On the 100th anniversary of the 1906 San Francisco earthquake, seismic experts see progress in Utah's preparedness for such a disaster, thanks in large part to technological advances.

But in some ways, Utah is as vulnerable as California was when it was jolted awake at 5:12 a.m. on April 18, 1906, by one of the biggest earthquakes in the state's modern history.

While initial estimates pegged the death toll at about 700, later research suggests as many as 3,000 of the city's 400,000 residents died directly or indirectly from the quake, which revised estimates have since placed between a magnitude 7.7 and 7.9, according to the U.S. Geological Survey.

Many of the structures that failed when the 1906 quake rocked the Bay Area were unreinforced masonry buildings. Despite this deadly example, similar brick structures - including many older homes - continue to dominate quake-prone Utah's building stock 100 years later.

When the "Big One" finally strikes the Salt Lake Valley, computer simulations indicate that hundreds of buildings could collapse while thousands of others would become uninhabitable. There could be some 2,200 deaths and many more injuries, not to mention severe disruption of power and water service.

Areas such as West Valley City, while not near the main Wasatch Fault segments, are at risk for significant damage due to the soft soils that lie below many homes. Certain ground motions could turn that soil into a liquid-like substance.

Utahns can take some comfort in that the Wasatch fault system, because of the geology involved, cannot produce a quake as strong as the one that struck San Francisco. In theory, the faults that snake beneath the Salt Lake Valley could produce a magnitude 7.5 ground shaker, but most estimates indicate the long-awaited quake would probably register no more than 7.25, said Jim Pechmann, a researcher with the University of Utah Seismograph Stations. A magnitude 8 quake, for example, is 10 times more powerful than a magnitude 7.

"We don't expect something as big as 1906 happening here," he said.

Grim scenarios: The fact that researchers can even place limits on quake estimates puts them in a better position than the scientists of a hundred years ago who knew little about fault system dynamics.

"The 1906 earthquake was really the birth of modern earthquake science," said Kristine Pankow, of the U.'s seismograph stations.

Decades of accumulated knowledge about how different fault systems operate now shape computer earthquake simulations.

Utah planners are using software known as HAZUS to examine post-quake scenarios that could hit the state, said Bob Carey, earthquake program manager for the state Department of Emergency Services and Homeland Security. The Salt Lake stretch of the Wasatch fault runs close to the western edge of the mountain range of the same name. Ancient quakes contributed to the valley dropping and the mountains rising. For planning purposes, many simulations assume a 7.0 magnitude earthquake. The state could feel a bigger one, but the resulting casualty and damage estimates would not increase too much, he said.

If a 7.0 hit the Salt Lake segment of the Wasatch Fault, the area could experience about 2,200 deaths and 1,100 serious injuries requiring hospitalization. Another 6,800 might suffer moderate injuries while 21,000 would experience slight injuries.

The disaster could cut electricity to 283,000 of the county's 295,000 households after Day 1. The lights would remain off for about 271,000 households after Day 30.

Similar problems would affect water service. About 227,000 households would go dry after Day 1, with 30,000 households remaining waterless through Day 30, Carey said.

Of Salt Lake County's estimated 240,000 buildings, about 91,000 would suffer extensive to complete damage. Building damage-related losses could total $28 billion.

The HAZUS software does more than spit out disaster scenarios. If a quake strikes, planners can load maps of ground-shaking severity to help determine where help and resources need to be immediately focused.

What lies beneath: Even before HAZUS, scientists began learning more about quakes as San Francisco in 1906 became a grim science laboratory. Pankow said scientists realized that what is underneath a building is just as important as how far it is away from the fault.

In San Francisco, structures built on bedrock tended to fare well in the quake. Many homes and buildings sitting above soft soil suffered heavy damage, she explained.

As the city searched for more land to build on, people began dumping dirt into the bay. This soft soil mixed with a high water table spelled trouble when the quake struck.

The Salt Lake Valley is in a similar situation, though nature provided the sandy soil found in many parts of the area, including western parts of Salt Lake City. With bedrock generally lying too deep to reach, many homes and buildings sit on this soil that could turn into a liquidlike substance when the ground shaking starts.

Combine this potentially liquefied soil with unreinforced buildings and the valley could face serious problems following an earthquake.

"It's not really the earthquake that kills people," Pankow said, "it's the buildings that kill people."

Unreinforced structures face the danger of total collapse or the destruction of key structural elements inside the building.

In the 1930s, California began to institute seismic regulations, which essentially banned new construction of unreinforced buildings, said Pechmann. Two of California's most recent significant quakes, the magnitude 6.9 in Loma Prieta in 1989 and the 6.7 quake in Northridge in 1994, killed 63 and 61 respectively, according to the California Department of Conservation.

It was not until the mid-1970s that Utah finally followed suit with building codes to deal with earthquake hazards. But all those earlier decades of building with brick leave an expensive legacy not easily overcome for the state.

"We're still stuck with this large inventory of old buildings not built to seismic regulations," Pechmann said.

Places like Los Angeles spent years going after owners of unreinforced building to either retrofit them or tear them down to combat this problem, he said.

Salt Lake City has had its share of high-profile buildings retrofit, including the City and County Building and the Capitol, but most older buildings remain vulnerable to quakes.

All quiet on the Wasatch Front: Part of the challenge in raising public awareness is that the Wasatch Fault has been relatively dormant since the Mormon pioneers settled the region. The largest Utah quake in recorded history was in 1934 when a magnitude 6.6 quake rocked Hansel Valley, north of the Great Salt Lake.

Experts believe the Wasatch Fault is overdue for a major quake, as stress has been building for centuries within the fault system.

Faults can be thought of as rubber bands - they can only be pulled only so far before they snap. It's not a question of if the snap will happen, experts say, it's a matter of when.

glavine@sltrib.com