Waiting for `The Big One' to Shake San Francisco: QuickTake Q&A

The Marina district disaster zone after an earthquake hit San Francisco in 1989.

Photographer: Otto Greule Jr/Getty Images

What happens to the San Francisco Bay area when the next major earthquake strikes? That’s the question seismologists are pondering as a key milestone approaches for a quake on the so-called Hayward Fault, which runs along the East Bay waterfront occupied by Oakland, Berkeley and other towns. San Francisco is the second-most-densely populated city in the country, after New York; the Bay area is home to seven of the 10 largest U.S. technology companies by market capitalization and more than 25 percent of the country’s technology workers. Three years after an unexpected temblor shocked the wine country of Napa Valley, northeast of San Francisco, the wait continues for "The Big One."

1. What would constitute ‘The Big One’?

The term is commonly used to describe something on the order of the devastating 1906 earthquake that destroyed 80 percent of San Francisco and killed about 3,000 people. Scientists generally consider any earthquake of magnitude 7 or higher to be a major earthquake; the 1906 quake, which occurred on the San Andreas Fault, had an estimated magnitude of 7.8.

San Francisco and Silicon Valley are flanked by the Hayward and San Andreas faults.

2. Which fault line is being watched the most?

Hayward is considered the one most likely to cause the next major earthquake. Its southern section, near the cities of Fremont and San Jose, has been estimated to slip approximately every 160 years; it’s been almost 150 years since the 1868 Hayward earthquake, with an approximate magnitude of 6.8, which killed about 30 people. But the truth is that an earthquake could happen along any of several faults in or near the Bay area, including the San Andreas and West Napa faults. The so-called American Canyon quake, magnitude 6.0, that struck Napa in August 2014 surprised seismic experts, merchants and tourists. It caused extensive property damage and killed one person.

3. How bad would it be?

A magnitude 7 quake on the Hayward Fault could kill hundreds and injure more than 60,000, while another 7.8 temblor on the San Andreas might cause 2,550 deaths and more than 220,000 injuries, according to estimates by the California Governor’s Office of Emergency Services and the Federal Emergency Management Agency. As in all earthquakes, older buildings, highways and other elevated infrastructure, and power and gas lines are of particular worry. The Loma Prieta quake of 1989, magnitude 6.9, severely damaged the Embarcadero Freeway that then ran along the Bay waterfront on the San Francisco side. The highway was later dismantled, paving the way for a renaissance of business and restaurant development along the waterfront.

4. What’s been done to prepare?

Because building codes were strengthened in the 1970s, newer buildings, including office towers, would probably fare well. The California Apartment Association says as many as 100,000 buildings could be facing retrofits, and the state offers retrofit grants to homeowners. A school bond issue approved by voters in November 2016 included money to make school classrooms stronger. There’s been progress as well on preparing for earthquake-sparked fires, such as those that followed the 1906 earthquake. San Francisco authorities have maintained and expanded a century-old system of underground cisterns to guarantee access to water in an emergency.

5. Which areas remain most vulnerable?

Areas that were built on fill, including mud and sand, by early settlers and residents who rushed to develop the city. These areas include the Marina District facing the Golden Gate Bridge, areas of the financial district east of Montgomery Street, many commercial blocks in the corridor south of Market Street, and some parts of the city’s Mission District. (A common perception that glass from office windows might shower down on the financial district is probably mistaken, thanks to innovations in structural engineering.) There’s also concern about the structural integrity of the Transbay Tube under the San Francisco Bay, which is used by Bay Area Rapid Transit commuter trains. The tube was built on sediment and might shift in a major quake, causing leaks that could render the vital transportation link unusable for more than 400,000 daily commuters. A $1.3 billion earthquake-safety upgrade is scheduled to be completed in 2022.

6. What would be the impact on Silicon Valley?

“The Valley” occupies the peninsula of land south of San Francisco. One thing this stronghold of the U.S. technology industry has going for it is new construction. The steel towers, low-rise commercial buildings and single- or double-story homes of Silicon Valley towns like Cupertino, Palo Alto and Sunnyvale would have a good chance to survive an earthquake. Many technology companies in the Valley and in San Francisco have detailed earthquake plans aimed at ensuring staff safety and business continuity. Still, both Silicon Valley and the city lie between the San Andreas and Hayward faults, and damage in a major quake could be extensive.

7. Could there be a tsunami?

That’s thought to be unlikely, even though portions of the San Andreas run under the Pacific Ocean close to the coast. Tsunamis tend to be generated by vertical motion of the sea floor, and "most of the motion in a San Andreas-style earthquake is horizontal, not vertical,” said Jack Boatwright, geophysicist and coordinator for Northern California earthquake hazards at the U.S. Geological Survey. “You need the vertical motion as the pusher for the tsunami -- the plunger, if you want to call it that.”

8. What about San Francisco’s famous bridges?

The Golden Gate, built between 1933 and 1937, is an engineering marvel that draws tens of thousands of tourists per year. It is thought to have good earthquake survivability, thanks to its durable suspension design and upgrades to supports. Controversy has surrounded a $6.4 billion replacement of the eastern span of the San Francisco-Oakland Bay Bridge, which was completed in 2015 and connects the city with the East Bay. Some steel rods that anchor the bridge’s massive main cables shifted after installation, raising questions about the quality of the engineering and construction. “I really hope with the amount of money that was spent building the eastern span, that it’s seismically strong, and I think it is,” said the USGS’s Boatwright. “The western span is actually quite strong. The expectation is that since the eastern span is post-1989, it’s built to withstand strong shaking.”

9. Could the ‘Big One’ strike elsewhere?

Nothing about earthquakes is certain, including where they’ll occur. For sure, Los Angeles and other population centers in southern California are always on alert for the next huge quake. So are at least some people in the Pacific Northwest, where the Cascadia fault line is viewed as capable of producing among the most powerful and damaging earthquakes in history. And the U.S. Geological Survey, in 2014, said 16 states -- among them, Illinois, Missouri, South Carolina and Tennessee -- have a "relatively high likelihood" of experiencing "damaging ground shaking from an earthquake in 50 years."

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