Were something like that 1700 quake to occur today — and it certainly could, seismologists say — enormous destruction and loss of life would result in a region that is home now to big cities and millions of people.

The magnitude-8.8 earthquake that rocked Chile and sent tsunami fears across the Pacific on Saturday — nearly seven weeks after the enormously deadly quake that destroyed parts of Haiti— serves as a vivid reminder of the perils posed to the United States by countless fault lines and shifting plates.

"It's not a matter of if, only of when an event like this strikes the people of the United States," says Marcia McNutt, director of the U.S. Geological Survey. "Shame on us if we don't prepare."

As of Sunday, the death toll from the earthquake in southern Chile stood at about 700. By comparison, the Jan. 12 earthquake in Haiti killed about 230,000 people, the Haitian government says.

Chilean officials said 500,000 houses were destroyed or badly damaged, and President Michele Bachelet said "a growing number" of people were listed as missing. Looters ransacked stores, and a curfew was imposed in Concepcio´n, where an estimated 60 people were trapped in a collapsed apartment building.

Haiti's earthquake was shallower and closer to a major city, Port-au-Prince, than was the Chilean quake, which accounted for much of the devastation in Haiti. Stricter building codes and better enforcement of them played a major role in reducing the loss of life in Chile, says Andres García, manager of AGR Analysis, a construction and building management company in Viña del Mar, Chile.

"Chile has been building according to the best standards in the world for at least 20 years," García says. "As the technology and techniques have gotten better, the rules have gotten stricter. And that's what has minimized the loss of life this time around."

Chile sits on the so-called "ring of fire," a system of geological faults that circles the Pacific Ocean, and is frequently rattled by earthquakes. In 1906, a magnitude-8.6 quake near Valparaiso killed 20,000 people. A magnitude-7.8 quake near Chillan killed 28,000 people in 1939.

The quake on May 22, 1960, near Valdivia shattered the records for the strongest quake ever. With a magnitude of 9.5, it sent tsunami waves racing through the Pacific to as far away as Japan and the Aleutian Islands of Alaska. About 5,000 people were killed.

After that quake, Chile imposed strict rules about the quality of building materials, García says. It also invested heavily in research to find weak points in the soil under major cities.

"Chile has one of the most modern building codes in the world, and now we're seeing how the rules pay off," says Juan Felipe Heredia, a Mexican civil engineer who has designed buildings in South America.

Chile's economic strength also gives it an advantage. It is the wealthiest country in Latin America, with a per-capita income of $14,700.

"In Chile, they've been preparing for this moment for a long time," says Uriel Texcalpa, an architecture professor at Mexico's Iberoamerican University. "Haiti is just beginning. They're starting from zero."

Chile offers more lessons for U.S. planners than Haiti does, McNutt says, given similarities in building codes and earthquake awareness. Although the U.S. has made preparations more stringently than anywhere else, McNutt sees an Achilles' heel in the aging U.S. infrastructure of bridges and overpasses.

"I look at the reports of collapsing bridges and highways in Chile and worry what would happen here," she says.

In 2009, the American Society of Civil Engineers warned that 26% of the nation's bridges "are either structurally deficient or functionally obsolete." The engineers' report, based on Department of Transportation figures, showed that one in three urban bridges are either broken or obsolete, and suggested a $17 billion yearly shortfall in maintenance spending nationwide.

"Clearly infrastructure is a legitimate worry," says geotechnical earthquake engineer John Christian of Waban, Mass., a member of the National Academy of Engineering. "Engineers worry that we have plenty of buildings that fall down on their own, even without earthquakes."

California's San Andreas fault poses a much-worried threat. Californians have focused on preparing for a Big One for more than a century, since a 1906 quake in Northern California motivated people to take the hazard seriously, says Susan Hough, geologist at the U.S. Geological Survey in Pasadena. A 1933 quake centered at Long Beach led to passage a year later of tougher building requirements for schools.

"Since that time, the building codes have continued to evolve as we learn more about what buildings are dangerous and how the ground shakes under earthquakes," she says.

State and local building codes have prevented new construction of the most vulnerable building styles, unreinforced brick or concrete structures, she says. The 1994 Northridge earthquake in the San Fernando Valley of Los Angeles exposed weaknesses in steel welds and buildings.

