Op-Ed: What qualifies a quake as The Big One? What happens afterward

On this date in 1906, the Big One hit northern California: An earthquake ruptured the San Andreas fault from south of San Jose all the way to Cape Mendocino. In San Francisco and Santa Rosa, the majority of their buildings were lost during the quake and the massive fires it triggered.

What made that the Big One? Early in my career as a seismologist, I answered that question quite literally: It was a strike-slip earthquake on the San Andreas fault, the largest we’d expect in California, and the type that happens, on average, every 100 years or so.

Over time and from talking to countless others, however, I have come to a more human-scale understanding. A Big One isn’t defined by the initial event, but what happens afterward. What made the San Francisco Earthquake a Big One wasn’t the 7.8 magnitude shaking, but that the region was so damaged that it took Bay Area communities decades to recover.

In human history, Big Ones of one kind or another have wiped out large cities (Pompeii by the eruption of Mt. Vesuvius in 79 A.D.), imperiled whole countries (Iceland’s Laki volcanic eruption in 1783), and contributed to downfalls of governments (China’s Great Tangshan Earthquake in 1976). When thousands of homes are destroyed or lives are lost, that is undeniably a disaster. But when the damage is so pervasive that it alters the fundamentals of society, that’s a Big One.

This more holistic understanding of the Big One — and the history of how other communities have responded to their Big Ones — has changed how I think about earthquake preparation in particular. It is easy to think all that really matters is to keep people safe from fatal injury during the shaking. But here’s the thing: You are almost certain to survive the Big One — but will the city? Even in Pompeii, 90% of the residents escaped with their lives, but the city disappeared and was lost to human memory for more than 1,500 years.

Similarly, when we get a major earthquake on the southern San Andreas fault, 99.99% of residents are expected to survive. We demanded safe buildings in Southern California, and our engineers have delivered. Still, hundreds of thousands of those buildings will be rendered completely unusable. A quarter-million families will lose their homes. Even if your house is intact, how long will you be able to stay there without water, sewers or power?

A report released by the U.S. Geological Survey today underscores how we haven’t yet addressed this society-crippling aspect of seismic risk. Looking at the Hayward fault — which runs through the East Bay beneath Richmond, El Cerrito, Berkeley, Oakland, San Leandro, Hayward, Union City, Fremont and San Jose — the USGS estimates that that an earthquake would leave 1 in 4 new, built-to-current-code buildings in the Bay Area unusable.

If the buildings were 50% stronger, however, just 1 in 20 would be labeled with a yellow or red tag prohibiting entry after that earthquake.

Making buildings 50% stronger does not mean they cost 50% more to build. In fact, estimates are that construction costs would only be 1% higher.

Assemblyman Adrin Nazarian (D-Van Nuys) has introduced AB 1857 to move California in the direction of what is called functional recovery. That means a building can be restored to regular use after a seismic event, instead of needing to be completely rebuilt. The bill directs the Building Standards Commission to pull together state and local agencies along with representatives from the insurance and construction industries to devise these stronger building codes by January 2020.

Other steps are being taken to make sure that Southern California has the resilience to endure the next major earthquake. The Los Angeles Department of Water and Power, for instance, is installing pipes that can withstand earthquake shaking. Long Beach has started to inventory its seismically vulnerable buildings. Moorpark has passed an ordinance to strengthen its cellular towers.

Government has its role in figuring out how to prepare for a Big One, but so do our local organizations. Will we choose to stay in Southern California when it is struggling to recover or will we give up and leave? Our churches and mosques, schools and synagogues, book clubs and hiking groups are places where we connect to one other. If these community organizations are ready to help their members after the earthquake, those people will be able to stay and be part of our functional recovery.

In a geologically active place like California, earthquakes are inevitable. The question is how can each of us commit to ensuring that our communities bounce back.

Lucy Jones was a seismologist with the U.S. Geological Survey for 33 years and founded the Dr. Lucy Jones Center for Science and Society focused on creating resilient communities in 2016. She continues to work at Caltech and has just published a book, “The Big Ones.�

Follow the Opinion section on Twitter @latimesopinionand Facebook