"The first thing for people to realize is that Loma Prieta was not the big one," warned Richard Allen, head of the U.C. Berkeley Seismological Lab.
The Loma Prieta quake in Oct. 1989 left 16,000 homes uninhabitable, knocked out a section of the Bay Bridge and caused the collapse of a double decker freeway in Oakland.
The disaster prompted an explosion of research in the Bay Area and a lot of science-based predictions about what will happen when the "real big one" hits. David Schwartz, geologist emeritus with the U.S. Geological Survey (USGS), describes the Bay Area as a "tectonic time bomb."
ABC7 ORIGINAL SERIES: 'The Earthquake Effect' provides in-depth coverage on Bay Area's readiness for the next major earthquake
"It is going to be the challenge of all our lives when we have this earthquake happen here" according to Mary Ellen Carroll, head of the San Francisco Office of Emergency Management.
The last time the San Andreas Fault unleashed its full power in the Bay Area was the Great San Francisco Quake of 1906, believed to have been magnitude 7.9.
That earthquake left an estimated 3,000 people dead and 225,000 homeless. The USGS calculates the 1906 quake released 16 times more energy than the Loma Prieta quake.
Now scientists say the network of faults running under the Bay Area is locked and loaded. The USGS calculates there is a 72 percent chance of a major quake here by the year 2043.
VIDEO: The catastrophic fall and slow rise of the Bay Bridge after Loma Prieta
"Regardless of where you live in the Bay Area, you're not far from a fault, and there are enough faults that, if any one of them has a major earthquake, it's going to affect the entire Bay Area," according to USGS Geologist Belle Philibosian.
Schwartz believes many people will be surprised at the amount of damage that occurs in a major quake, despite years of improved engineering, retrofitting, rebuilding and planning.
He points to what he calls a "smaller" quake, the magnitude 6.0 temblor that hit Napa in 2014. Damage estimates ran up to a billion dollars. But in a magnitude 7.0 or a 7.9 like the one in 1906, Schwartz expects the damage to be spread over a much, much wider area.
"If you have a minute and a half or two minutes of shaking, it's really unclear what that's going to do to a lot of structures that are out there," he added.
The reason California is at such high risk for earthquakes is that we are right on the edge of two huge tectonic plates in the earth's crust -- the Pacific Plate on the west and the North American Plate on the east.
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The San Andreas Fault runs between the two plates, right through California. The Pacific Plate is constantly moving north. The movement is usually so slow we don't feel it, but sometimes the pressure builds and the ground shoots forward faster, causing an earthquake.
Scientists believe the continued movement of the plates over millions of years will eventually lead to Los Angeles being right alongside San Francisco.
Many researchers believe the Hayward Fault is actually a bigger threat to public safety than the San Andreas. Some call it the "most dangerous fault in America."
The Hayward Fault runs just east of the San Francisco Bay, passing through 11 cities -- San Jose, Fremont, Union City, Hayward, Castro Valley, San Leandro, Oakland, Berkeley, El Cerrito, Richmond and San Pablo.
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Images from ABC7's SKYMAP7 clearly show why the danger is so great. The Hayward Fault is underneath some of the most heavily populated areas in the Bay Area, with about 300 buildings directly on the fault itself.
"So when it moves, it moves two feet, or three feet or six feet, those structures are going to be stressed and many of them are going to fail" said Schwartz.
Some of the buildings along the fault are iconic structures including the Mormon Temple and Claremont Hotel in Oakland and the UC Berkeley Football Stadium.
Many of the major structures near the fault have had major seismic improvements, but most of the structures are homes that were built before new, tougher building standards. Experts say there is way to know how many of them will perform when a big earthquake hits the fault.
The Hayward Fault also crosses a lot of critical infrastructure including roads, utility lines and water mains.
The last time a really big earthquake hit the Hayward Fault was in 1868. Back then there were about 25,000 people in the area around the fault. Now there are about two million, most of whom probably have no idea what is happening right below them.
"Hayward Fault is pretty unique in that it creeps, so it actually is moving very, very slowly, all the time" explained Angeline Catena with the Math Science Nucleus.
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Over the past million years, that non-stop movement actually created the East Bay Hills, and the movement is not stopping.
Catena took us to Fremont's original City Hall that sits right on the Hayward Fault. Back in 1972, a huge crack appeared in the floor and it has been growing ever since. Fremont's city government was moved to a new building and the old building was never repaired, so the crack remains as an ongoing record of how the fault keeps growing, moving in three different directions.
Nearby is a muddy Tule pond that was once the epicenter for research on the Hayward Fault. Schwartz and other scientists spent years digging trenches and analyzing data from the pond. That research determined that over the past 1,700 years, there have been 12 large earthquakes on the Hayward Fault.
Research at the pond is over and now a BART extension to San Jose runs right across it, directly over the fault. BART has spent millions of dollars of voter approved bond money on seismic upgrades all over the system.
Schwartz also showed us around downtown Hayward where the fault's signature is especially obvious, from some buildings slowly sliding apart, to others completely abandoned.
