Just before 3pm, I heard an explosion and felt a rumble. My first thought was that there had been a large explosion at SFO airport, about ten miles north of San Mateo - where I am located.
I grew up near Houston Texas, where refinery explosions are not all that rare. I was sitting at my computer, when all of a sudden, I heard a huge CAHRUMPPP! Followed by rumbling, which seemed to get stronger after a second.
My first impression was actually that something had exploded. Possibly at the San Francisco Airport (SFO) which is only about ten miles north of me.
I rushed to the door, because that's a good idea if it IS an earthquake, but I also expected to perhaps see smoke in the sky.
Outside, several other neighbors were making the same exit. We all held our hands up in the air in that universal "WTF" gesture. I spoke briefly with another neighbor - we both concluded it SOUNDED like an explosion, not an earthquake, but we had all felt the foundations shake.
Back in to the internet - and sure enough, less than three minutes later the USGS site had a 3.8 quake (preliminary) listed 2 miles southeast of Pacifica. This is a small coastal town south of San Francisco - Hiway one runs through it. And the San Andreas fault pretty much as well.
One hundred and five years after the great quake of 1906. Grim reminder to check the earthquake readiness kit.