Where in California there is a high risk of a strong earthquake: the most dangerous faults

Living in California means living in an earthquake state. Every year there are thousands of earthquakes, most of which are too small for residents to feel them. Fox5.

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But after the February earthquakes in Turkey and Syria, which killed more than 47 people in the region, questions about a “big” earthquake in California began to arise again.

To understand which faults can trigger a major earthquake, experts say it's important to understand what counts as "major."

When people think of a big earthquake, an earthquake with a magnitude of 8,0 on the Richter scale may come to mind.

But as Dr. Pat Abbott, professor emeritus of geology at San Diego State University, said, the threshold for what a major California quake might look like may not be so simple.

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The last "big one" level quakes in California's recorded earthquake history are the 1857 earthquake in the central third of San Andreas and the 1906 earthquake in the northern third. Both were below the magnitude 8,0 threshold.

Although there have not been many earthquakes larger than 7,5 in recent history, this does not mean that there have not been large earthquakes since then.

For example, the 1989 San Francisco earthquake and the 1994 Northridge earthquake were devastating. They caused significant damage to infrastructure and significant loss of life. But both of these earthquakes were magnitudes less than 7,0.

"Now, technically it's not 'big quakes', but when you look at what happened, every single one of those quakes was terrible," Abbott said.

The impact that a strong earthquake has, whether it is a magnitude 6,5 or 8,0, depends largely on proximity to urban areas.
“In some respects, the ‘big ones’ in terms of damage and death will be those earthquakes that move through a city, not those that are far away,” he continued. “There’s the size of the earthquake, and also where you are compared to where the fault is moving.”

Southern San Andreas is of particular concern to experts when they look for locations that are expected to experience a quake greater than 7,5, affecting areas such as San Bernardino, Palm Springs and Imperial County.

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This 1126 km section of the fault has not seen an earthquake of this magnitude since about 1690, Abbott said.

A 2008 forecast indicated that a potential 7,8 magnitude earthquake at the southern end of the fault could kill up to 1800 people.
But scientists are also particularly concerned about a smaller fault that is small for a major quake: the Hayward Fault, which runs right under the cities in the Bay Area.

“Part of the concern is not just the size. The whole East Bay area is full of old towns with a lot of old buildings,” Abbott said. “We could have terrible loss of life from even a magnitude 6,5 earthquake.”

With these two areas of concern to seismologists, there are a number of potential scenarios that could play out in the Earth's crust. Some scenarios may be very similar to a catastrophic 7,8 earthquake in Turkey and Syria due to the similarity of the premises.

“What challenges us is that these earthquakes are so rare,” said Alice Gabriel, assistant professor of seismology at the Scripps Institute for Geophysics and Planetary Physics. - It's just a very long time. Now we have events in Turkey that were very devastating, but we also learned important lessons.”
One such lesson is the possibility of a "double" quake, or additional quakes triggered by nearby faults.

“What we saw in Turkey is really difficult to predict. The earthquake was not a one-time occurrence,” Gabrielle said. “Magnitude 7,8 was the first event, but nine hours and a few minutes later it triggered a second event that was almost as large.”

The Hayward Fault could be a candidate for a secondary quake that would be dynamically triggered if a 7,4 magnitude quake hits Southern California, given how much stress has already built up between the plates, she said.

Despite what can be learned from events such as the Turkey earthquake, it is still impossible to determine exactly where a "big quake" might occur, when it might occur, or what it would look like. Though experts say California probably won't see as much destruction and casualties as Turkey due to concerted government earthquake safety efforts.

"No one can really tell you if one fault will rip due to movement in another fault," said Jose Lara, head of seismic hazards for the California Governor's Office of Emergency Management.

“But California is in a much more resilient position to withstand an earthquake because of our many years of mitigation efforts that have been put in place,” he continued.

These efforts include regularly updating and enforcing state building codes, as well as focused work to develop disaster plans that will guide the government's response, from personnel deployment to emergency supply chains.

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Lara stressed that no matter how the next large-scale earthquake ends up, people should be prepared for the fact that a strong earthquake can occur near their home.

Lara and other experts said the best way to prepare is:

• Identify and secure items in your home or workplace that could fall during a major earthquake, such as cabinets or ceiling fans.
• Make a home preparation plan.
• Provide an emergency kit containing items such as flashlights, money or food in case critical infrastructure is damaged.
• Download app MyShake for earthquake warnings in California.

“Even though Californians improve building standards after each earthquake and things become safer, there are still risks,” Abbott said. “California is getting safer all the time, but there are still many problems.”

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