SAN FRANCISCO -- A century later, the people who felt the ground rock that Wednesday morning tell the story best.

"The prelude, or opening, was a very low rumbling noise, like distant thunder."

"The solid earth took on the motions of an angry ocean."

"Buildings were tumbled over on their sides, others looked as though they had been cut off short with a cleaver."

"From the moans and cries coming from below it was evident that a considerable number of people were trapped."

"And all the work of less than a minute!"

A magnitude 7.8 earthquake had struck the capital of the western United States, a shining city built on the promise of silver and gold with little thought to the destruction rock could also produce under enough pressure. Although the eyewitnesses didn't know it at the time, after April 18, 1906, neither San Francisco, nor an America just learning about the natural and man-made hazards of urban life, could ever again be so willfully innocent.

The Great Quake and the resulting fires that took what the early-morning temblor spared comprise one of the costliest disasters in United States history. While no one knows how many people died in crowded rooming houses and other structures that collapsed and burned, reliable estimates put the death toll above 3,000, and possibly as high as 6,000.

As the first modern disaster documented on a mass scale, San Francisco's calamity stands as an enduring benchmark to which all other scenes of decimation are compared.

Like residents of bombed-out Dresden during World War II, and last year's victims of Hurricane Katrina, San Franciscans saw their city wiped out in a matter of days. It gave birth to seismic science, shaping cities the world over where the earth's crust is likely to shift with devastating results.

Even today, with the lessons of Katrina and facing better than even odds that another Big One will hit Northern California by 2032, historians and scientists work to separate facts from the folklore that envelopes the iconic event like so much sentimental fog.

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From the Ferry to Van Ness, you're a God-forsaken mess, but you're the damndest finest ruins, nothing more or nothing less. -- Poem by San Francisco businessman Larry Harris, 1906.

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Six years before the earthquake, San Francisco officials chose the design for a municipal flag. The dominant symbol was a phoenix, the mythical bird that rises from the ashes to live again. With its downtown destroyed, more than half its 400,000 residents homeless and personal fortunes in the balance, the city labored to live up to that image by rebuilding as quickly as possible.

"The strength and indefatigability of San Francisco -- that to me is the most obvious aspect of the earthquake," said California State Library curator Gary Kurutz. "Yes, it was a disaster, but people did see it with a sense of adventure, too."

Indeed, it can be humbling to realize how few of the landmarks visitors recognize in San Francisco in 2006 existed a century ago. From the Portals of the Past columns in Golden Gate Park -- the salvaged entranceway from a flattened mansion -- to the Coit Tower, erected to honor the firefighters its benefactress admired, the city is both a monument to and an outgrowth of 100-year-old events.

Chinatown, for instance, did not have its Far East-inspired appearance until after the earthquake. When the city tried to use the disaster as an excuse for driving out Chinese immigrants, a savvy Chinese businessman came up with the idea of using the ersatz architectural theme to turn Chinatown into a tourist draw.

The rebuilt City Hall (the old one, after two decades in the making, was reduced to a steel skeleton in 28 seconds) not only features the world's fifth-largest dome and stands taller than the U.S. Capitol, but has a shock-absorbing foundation that was installed in 1999 to protect it from the next earthquake.

Mission Bay, home to the Giants baseball stadium and one of the fastest-growing parts of the city today, is former marsh that became a landfill for the disaster debris.

But where some see a reason for civic pride, others recount an untold tragedy.

In their rush to rebuild, San Francisco's elite did no better job planning for the future than they had before the earthquake, said environmental historian Philip Fradkin, author of "How San Francisco Nearly Destroyed Itself." Thousands of acres of forest were raided for lumber and thousands of horses were worked to death to protect the city's reputation as "the Paris of the Pacific," he said.

"After 1906, San Francisco loses its dominance in the West. Industry fled, and what it became was a poor imitation of itself through tourism," said Fradkin. "It takes that image of being a gay, giddy city and trades on that image."

Charles Wollenberg, a history professor at Vista College in Berkeley, thinks a lot of the growth that happened elsewhere in California after the earthquake would have happened eventually anyway.

Los Angeles, he notes, owes its development to the entertainment and aerospace industries. But the flow of terrified refugees who fled the burning city on ferries bound for Berkeley and Oakland hastened development in other earthquake-prone communities in the San Francisco Bay area.

