Wow, we just had quite a little tremor here in San Diego. It was pretty scary, even though it didn't last very long at all, because it was just a little rumbling, than one very strong jerk. Didn't really feel like any earthquake I've been it.

I assume this is the culprit. 3.9 sounds about right, especially considering the relative proximity; usually, the epicenter is well out into the ocean or desert. I guess I shouldn't have been surprised, as there have been quite a few earthquakes in Imperial County lately.