Might be something about the geology in this area … a couple months ago city crews were jackhammering in the street near our house in that vicinity and the house shook like an earthquake. Qwest crews are often working in something underground that’s accessed nearby and we joked uneasily that it’s probably a huge hollow underground vault stretching for blocks. The OP is probably best advised to drive a few blocks’ radius if she/he hasn’t already, to look for a construction site or evidence of recent work.