JUST before dawn on 18 April 1906, the ground shook for one long minute beneath the young city of San Francisco, home to one-quarter of America’s population west of the Rocky Mountains. The magnitude 7.9 earthquake rattled much of the west coast and was felt as far inland as Nevada. Together with the firestorm that followed, it devastated San Francisco. Some 3000 people died and at least 225,000 of the city’s 400,000 residents were left homeless.
The quake, with an epicentre about 3 kilometres offshore from San Francisco, was one of the worst urban disasters of the 20th century. It also gave birth to serious seismology. “The 1906 earthquake was really the beginning of earthquake science,” says Keith Knudsen, a geologist with the California Geological Survey in Menlo Park (see “How seismology was born”). Today we know that the quake was caused by a complete rupture of the 500-kilometre northern San Andreas fault, ripping north and south from the epicentre at more than 13,000 kilometres per hour.
In the relative seismic silence since 1906, the city has been rebuilt, and the heavily urbanised San Francisco Bay Area has become an indispensable engine of the US economy. The calm may not last much longer. Over the course of the past 100 years numerous geological faults that traverse the region have been storing strain, making the next big earthquake a question of when, not if (see Map).
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For a city that knows just how shaky the ground beneath it is, San Francisco is shockingly unprepared. Bay Area seismic scientists and engineers hope the centenary and the recent memory of hurricane Katrina will spur residents to do what they can to ready themselves for an earthquake. “Our message is a pretty simple one,” says Tom Brocher of the US Geological Survey (USGS) in Menlo Park. “The earthquake will come without warning, so you have to be prepared now.” The fear is that without more planning a repeat of the 1906 earthquake would unleash horrors to rival those of a century ago, according to two studies conducted by federal and state agencies, including the Association of Bay Area Governments, the USGS and the California Geological Survey.
Depending on the time of day the quake hits San Francisco, an estimated 1500 people in the city would be killed by falling buildings and fires. Thousands would be hospitalised and almost 360,000 would be made homeless. Nearly 40 per cent of the city’s private buildings could be completely destroyed. That is because about 84 per cent of them pre-date modern building codes from the 1970s, including the charming Victorian structures that epitomise San Francisco. In the wider Bay Area about 6000 would die.
“People think the city wouldn’t let them live in an unsafe building. The damage to come may be a rude awakening”
One particular problem is that more than half the city’s housing sits above a “soft storey” – a bottom floor comprising a garage or a shopfront with large windows that will offer little structural support during shaking. The parts of the city built on land reclaimed from the bay are prone to strong shaking and liquefaction during a quake – the soil turns into something like quicksand and loses its ability to support structures (see Diagram).
The damage projections are “worse than I ever imagined”, says Laurence Kornfield, San Francisco’s chief building inspector. They are especially dire for the western part of the city, which lies closest to the San Andreas fault and was largely uninhabited in 1906. “People think that the city wouldn’t let them live in a building if it wasn’t safe,” says Kornfield. But it does, and the extent of the damage to come may be a rude awakening for many residents.
The city’s lowest-income residents are most at risk. More than 60 per cent of San Franciscans rent, and most of these people live in units subject to rent control. Landlords cannot pass on the cost of retrofitting these to tenants, and so have little incentive to make their buildings safer.
Fire, which caused about 80 per cent of the damage in 1906, remains a danger today. Though the region’s gas and electric company has almost finished upgrading gas pipes, its responsibility ends at the meter. Inadequately secured water heaters in homes could topple or break during a quake and ignite fires. These would be hard to control among the dense rows of wooden houses that characterise parts of the city.
Firefighters helpless
The knowledge that the 1906 quake broke water mains and left firefighters helpless compelled San Francisco to install alternative water systems for fighting fire. But when the 1989 Loma Prieta quake struck near Santa Cruz, about 100 kilometres south of San Francisco, it ruptured the auxiliary system’s water pipes and drained a massive holding tank in a matter of minutes. Given that a quake like the one in 1906 would release 30 times as much energy as Loma Prieta, the integrity of water systems is a cause for concern.
Another concern is the transport infrastructure. San Francisco has doubled in population since 1906. Suburbs have crept over many Bay Area faults, and a tide of commuters now crosses the faults to go in and out of the city. In a repeat of the 1906 quake, these arteries would be broken. The eastern span of the Bay Bridge, which connects Berkeley, Oakland and other cities across the bay to San Francisco, is especially vulnerable. A section of this bridge collapsed in the Loma Prieta quake, and it is likely to fare far worse if a quake strikes nearer San Francisco.
The other main route across the bay, the underwater tube for the Bay Area Rapid Transit (BART) subway system, is also vulnerable. One of the problems is that the soil covering the tube was not compacted when the tunnel was built in the early 1970s, leaving it prone to liquefaction. The trains within the tube hold about 2500 people during rush hour. In the worst case, the tube would rupture and fill with water and downtown San Francisco’s BART stations, which are below sea level, would be flooded. Even without catastrophic failure, the tube would be out of commission for two to three years.
Many other important structures rest on liquefiable soil, including a large stretch of the main highway leading south out of San Francisco; the Oakland and San Francisco airports; and the Oakland port. Though the huge cranes for loading and unloading ships are anchored on piles, the roads and docks themselves may be rendered impassable, cutting off access to the port.
People injured in the quake may arrive at city hospitals to find them damaged or destroyed. Though a California law passed in 1994 requires that hospitals remain functional after a large earthquake, the city’s only trauma centre at the San Francisco General Hospital was classified as being at “significant risk of collapse and a danger to the public after a strong earthquake”. According to assessments of all hospitals, only 14 per cent of Bay Area hospital buildings are expected to be structurally functional after an earthquake, and this figure falls to less than 1 per cent if non-structural factors such as emergency power are taken into account.
