Yesterday morning at 5 a.m., a 3.6 magnitude earthquake shook the D.C region. I missed it, asleep for a change. But it made me think about other minor earthquakes I’ve experienced. One was in New York City. That tremblor woke me out of a deep sleep; I thought the boiler in our apartment building had exploded.

The mind seeks explanations — a passing jet, a roll of thunder — because if the earth is not solid and grounded, then what is? But such is the nature of life that even that which we think is certain sometimes turns out not to be.

Scientists interviewed about yesterday’s quake said it was nothing to worry about, that it’s just part of the “creaking and grinding” of our old planet, like the “settling of an old house,” the newspaper said. Even the ground beneath our feet needs to shift and stretch once in a while.