Yeah, the sound is so low, and so deep and coming right out of the ground. Nothing else like it, people try to say it's like this, or that, but it's not, it's unique.
And remember, earthquakes are measured with a Richter Scale, so that each number is double the strength of the one before it, so a 6. is twice as strong as a 5. The Loma Prieta (89 World Series) had a magnitude of 6.9, and the big one in 06 is estimated to be around 8.2 - 8.3.
I was on the 16th floor of an office building once for a 5.2, talk about rocking.