“The city’s only trauma centre is classified as likely to be a danger to the public after a strong earthquake”
About 100 kilometres to the north-east, levees on the Sacramento river, which flows into San Francisco Bay, are likely to break in a quake, causing local devastation. For this to happen, the quake would have to occur on one of the faults nearer the river delta, and the effects on California would be considerable, ruining the freshwater supply for 22 million residents.
If the next big earthquake holds off for a few years, at least the infrastructure should fare better. A new suspension bridge is under construction to replace the ailing segment of the Bay Bridge. Retrofits are under way for BART and for the water system, including the pipelines that carry water across several faults from Yosemite National Park to the Bay Area. And by law, hospitals in danger must rebuild by 2013.
But little is in the works to solve the problems of domestic buildings. The ground will unquestionably rumble again, and many of its effects are predictable. So how can the city and its residents be so unprepared? Part of the problem is that no one really comprehends the scale of the impending disaster. “People don’t get it. It’s unbelievable, the horror of an earthquake,” says Kornfield, who was in Japan during the 1995 Kobe earthquake. It killed over 5000 people, caused $150 billion in losses and is viewed by many as a good example of what a strong quake could do to the Bay Area.
“We are all unable to objectively think about the true scope of how bad it could be,” says Mary Lou Zoback of the USGS. Part of that problem lies with politicians. “It is hard to get an elected official to concentrate on tomorrow’s problems when there are so many problems today,” says David Prowler, a San Francisco consultant who managed the city’s building assessment project.
At least the devastation of New Orleans by hurricane Katrina has instilled a sense of urgency. “Katrina has had a big role in showing what the effect of a major catastrophe can be, and the fact that it is very difficult to come in afterwards very quickly,” Brocher says.
For Kornfield and others, it is not the immediate emergency response that is most troubling, it is the recovery process. They point to the problems that will be faced by the long-term homeless, the large numbers of small businesses that would be ruined, the lines of building owners seeking permission to demolish buildings and escape rent control, and the loss of San Francisco’s Chinatown or its Bohemian residents who may not have the resources to live in a new, rebuilt and possibly more expensive city.
How seismology was born
The famously comprehensive Lawson report prepared by Andrew Lawson of the University of California, Berkeley, after the 1906 San Francisco earthquake “set the agenda for earthquake research for the next century”, says Bill Ellsworth of the US Geological Survey in Menlo Park, California.
Before 1906, little was known about earthquakes. A proposed report about the magnitude 6.9 earthquake in the Bay Area in 1868 came to nothing. George Davidson, who worked on the Lawson report, blamed business interests, saying they wanted to downplay the seismic danger of the area, but Stephen Tobriner, an architectural historian at the University of California, Berkeley, says the report did not materialise because the man in charge died a year after the quake.
There is plenty of evidence that architects and engineers took local earthquake danger very seriously after 1868. Tobriner showed that structures built between 1868 and 1906 had iron reinforcements threaded through their bricks and that steel frames were far stronger than was typical for the time. Despite their best intentions, “they didn’t understand how big earthquake forces could really be”, Tobriner says. He notes that the seismologists weren’t helping. In 1904, Lawson himself wrote in the student newspaper: “Earthquakes in this locality have never been of a very violent nature…There is not occasion for alarm.”
After the 1906 quake, it was Lawson’s team that mapped the length of the San Andreas fault, recording fences and streams that moved by more than 3 metres (some of which can still be seen today), and showing that the fault was one continuous geologic feature. The report catalogued the damage to everything from chimneys to headstones, and even the direction that milk sloshed out of buckets.
Perhaps the most significant outcome of the report was the “elastic rebound hypothesis”, to explain why earthquakes occur. It pictures the Earth’s crust deforming elastically until a fault finally slips, causing an earthquake and releasing the pent-up strain. At the time no one knew why the crust moved in the first place. The theory of plate tectonics was still six decades away.
What the future holds
While a repeat of the 1906 earthquake is possible, seismologists are worried about other equally dangerous scenarios. In the 70 years leading up to the 1906 earthquake, the Bay Area was rocked by 17 quakes of magnitude 6.0 or greater. “If that had happened since 1906, we probably wouldn’t have the huge Bay Area population that we do,” says Mary Lou Zoback of the US Geological Survey in Menlo Park, California.
But the seismic silence is due to end. The huge 1906 quake released the strain on all the parallel faults in the area, and in the aftermath the area enjoyed what geologists call a “stress shadow”. Now continued movement of the Pacific and North American tectonic plates has reloaded the faults. “All of our models suggest we’re out of the stress shadow,” Zoback says.
The USGS predicts a 62 per cent chance of a quake at magnitude 6.7 or greater on a Bay Area fault before 2032. Most likely is an earthquake of around magnitude 7 across the bay from San Francisco on the heavily urbanised Hayward fault, which runs between the goalposts of the football stadium at the University of California, Berkeley, through Oakland, and south towards San Jose.
A Hayward quake would be as catastrophic as a repeat of the 1906 quake. More than 1700 roads crossing the fault would probably be closed and nearly 360,000 people could be left homeless. San Francisco would not be immune from a Hayward quake. The financial district, which sits on the city’s shake-prone reclaimed land, is about equidistant from the San Andreas and Hayward faults. “We are really worried about the Hayward fault,” Zoback says.
