Photo: Estelle Chaussard Image, UC Berkeley
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If an earthquake with the same 7.1 magnitude as Friday night’s Ridgecrest temblor shook the Bay Area, the impact would be significantly more devastating than what unfolded in Southern California, experts say.
The recent quake struck a remote, sparsely populated area consisting of mostly undeveloped desert land, except for the the city of Ridgefield. It has a population of 30,000 and few multistory buildings. As a result, damage is less severe and there were no deaths. By contrast, the Bay Area is home to more than 7 million people with multiple major cities and vast suburbs connected by a complex highway and transportation system.
“In the Bay Area, we have a dense urban environment and many multistory buildings, many that haven’t been built to code,” says Ole Kaven, a seismologist with the U.S. Geological Survey. “The exposure due to people and structure in the Bay Area is much greater, and we would expect much more damage.”
What’s more, much of the Bay Area is developed on bay mud and artificial fill in an area where the water table is high due to the proximity of San Francisco Bay and the Pacific Ocean. This scenario could result in more violent shaking than what occurred in Southern California. Structures built on fill would suffer from the amplification.
“The loose, weak soil plus the water table can cause liquefaction,” says Anne Wein, a disaster scientist with the USGS. “It messes up foundations and all our water pipes. All the ground sinks and moves.”
A handful of major faults cut through the Bay Area, but the one most likely to see a major event is the Hayward, the 52-mile fault line stretching from San Pablo Bay in the north, to just east of San Jose in the south.
“We think a probability of a 6.9 or larger in the next 25 to 30 years is most likely on the Hayward Fault,” says Kaven. “The San Andreas ruptured in 1906, but we have’t had anything significant on the Hayward Fault in a long time.”
Due to the Hayward Fault’s vulnerability, the USGS imagined the outcome of a magnitude 7.0 tremor on the Hayward Fault in a 2018 report. It offers a clear picture of what a Ridgecrest-strength earthquake would look like in the Bay Area.
The “Haywired scenario” estimates up to 800 people could die and 18,000 could be injured if the fault were to rupture. Homes would be engulfed in flames, with an estimated 52,000 homes burning. Twenty-thousand people could be stuck in elevators and 1,500 could be trapped in fallen buildings.
This tragedy is, in fact, quite possible. The USGS says there is a 72 percent chance of one or more magnitude 6.7 earthquakes occurring in the Bay Area before 2043. Kaven says the Bay Area has been in the midst of an earthquake drought; earthquake activity has been underdeveloped in the past 100 years, compared to previous centuries.
“We should expect much more energetic activity,” he says. “It’s a great reminder for us that this type of an event could happen on the Hayward and we can prepare for it and we should.”
The magnitude 7.1 earthquake jolted Southern California the day after the region was hit by a 6.4 earthquake, which seismologists are calling a foreshock. It was the largest earthquake in the area in 20 years. Thousands of smaller aftershocks followed.
On Monday, Kern County spokeswoman Megan Person said inspectors will need several more days to assess all the damage in the desert community. Dozens of properties will likely be found damaged.
Person said 100 people were staying at Ridgecrest’s main shelter as of Sunday afternoon. An additional 43 evacuees were camping on the lawn outside, afraid to go indoors.
It could be days before water service is restored to the nearby desert town of Trona, where officials are also still assessing damage.
San Bernardino County spokesman David Wert said Monday that 10 residences have been red-tagged as uninhabitable, but he expects that number to rise. He says he has seen homes that shifted 6 feet (nearly 2 meters) off their foundations during the magnitude 6.4 earthquake Thursday and a magnitude 7.1 quake Friday.
Amy Graff is a news producer for SFGATE. Email her at email@example.com.
This post was originally posted at https://www.sfgate.com/bayarea/article/Haywired-Report-San-Francisco-Bay-Area-7-0-14079313.php.