While all of San Francisco sits on top of the San Andreas fault, and suffers damage caused from many other faults, there are many parts of the city where damage is even worse due to liquefaction
If you live or work in the Marina, SOMA, FiDi, the Mission, Mission Bay, Dogpatch, or Hunter's Point, you likely do.
Liquefaction risk in SF
San Francisco's coastline looks different now than it did 150 years ago. As SF developed and grew quickly, the city expanded our scraggly coastline into a smooth, straight line by dumping landfill and sand into the Bay. Turns out, that isn't the most stable base to build your city on.
The Financial District, for example, has at least 60 buried ships and countless abandoned buildings underneath the modern skyscrapers of our beautiful skyline. What used to be the coastline is now a half-mile from the Bay. Parts of the Marina were created when a lagoon was filled with dune sand and building rubble from the Great Earthquake of 1906 to create space for the Panama-Pacific International Exposition. Mission Bay was previously an actual bay, until the city filled it in to expand the city.
What is Liquefaction???
Those filled-in parts of the city are built on soil that isn't as densely packed, so they are much more susceptible to damage. When a fault ruptures, seismic waves travel through the ground. These waves cause the shaking we feel, but they also change the properties of the ground we stand on. When this soft soil starts moving during an earthquake, it becomes like quicksand (liquefaction) and shifts the the structural integrity of buildings, which can lead to water and gas pipes bursting. If you live in one of these areas, it is even more important that you are prepared to act quickly, and potentially evacuate your building quickly.
So liquefaction is when the strength and stiffness of a soil is drastically reduced by earthquake-shaking or other rapid loading. It occurs when the space between individual particles of soil is filled with water, exerting a pressure on the soil that influences how tightly the particles are pressed together. Prior to an earthquake, the water pressure is relatively low. However, the shaking can cause the water pressure to increase to the point where the soil particles move easily with respect to each other (imagine shaking a bowl of oatmeal).
What Happened Last Time?
Liquefaction has been responsible for tremendous amounts of damage during earthquakes around the world, and specifically in the Bay Area. The 1989 Loma Prieta quake devastated the Marina in particular because of it's composition, and led to the 1990 Seismic Hazards Mapping Act requiring the California Geological Survey to map and publish liquefaction risk areas. Knowing is half the battle, and it's allowed us to fortify the buildings that are in the most compromising locations.
Sometimes the biggest challenge in convincing citizens to think serious about earthquake prep is that we as humans have short memories. We can talk about the 1906 Great Earthquake in SF, but we can picture it.
Want to know what a relatively minor earthquake looks like? Check out a few YouTube videos of Loma Prieta damage - you'll see the buildings we recognize on color video up in flames. You'll see overpasses that look like the ones we drive everyday, crumbling and destroyed.
How to Prepare
The good news is that getting earthquake prepared doesn't need to be hard. The are two main steps - first is having a plan, and the second is having the right supplies:
1) Sit down with your family or roommates tonight and talk through the basics... How will you meet up when there is no cell/internet service? Who is your out-of-town contact? Where are the danger spots in your home?