On this day in 2011, at about ten to two in the afternoon, a magnitude 5.8 earthquake broke loose under Louisa County, Virginia, some sixty kilometres northwest of Richmond. It was felt by more people than any other earthquake in United States history.

Here is the kitchen-table version. Tap a cracked old saucepan and you get a dull thud; the flaws swallow the sound. Tap a good solid one and it rings, and keeps ringing. The crust under eastern North America is the solid pan: ancient, cold, comparatively unbroken, so seismic waves travel through it for hundreds of miles instead of dying out nearby. A moderate quake in California is a local event. The same quake in the Piedmont reaches Toronto and Atlanta. Aftershocks followed, the largest around magnitude 4.5.

The part I keep coming back to is the people at the instruments. Their own offices were swaying, and they still logged the arrival times, located the epicentre and got the numbers out. I once dropped an entire rack of samples because a lorry went past the window, so I know what steady hands are worth.

The photograph below shows the marble pyramidion at the top of the Washington Monument, with a jagged crack through one slab. It was repaired over the following years, panel by panel, by people on ropes.