During a day sail it is always too rough to see seismic waves. 

After all the discussion about Tsunami’s entering the SF Bay which appeared in the March 2018 Latitude 38, I felt compelled to report here in our blog, that under the right conditions seismic waves from local earthquakes can actually be observed inside San Francisco Bay.

Everyday in California there are dozens of minor earthquakes above 1.0 on the rector scale too small to be felt ashore (it usually takes a 3.0 to be felt ashore). Yet, we don’t usually notice the seismic waves out on the bay, because they get lost in all the daytime chaos of wind waves, ferry wakes, steamer wakes, current eddies, and other yacht wakes, which are usually present during daylight when we are usually out on the bay. But, under the right conditions seismic waves can easily be seen on the bay.

For example, on Thursday evening. November 2, 2017, at 11:00 PM we had a windless, moonlit night providing perfect conditions for observing seismic waves.

Our Schooner SEA RAVEN had departed from Brisbane Marina at 9:30 PM. We motored leisurely Northward at about 4.5 knots, bound for Richardson Bay where we planned to drop anchor for the evening, preparatory to our fall cruise out to Drakes Bay and the Farallones the following morning.

By 11:00 PM, the South Bay had become a mirror reflecting the full moon. Being a week night, the ferries had made their last run from Oyster Point at 8:50 PM, and from SF to Oakland/Alameda at 10:00 PM, and there were minimal currents with high slack at the Golden Gate occurring at 11:40 PM. So, the South Bay was flat calm with no wind waves, and no other boats were out at this hour on Thursday night.

Yet, unbeknownst to us as we glided northwards, there was a 1.2 earthquake, 7 km SSE of Ridgemark, CA. (which lies just SE of Hollister, CA 150 miles to the SE of our location) at 10:45 PM. We were in mid-channel due West of the former Alameda Naval Air Station headed ing for the Oakland Bay Bridge when, at approximately 10:47 PM, we encountered two dozen four foot high swells about 50 yards apart coming up on our starboard quarter from the South East.

These were not anything like the waves created by a steamer wake, but were instead just two dozen very regular four foot high swells that stretched from horizon to horizon, in parallel lines, where steamer wakes come at you in a series of about three. So, I wrote this strange occurrence in the log book.

The consensus of all six of us on board was to suspect that they were seismic waves……. Later, I found the specific earthquake that caused these seismic waves, listed on the USGS website’s earthquake calendar (which you can search by date, location and magnitude). Unfortunately, for some months I had been looking for a 3.0 or higher trembler, not realizing that the bay was sensitive enough to record a 1.2……I also later learned that seismic waves travel at around 2 km per second which is nearly 75 miles per minute and these had came from roughly 150 miles away in approximately 2 minutes. All of which ads up.

Alan Hugenot, Master Schooner SEA RAVEN