A repeat of the most powerful earthquake in San Francisco’s history would knock out phone communications, leave swaths of the city in the dark, cut off water to neighborhoods and kill up to 7,800 people, according to state and federal projections.

If a quake like that were to strike along the San Andreas Fault today, building damage would eclipse $98 billion and tens of thousands of residents would become homeless.

Thursday marks the anniversary of the 1906 quake, a 7.9-magnitude event that turned San Francisco streets into waves, flattening much of the skyline and igniting fires that raged for almost four days. The quake ruptured 296 miles of fault line — from Cape Mendocino to San Juan Bautista.

Since 1906, the fault has remained locked from Point Arena through the Peninsula. The 1989 Loma Prieta earthquake hit 50 miles south of San Francisco, on a remote segment of the San Andreas Fault, and ruptured only 25 miles.

While nearly 113 years have passed without a major earthquake on the fault from the Peninsula north, the quiet will end. It could happen tomorrow or next week or decades from now. When it does, hundreds of thousands if not millions of people will feel the ground shake like they’ve never felt before.

For now, all the experts can do is model, project and, like the rest of us, prepare for the worst. The Bay Area had a small reminder of the risk when a 2.9-magnitude quake, centered about 2 miles east of Berkeley, struck Saturday morning.

At typically active sites along five major faults in Northern and Southern California, including parts of the San Andreas and Hayward faults, no big earthquakes have occurred in the past century. Based on current understanding of earthquake recurrence, that’s a statistical anomaly, according to a report this month from U.S. Geological Survey scientists.

According to a 1,000-year earthquake history of those fault branches, the chances that not a single site would experience a major temblor in any given 100-year period is about 0.3%, the paper said. (The site where Loma Prieta hit was not analyzed.)

Earth’s tectonic plates constantly slide past and bump into each other — not always smoothly. An edge that gets caught up against another will break free eventually as stress builds up, and the release of such friction produces earthquakes.

“It may be that the whole system is on edge and ready for a more active cycle,” said Glenn Biasi, co-author of the paper and a supervisory geophysicist with the USGS. “The whole system is loading, and we don’t know how it’s going to unload. It hasn’t been dumping stress on the faults we know carry the most slip.”

Building codes have improved dramatically since 1906, of course — engineers now must design flexible, fire-resistant structures that can withstand horizontal forces. Researchers also have made key earthquake science discoveries, and emergency response procedures today are far more sophisticated.

But the Bay Area’s population has increased tenfold over the last century, and with more humans come more sources of vulnerability: water-treatment facilities, petroleum refineries, chemical plants and critical infrastructure, like the Transbay Tube that runs beneath the bay. Beyond shaking the ground, powerful earthquakes can set secondary disasters in motion: fires, tsunamis and hazardous material releases.

Most people would survive a magnitude 7.8 quake, which the report used as a basis for its models. Death toll estimates range from about 200 to more than 7,800 — but it’s the quake’s aftermath that is of greatest concern, said Bijan Karimi, acting deputy director of the San Francisco Department of Emergency Management. A third of households will likely be without power for at least three days, and half are expected to be without water for at least a month.

Karimi said people should be prepared for at least 72 hours on their own, making sure they have enough supplies on hand for themselves and their families.

“There’s only so many emergency responders to go around,” he said. “We really want to have individuals relying on themselves and their neighbors.”

The death, injury and damage projections for a major quake are based on Federal Emergency Management Agency modeling software, which incorporates census tract-level data, ground-shaking models and the ages and values of buildings, said Michael Bishop, a FEMA risk analyst. But the projected losses might be underestimated. The model uses census data from nine years ago and assumes that structures are not compromised, as could happen when aftershocks hit weakened buildings, Bishop said.

Officials in California’s Office of Emergency Services must submit a report to FEMA every five years to qualify for emergency federal aid, and last year’s report catalogs the state’s major threats while also predicting where systems might break down.

The plan notes there is no statewide inventory of fire, police, ambulance, emergency communications and other critical facilities. Most were built before new state regulations on the design and construction of essential services structures went into effect. Because of the ages of these structures, “they are not expected to be reliably functional after earthquakes, delaying emergency response and in some cases posing significant risks to life,” the report says.

Still, California has shored up many of its earthquake-response measures. In the event that cell phone towers and other communication infrastructure are knocked out, for instance, officials can use satellite systems and “multiple layers of redundancy” not available in past disasters, said Tina Curry, deputy director of planning, preparedness and prevention in the Office of Emergency Services.

“We have a lot of things that have improved,” she said. “It’s still going to be a very devastating and disruptive event.”

In some cases, local governments have developed their own backup systems and contingency plans.

In Alameda County, 50 amateur radio experts have volunteered to help set up wireless communications, said Paul Hess, emergency manager for the county sheriff’s office. Loon, a subsidiary of Alphabet Inc., has plans to send balloons into the stratosphere that could get the internet back in operation, Hess said. If all else fails, officials will send couriers into zones where people aren’t responding to deliver messages and information.

Hess said his No. 1 concern is not communications, but water. Most people he talks to at training events do not have an adequate supply.

“Our biggest challenge is trying to get people out of their day-to-day schedule inertia and take the time to get prepared at home, at the workplace and in their vehicles so that no matter where you’re at you have the supplies to deal with the effects of an earthquake,” Hess said.

People who stock water, tools and bandages — even long strips of old bed sheets or shirts — will help supplement emergency responders, Hess said. In many areas, people will have to depend on themselves and their neighbors for aid.

“In a typical neighborhood, you may have half a dozen people suffering injuries — severe bleeding, broken bones, hit on the head and in shock,” Hess said. “If you know how to handle those, you can become a support rather than running around like a chicken with your head cut off.”

Jennifer Strauss, on the leadership team of ShakeAlert, the earthquake early-warning system, also worries most about preparedness. Buildings may be more resilient now, she said, but they won’t do much for people sitting next to windows or beside a bookshelf that hasn’t been bolted down.

California’s other main disaster risks — wildfires and floods — are influenced by predictable weather events. Earthquakes, though, hit everyone at roughly the same time and come with little to no warning. ShakeAlert could buy the public precious seconds.

Just like school children go through drills, adults need to train too, Strauss said.

“When you get into an office building with a bunch of 30-, 40-, 50-year-olds, if people are not taking the drills seriously, because we have these long time periods with no events, you’re not going to actually drop, cover and hold when you feel ground shaking,” she said. “You’re going to spend precious seconds thinking about how an earthquake is happening.”

While predicting earthquakes is impossible, scientists estimate there is a 72% chance that a quake greater than or equal to a magnitude 6.7 will hit the Bay Area before 2043.

Why the current inactive period has lasted so long is unclear, said Biasi, co-author of the new paper, which examined a 1,000-year earthquake record and was published this month in Seismological Research Letters. That no major temblors have occurred across so many sites on both ends of the state is unexpected. The faults are not supposed to coordinate with each other, Biasi said.

If there is a missing variable for understanding earthquake recurrence, scientists are not sure what it may be. Biasi and his colleagues said it could be that fault branches in Northern and Southern California are somehow communicating with each other through unknown structures.

“Whatever is going on now doesn’t seem to have a precedent in the last 1,000 years. Something’s missing in the way we’re handling the data,” Biasi said. “The system isn’t bleeding any energy off. It can’t do that forever.”

Kimberly Veklerov is a San Francisco Chronicle staff writer. Email: kveklerov@sfchronicle.com Twitter: @kveklerov