The earthquake is coming.
Living in the Bay Area, I’ve heard this again and again, but when I recently moved and the landlord couldn’t tell me how old or earthquake-safe the house was, I decided to do some digging into how hazardous it is to live and work in this neighborhood.
I discovered that it depends on which part of the Mission we’re talking about.
Maps from the United States Geological Survey (USGS) and the California Geological Survey (CGS) show that parts of the northern Mission and its southeastern corner may be somewhat more dangerous than the rest of the neighborhood.
The USGS Liquefaction Susceptibility Map (the Mission District portion is re-created above), shows zones in the area that are generally more prone to liquefaction than others. The grounds designated as having “very high” susceptibility are made up of loosely packed sandy or silty materials saturated with water. If the ground in these areas is shaken hard enough during an earthquake, it can lose strength and stiffness, acting like a liquid instead of a solid.
“It has to be saturated and shaken hard enough that it basically loses strength and sand particles are suspended in water,” said Danielle Hutchings, ABAG’s earthquake and hazards program coordinator.
“It’s almost like if you imagine quicksand, or you’re at the edge of a beach. Water comes up and gets in between sand particles, and they can no longer support the weight above,” she said.
“The movement tends to mostly affect linear features, destroying pipes, roadways, building foundations and airplane runways.”
The scale of damage depends upon which fault the earthquake takes place along, where the epicenter is and the quake’s magnitude.
Tim McCrink, senior engineering geologist at USGS, said that for very loose, saturated sand, we should only encounter widespread liquefaction if the area is right along the fault line and the magnitude is 4 or 5 or above. If the area is further from the fault line, a magnitude 5-plus quake could still cause longer shaking periods, leading to liquefaction damage.
“Knowing the history of San Francisco earthquakes,” said McCrink, “there’d probably be enough shaking in the Mission to cause liquefaction damage.”
During the 1906 quake, which had a magnitude of 7.8 and an epicenter two miles west of San Francisco, liquefaction-related damage to water supply pipelines prevented containment of the fire that destroyed about 500 city blocks. Thus, liquefaction can be indirectly blamed for 85 percent of the total damage to San Francisco in 1906, according to an ABAG report.
A historic context statement prepared by the San Francisco Planning Department states that the northern Mission was “utterly devastated” in 1906 and was completely rebuilt. “In contrast, the southern Mission was spared destruction in 1906 and thus retains generally intact pre-disaster residential architecture from the latter half of the Nineteenth Century, mixed with Twentieth Century construction.”
According to the USGS website, during the 1989 Loma Prieta earthquake, which had a magnitude of 6.9 and an epicenter in the Santa Cruz Mountains 60 miles south-southeast of San Francisco, liquefaction of the soils and debris used to fill in a lagoon in San Francisco’s Marina District caused major subsidence, fracturing and horizontal sliding of the ground surface.
Old creek beds may be the cause of sandier soils in the northern Mission. For our map of the Mission’s historical waterways, go here.
“In the past, Mission Bay was an actual bay, so there could also be historical beaches or other loose soils that surrounded it that could now be part of the North Mission,” said Michael Smith, ABAG’s GIS coordinator.
Areas where fill was used to level out uneven topography and the water table is high can also be more susceptible to liquefaction.
The closer people are to the boundary between two zones, said Smith, “the greater the chance is that they may fall into either zone, and as a result, it is more important that they consult a professional for a site evaluation prior to making any important decisions.”
Owners are not required to tell their tenants about potential earthquake hazards, McCrink said.
Owners of buildings constructed before 1989 — most buildings in the Mission, except for the newest condos — are only required to disclose liquefaction or landslide hazards to buyers. This is in accordance with the CGS Seismic Hazards Zonation Act implemented after the 1989 quake.
Contractors who want to build in a high liquefaction hazard zone must hire an engineer to help mitigate damage by strengthening the ground or foundation.
“But this is an incredibly expensive process, and retroactively it’s not cost-effective for one building,” Hutchings said.
It’s easiest to reinforce a building against liquefaction damage when it is under construction rather than after it is built, she added.
The most that tenants can do to prepare for an earthquake, Hutchings said, is to prepare emergency kits, know where to relocate and have plenty of water, food and supplies — enough for at least a week.