Do You Live On One of SF's Liquefaction Zones?
If you live or work in the Marina, SOMA, FiDi, the Mission, Mission Bay, Dogpatch, or Hunter's Point, you likely do.
How SF Set Itself Up for Damage
San Francisco's coastline looks different now than it did 150 years ago. As SF developed and grew, the city expanded our scraggly bay coastline into a smooth, straight line. As SF historian Katelin Ghormley explains, the red areas are mostly pre-WWI landfill. If you remove them, you see the natural coastline & what it looked like before the Gold Rush of 1849. Originally, there were a ton of natural coves, but with the massive influx of people, it resulted in a combination of planned and unintentional filling. The financial district for example, has 60 buried ships that we know of underneath the buildings that were abandoned as men headed to the gold mine fields.
The Richmond and Sunset
The yellow areas are naturally occurring loose fill, most notably the Richmond district on the west side that used to be sand dunes. Not as bad as the red areas, but worrisome. You know about the Sutro Baths right? Well, Mr. Sutro bought up the entire Richmond district (near his baths) while it was still dunes during one of SF's big housing crisis'. Of course you can't build a house on sand, but he terraformed it and made a huge profit. Unfortunately, it's still all sand underneath all that topsoil.
The Marina, Soma, Fidi and More
Parts of the Marina, for example, were created when a lagoon was filled with dune sand and building rubble from the Great San Francisco 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. The area previously known as the Barbary Coast in the Financial District is located a half-mile from today's coastline. 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 this soft soil starts moving during an earthquake, it becomes like quicksand (liquefaction) and can shift the the structural integrity of buildings, and lead to water and gas pipes bursting.
What Happens When the Ground Liquefies?
It's when the strength and stiffness of a soil is 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).
Photo: The Marina District experienced liquefaction during the 1989 earthquake
Liquefaction has been responsible for tremendous amounts of damage during earthquakes around the world, and in the Bay Area. The 1989 quake devastated the Marina in particular because of it's composition, and led to 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.
How to Prepare
There are two important elements of being prepared. The first is having a plan, the second is having the right supplies.
1) If you haven't taken the time to build your family earthquake plan, download the 3 Step Earthquake Prep Guide. This guide has three simple steps you can take tonight to be prepared for the big one.
2) If you don't have the government recommended food, water, first-aid, and shelter supplies we recommend an earthquake kit. If you don’t have time to track down the best tools and materials to do it right, we’ve made one for you. It’s The Earthquake Bag and it’s our mission to make sure everyone either buys one of ours or builds one of their own. Everyone is safer when we all are more prepared individually.
Whether you live in a liquefaction risk area or not, make sure that you have the tools you need, food and water, and a plan by the door or under the bed. We know San Francisco will experience major earthquakes again, and we are smart enough to be prepared for whatever happens.
- Skyler Hallgren