Um, so what is an earthquake anyway?

EARTHQUAKE! Now with pictures, too!
Community Newspaper Group / Dan MacLeod

Did Tuesday’s tremors rock you to the core? Maybe because you don’t know what an earthquake is. Here’s our primer:

So what’s an earthquake?

The surface of the Earth is divided into large tectonic plates that are in constant motion. The plates rub against or sink beneath each other — and when the pressure builds, they suddenly break apart at a fault line, releasing shock waves that cause an earthquake. Usually quakes occur along plate boundaries, but because Virginia is in the middle of a tectonic plate, Tuesday’s quake was caused by seismic activity within the plate itself.

So why did we feel it here?

The tremor was felt across the East Coast because rock formations here are older and harder — therefore transmitting waves more quickly than in the West.

But that doesn’t happen here very often, does it?

No, it’s doesn’t. The Eastern Seaboard has fewer fault lines and they aren’t very active. The last time a major tremor hit New York was in 1884, when a 5.3-magnitude quake rattled Brooklyn. Last year, the city barely felt a 5.0 quake located around Ottawa, Canada.

— Kate Briquelet