Isn't that quite a story on the East Coast? An earthquake in D.C. Who'd have thought?
Anyway, Mr. Rewrite spent some time in the Bay Area, so he's up on earthquake terms. That's why he's rattled by media accounts calling this a tremblor. The correct term, believe it or not, is temblor. He understands the confusion, since earthquakes result in tremors. But don't risk offending a scientist by botching the term.
Paul Brians' Common Errors in English Usage offers this take on temblor/tremblor.
The AP Stylebook's entry on earthquakes lists tremblor as an erroneous alternative to temblor.