"It sped things up and happened overnight in a sense," Wollenberg said.

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An enumeration of the buildings destroyed would be a directory of San Francisco. An enumeration of the buildings undestroyed would be a line and several addresses. An enumeration of the deeds of heroism would stock a library and bankrupt the Carnegie medal fund. An enumeration of the dead will never be made. -- Jack London, 1906.

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From the beginning, it was hard to pin down the extent of damage done by the earthquake and the amount caused by the fires that raged for three days afterward. Blazes erupted almost immediately from cracked chimneys, broken gas lines and toppled chemical tanks, but with the pipes that carried water into the city from 20 miles south ruptured by the quake, firefighters were nearly helpless.

In the last two decades, historians have uncovered evidence showing that civic leaders at the time deliberately tried to minimize both the earthquake's impact and the role human error played in fueling the conflagration. To avoid scaring off Eastern investors uncomfortable with the capricious whims of nature, they decreed the disaster would only be referred to as the Great Fire, an obfuscation that never stuck.

Without a way to extinguish the flames, meanwhile, the Army resorted to blowing up buildings to create a firebreak and instead ended up spreading the firestorm. Similarly, some people purposely set fire to their quake-damaged homes, which were insured against fires but not earthquakes, according to Gladys Hansen, a retired city archivist.

Hansen has spent decades trying to come up with an accurate death count -- for decades, the official toll has stood at 478 deaths in San Francisco, 64 in Santa Rosa and 102 deaths in and around San Jose.

She thinks Complicating the task is that turn-of-the-century San Francisco attracted a transient, ethnically diverse population. She thinks the uncounted dead included many immigrants and members of the working class whose bodies were incinerated and forgotten.

"When you get new people constantly into an area, they are not sure of themselves. They don't know what the past is, and I don't think they care that much. They came here for the future," Hansen said, adding that the same could be said of people who move to earthquake country today without considering or preparing for the danger.

Another chapter in Great Quake history that continues to shift under the gaze of history deals with the great decorum that reportedly reigned in the city in the days and months after the earthquake. Photographs from the period frequently show survivors smiling as they cooked meals on the street and children playing at pristine refugee camps.

Less known is that the Mayor Eugene Schmitz issued an order authorizing soldiers and vigilantes to shoot anyone suspected of looting, or that xenophobic policies directed at segregating Japanese immigrants got so bad that President Theodore Roosevelt intervened to prevent a diplomatic crisis.

"It's not that there weren't heroic things happening, but at the same time there were a lot of terrible things happening," Wollenberg said. "You can find the same thing in New Orleans today. When a disaster of this magnitude occurs, you are going to find both the best and worst in people."

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"I am forced to conclude that when the earth shocks become so severe as to cause vibrations far beyond the range of our ordinary experience, our senses fail in comprehension." -- Joseph Harper, engineer, 1908.

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Another legacy for which 1906 will be remembered is the wealth of scientific information it yielded about earthquakes. California's Gov. George Pardee appointed a team of geologists and astronomers to photograph the damage, map the San Andreas fault and learn what they could about the ground under Northern California.

Their findings eventually would be used as the basis for the theory that earthquakes are caused when the earth's crust slips along a fault and lead to the ongoing monitoring of faults for seismic activity.

The report also showed that earthquake damage resulted both from how a building was constructed and the type of soil it stood on, a discovery that led to building codes that have been refined as subsequent quakes supplied more data on how different kinds of construction fared. Like almonds and raisins, an understanding of what it takes for a home or high-rise to withstand a minute of intense shaking is one of California's biggest exports.

"The pressure on the engineering community at that time was to prove they could design buildings that wouldn't be damaged by earthquakes," said Chris Poland, a structural engineer who is chairing a conference for 2,500 seismic safety professionals during this month's centennial week. "We are still doing that."

After 100 years, he said, they have it about right. Poland is confident that "everything built since the 1970s is going to perform in a safe manner" when the next big quake happens. What is less certain is the fate of the 70 percent of all structures built before then and never retrofitted because of the cost.

To Fradkin, it's no surprise that Californians tend to take an "ahistorical" view of earthquakes and fires. He expects the same to be true a century from now.

"Landscape shapes character. This is a landscape that moves, convulses and is constantly being reshaped over long periods of time," Fradkin said. "It's the same characteristics of the population."