Now the biggest worry is widespread use of apartments built atop ground-level parking, with supporting poles that can give way in a quake of sufficient size, she says.

"People may be complacent about California," Hough says. "We haven't really seen a big earthquake that really tests the infrastructure."

Such a big quake may be overdue in the heavily populated Southern California region, says Jim Goltz, earthquake and tsunami program manager for the California Emergency Management Agency.

"We have hundreds of faults in Southern California, and we could have a really large earthquake on any of them," Goltz says.

Officials in California are also focusing on non-structural hazards that can be deadly in big quakes, such as big-screen televisions, water heaters, furniture and bookshelves that can become missiles in violent quakes.

This Oct. 21, California officials will hold their third annual "Great California Shakeout" event aimed at informing people what to do in such a quake. Their advice — "Drop, cover and hold on" — suggests people take cover under furniture to protect themselves from flying objects.

Elsewhere along the Pacific Coast:

• The state of Oregon last month announced 13 school buildings and 11 emergency management facilities would be retrofitted to withstand earthquakes. More than 1,000 such buildings are at high or very high risk of collapse during an earthquake, according to a 2007 report from the Oregon Department of Geology and Mineral Industries.

• Washington state's Department of Transportation announced in 2008 a two-year project to retrofit 19 bridges on Interstate 5 to current seismic standards, part of a plan to improve more than 900 bridges to withstand earthquakes.

• Naval Hospital Bremerton completed a seismic retrofit project in 2007 to improve its ability to withstand a large earthquake and provide medical care during and after a crisis, according to the hospital public affairs division.

The spate of recent earthquakes, starting with the magnitude-9.3 Indian Ocean event in 2004, follows a 50-year cycle of earthquake activity, McNutt says. The last cycle, in the 1960s, produced the two other record holders for recorded earthquakes — the magnitude-9.5 quake near Valdivia and a magnitude-9.2 quake in Alaska's Prince William Sound.

"We know earthquakes are not uniformly distributed in time; they cluster," McNutt says. "Now suddenly the earthquakes are lighting up again."

Even with the knowledge that a Big One is inevitable, retrofitting buildings and requiring better building practices is a tough sell, even in parts of the country where quakes are facts of life, says Mark Benthien of the Southern California Earthquake Center at the University of Southern California.

"Improvements to our building codes have often followed the earthquakes that we have had," Benthien says. "They are very difficult to pass in other times."

A report commissioned recently by the Earthquake Engineering Research Institute and the U.S. Geological Survey concluded that many of the deaths in Haiti's earthquake could have been prevented by using earthquake-resistant designs and construction, as well as improved quality control in concrete and masonry work of affected buildings.

"The massive human losses can be attributed to a lack of attention to earthquake-resistant design and construction practices, and the poor quality of much of the construction," according to the report. It added: "Indirect evidence suggests that the earthquake did not produce ground motions sufficient to severely damage well-engineered structures."

Chile shows that earthquake-resistant building codes don't mean that people will be able to return to buildings, "just that they won't fall on them," Christian adds. The unfolding scenario of millions of displaced Chileans would likely occur in the USA as well, after a major earthquake, he says.

"We could build things to completely survive earthquakes," Christian says. "They would all look like nuclear power plants. And cost as much."

No predictions are possible for when an earthquake will strike, but the pattern of recent events does worry U.S. planners.

McNutt points to Alaska as the closest copy to Chile on U.S. shores. With a very active fault in the Aleutians and a population hugging the coasts, Anchorage and Juneau are susceptible to similar "subduction" earthquakes, where the Pacific Ocean plate dives under the North American crust.

But it is Puget Sound, with its population and potential to funnel in a tsunami, which is being watched closely.

"Seattle is another area of concern," McNutt says.

Off Washington state's coast, large earthquakes have struck every 500 years or so, with the 1700 quake the last major one. The resulting tsunami tore cedar trees from the ground along Puget Sound and was written about in Japan. The Juan de Fuca plate moves about 40 feet in a century, which means about 120 feet worth of energy is coiled up in the fault now, says Brian Atwater, a U.S. Geological Survey expert on historical tsunamis.

"That gives you about a one in 10 chance of (another large quake) across the next 50 years," he says. "That's enough for society to make some serious decisions about how we build schools and hospitals."

Contributing: Oren Dorell