Hi Chris. I rather doubt that it was the “P” wave they felt. Compressional waves, also known as primary or “P” waves travel fastest, at speeds between 1 to 5 miles per second in the Earth's crust. Shear waves, also known as secondary or “S” waves, travel more slowly, usually at 60% to 70% of the speed of “P” waves. If the foreshock that was felt 20 seconds before the main event was the “P” wave this would mean that the quake occurred about 100 miles from San Francisco instead of the 3 to 5 miles from San Francisco.

However, and this is one huge however, if the magnitude of the quake was larger then the current estimate of M>7.9 to M>8.3 then it could have indeed occurred around 100 miles from San Francisco. This would mean the quake came close to being an M>9.0 quake. I don’t know of anything that would support this. Take Care…Don in creepy town