn't you know, early Friday morning a light earthquake struck the southern Illinois-Indiana border area, a strike-slip event of magnitude 5.2. Being on the stiff North American craton and at the quiet hour of 4:37 local time, it was felt very widely. Over the next eight hours almost 30,000 people sent felt reports to the US Geological Survey (you can too on this page), and the resulting map shows no variations that might indicate crustal structure. A good place to sample what people felt ison Sciencewoman's blog, part of the ScienceBlogs collection.

Everywhere is earthquake country. This particular quake was in the Wabash Valley Seismic Zone, one of those sleepy but significant earthquake-prone areas to be found in parts of the country you'd think were seismically quiet.