Last week’s early morning (3:39 a.m.) earthquake, which roused some floating home residents from a deep sleep, is a reminder that we are definitely on Terra Non Firma. The December 17 temblor was centered in El Cerrito—only 10 miles away—and was measured at a magnitude 3.6. The tide was about 3.5 feet so those who were floating slept through it, but those sitting on mud could feel the seismic waves as they passed through the soft Bay deposits that all our homes rest upon at low tide.
Our floating homes community sits between the San Andreas fault and the Hayward-Rodgers Creek fault in one of the most seismically active regions in the U.S. These faults mark the boundary between two major plates, the North American on the east and the Pacific on the west. These plates have been grinding past each other for about 30 million years. An earthquake occurs when the stress from the force of a moving plate overcomes the friction that holds plates together at the boundary. The accompanying U.S. Geological Survey map shows the faults and the predictions for earthquakes.
The last major earthquake in our region was the Loma Prieta event on October 17, 1989, 82 years after the Great San Francisco Earthquake. There is a 22% chance of a magnitude 6.7 earthquake occurring between now and 2043 on the San Andreas fault and a 33% chance on the Hayward-Rodgers Creek fault.
When an earthquake hits, seismic waves travel through the earth in all directions from the epicenter. The shaking we feel depends on the ground beneath us. During an earthquake, an observer standing on bedrock up in the hills would not feel as much shaking as one standing at the edge of the bay. This is because soft muds, like the Bay mud deposits under our docks, shake much harder than bedrock. The graphic above illustrates how the geology can affect the degree of ground shaking during an earthquake.
The 1989 Loma Prieta Earthquake was over 70 miles to our south and, although Marin County did experience some noteworthy ground shaking, there were no fatalities and damages in the county were only $1.6 million of the $10 billion Bay Area total. The tide was about 3 feet when the Loma Prieta earthquake hit at 5:04 p.m. on October 17, and some residents reported watching the docks “shake like a serpent”. Others felt their houseboats shake and damage occurred when piling guides and metal collars broke away from the hulls, in some instances, leaving certain houseboats secured to the dock by only their water, gas, and sewer lines.
The floating homes community has yet to experience a major local regional earthquake. Such an earthquake on the San Andreas fault, Hayward-Rodgers Creek fault, or one of the other regional faults could result in ground shaking far more violent than was experienced in 1989, and damage to houseboats and surrounding marina infrastructure could be significant. While we hope the next big earthquake hits at high tide when our houses are floating (seismic waves don’t travel through water), the worst-case scenario would be an earthquake during low tide when most of our homes are sitting on the Bay mud and thus susceptible to strong ground motion. Ground failure caused by liquefaction (when strong shaking turns Bay deposits into a sandy liquid), consolidation, and settlement are possible and could damage houseboats, docks and utilities including fire supply, electricity, sewer and water. We would likely be on own during a major earthquake as the local and county fire protection/rescue services would be pulled thin and busy responding to emergencies and evacuations in the hills of Sausalito and Mill Valley. That is why it is important to have an emergency preparedness plan so that when the earthquake hits, you can get yourself and your neighbors to safety.
Author Pete Hudson is a Certified Engineering Geologist and lives on Main Dock.
NEXT MONTH: What you and your neighbors can do to prepare for a major earthquake.