I have always wondered what happens in a marina when there is an earthquake (true confessions here, I am a geologist and I should know, but really don't). Mostly what we see is marinas getting blasted from tsunamis associated with the quake or submarine landslides. But given the very low density of water (and its elasticity) it can't transmit the type of waves that cause the ground to move up and down (called "S" or secondary waves). However water can transmit "P" or primary waves which are the very fast waves that arrive first. But because these waves are longitidinal or compressional, they really don't shake things all that much.
So my guess is that you felt the primary wave on your boat, which is cool. Alternatively, the up and down motion of the shoreline from the secondary waves may have resulted in sloshing of the boat in an irregular way. It was a pretty small quake (4.8), so who knows.
That's the depth of my knowledge on this. Fortunately, I never had to teach it at an advanced level.
My fascination with this is because our boat, tied up next to Fisherman's Wharf in San Francisco, is considered the family earthquake escape pod when the large quake comes (I work there, my daughter and boyfriend live there). San Francisco gets very large quakes with a recurrence interval of roughly a century. Since 1906 was the last really big one, we are just counting time until the next. Unlike our friends in Seattle, the San Francisco Bay--given its configuration and location of major faults on land--is unlikely to see a tsunami of any size following a quake. The plan is to crawl out of the mess, head to the tug, and motor back to Sacramento, with a hot shower, water and food on the escape pod.
Ranger Tugs 27: the Swiss Army Knife of boats.
Jeff
