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", right). 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, page 10).
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
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