"You can cover the fault up, but in the end the fault always wins" Schwartz said.
Braces and bolts tell the story of constant effort to prepare for the next big earthquake, but it's a neverending battle. Cracks are filled in and covered up, but they just keep coming back.
Experts say it is critical the public understand the danger that is coming. That's why the U.S. Geological Survey and its partners developed a project they call "HayWired." It is a science-based scenario showing what could happen if a magnitude 7.0 earthquake hits the Hayward Fault.
An animated video shows the scenario. In a quake with an epicenter underneath the city of Oakland, the rupture races 52 miles along with fault toward Fremont and Richmond, with speeds up to 7-thousand miles per hour.
In the scenario, the ground in Berkeley and Hayward shifts 3 to 5 feet, ripping through buried pipes and wires. Violent and extreme shaking lasts up to thirty seconds or longer causing extreme damage.
"The predictions in the HayWired scenario are grim: 800 dead and 18,000 injured. Of course, this is just one possible way a major earthquake may play out.
Back at the San Francisco Department of Emergency Management, executive director Mary Ellen Carroll explained that realistic scenarios of what to expect in a big quake are a serious planning tool.
"How we prepare for something that we haven't experienced is that we pretend," she said.
Carroll showed us around the command center where her team sometimes practices for major earthquakes, often in coordination with other similar departments in other cities and counties around the bay.
"We are looking at thousands of buildings lost, potentially hundreds of thousands of people that may be trapped in the city, depending on the time of day," she said. "There will be many injuries and deaths. There's just no way around that. It's not good scenario."
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And don't think you are safe just because you don't live or work right on a fault.
"The shaking intensity isn't right at the fault or just at the fault, it is over a pretty wide swath as you go away from the fault," according to Richard Allen at the U.C. Berkeley Seismological Lab. In a big quake he says, "You are going to feel strong shaking across the entire region."
For example, the 1989 Loma Prieta earthquake epicenter was in the Santa Cruz Mountains, but some of the worst damage was actually 50 miles away. The effects of a bigger quake could travel even farther.
The experts say you should expect major power outages, most communication including mobile phones and the internet will be down, thousands of people may have no water for weeks, maybe months. In addition to shake damage, major fires could cause even more destruction.
"The question is what will that be like for those of us who survive," Carroll said. "The steps that we take to prepare individually are so critical."
As bad as a major quake will be, every expert we talked to agreed, we are safer now than we were 30 years ago.
The Bay Area region has spent an estimated $80 billion on a wide range of seismic improvements since Loma Prieta.
Disaster recovery expert Mary Comerio says it is money well spent and she cites a long list of improvements.
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"We have required hospitals to be significantly upgraded all across the state, locally we have improvements to Hetch Hetchy, the water supply system and to BART," she said. "We have also had retrofit ordinances for brick buildings and soft story apartments. Many of our police and fire stations, 911 call centers, city halls have been seismically upgraded."
Caltrans has spent over $9 billion improving and strengthening the large bridges in the Bay Area, including the completely new eastern side of the San Francisco Bay Bridge which was finished in 2013.
"We've had tens of thousands of professionals come here over the last twenty years and help us with this," said Caltrans Bay Area Chief of Public Information Bart Ney. "It's the biggest thing that we've ever done as a state is prepare this region for the next earthquake."
Even so, a major earthquake is likely to do serious damage to many roads and Bay Area airports, so Caltrans has built what it calls Lifeline Routes. Lifelines are specific highways engineered to withstand the region's strongest expected earthquake.
"These are going to be the roadways that emergency services use to begin the relief for the area once this earthquake hits" according to Ney.
The only big bridges built to Lifeline standards are the San Francisco-Oakland Bay Bridge and the Benicia-Martinez Bridge. The other big bridges are expected to stand up in a big quake, but might not be usable for some time after.
One of the enduring lessons from Loma Prieta is that in a large scale disaster, many of us will be on our own for hours, maybe even days. Our preparation for disaster will make the difference, and so will regular people who step up when first responders are overwhelmed.
After Loma Prieta, those experiences led to the formation of a more organized citizen response for future disasters. Teams of volunteers are now constantly training all over the Bay Area, learning basic search and rescue and some first aid.
FROM THE ARCHIVES: ABC7's Peabody Award winning coverage of 1989 Loma Prieta Earthquake
Looking back at ABC7 News Peabody Award winning coverage of the terrifying hours after the Loma Prieta quake, it is the courage and heart shown by both first responders and ordinary citizens that stands out.
Volunteers helped fight fires, searched for survivors in rubble, staffed shelters and took displaced neighbors into their homes. Restaurants donated meals and union workers provided free labor to help repair homes. All proving over and over that when it comes to fundamental values in a crisis ,we in the Bay Area are made of the right stuff.
There is a lot that is inspiring about what happened after the Loma Prieta earthquake, but there is still plenty of reason to be concerned about what is going to happen to each of us when a major quake hits. The best thing you can do is be ready.
Take a look at ABC7's in-depth coverage of the Loma Prieta earthquake here.