Apartment buildings in the Greater San Diego Area may sit on soil that is prone to liquefaction, landslides or other instability when shaken by an earthquake.
If your apartment building is located in Mission Bay, Mission Valley, Tierra Santa, Linda Vista, and many other San Diego communities, it’s best to have your building checked by professional structural engineer.
These destructive reactions to earthquake shaking can happen when ground vibrations cause soil particles to lose contact with each other. In the case of liquefaction, the ground vibrates like Jell-O – significantly increasing the potential for damage to buildings.
You can check the City of San Diego website for an interactive map showing local liquefaction zones, landslide areas and earthquake faults – and zoom in to see if your building falls within those areas. Visit https://www.sandiego.gov/development-services/zoning-maps/seismic-safety-study to access the map.
When Soil Turns to Soup
Liquefaction. It’s a little like quicksand, but many times more deadly.
The United States Geological Survey defines it as the phenomenon that takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking.
When it occurs beneath buildings and other structures, the results can be catastrophic.
It’s what contributed to the devastation of the 1989 Loma Prieta earthquake – when soil and debris used to fill in a lagoon to build the Marina District subsided, fractured and caused horizontal sliding of the ground surface.
The Atlantic magazine describes it as “horrifying.”
“Videos of its occurrence look like found-footage documentaries of the Second Coming,” the magazine wrote. “Buildings seem to simply slip away, the earth gives out, and the once-steady structures slide into the morass.”
But the science behind the phenomenon is straightforward. Liquefaction tends to occur where water has been – near rivers, harbors, reclaimed land, marshes – or in the case of Mexico City – on a lake that was filled in to build a community.
Slips and Slides
Landslides are another concern for earthquake-prone communities.
Earthquakes are a major cause of landslides, according to the USGS, which classifies landslides into five types: falls, topples, slides, spreads, and flows.
Falls are abrupt movements of rocks or boulders that become detached from steep slopes of cliffs, but they can also be represented by falls in the earth itself.
Topples describe instances when a section of land, such as a cliff, will fall onto lower elevations beneath it.
Slides describe a downward movement of a mass of land, like large quantities of sand moving down a playground slide.
Spreads refers to a fracturing of the earth, followed by slippage, like a custard dessert that has not set.
Flows are the most destructive type of landslide to structures, as they often occur without warning, move too quickly to escape, and due to velocity and material are often very powerful and destructive, the USGS explains.
Guarding Against Risk
The more scientists and engineers learn about ground movement responses to earthquakes, the better we can guard against damage.
Stanford University launched an unprecedented study to repurpose underground fiber-optic cables to measure soil properties in urban areas. In telecommunications, data are transmitted in the form of light traveling through fiber-optic cables, which are bundles of thin, glass rods. To measure soil properties, these Stanford researchers are looking at the tiny defects in those cables.
These cables – while underground – are constantly twitching under the vibrations of ocean waves, trains, even traffic. Researchers shoot a series of light pulses into a fiber-optic cable and track how the reflected light wobbles – helping them to quickly determine how much the ground shakes. These measurements reveal a lot about the properties of the soil where the cable is buried.
We are bound to learn much more in the future about the way soils behave under seismic shaking.
Until then, it is wise to learn the type of soil your building was constructed on in the San Diego area know and the risks that soil may pose to your tenants and your rental income.
