Could the next big earthquake rock the U.S. economy?

The 6.1-magnitude earthquake that shook the Napa region of northern California early Sunday morning left scores of people injured and early estimates of over $1 billion in damage.

But it could have been a lot worse. Many buildings in the region withstood the moderately strong quake with minimal or no damage. According to the National Institute of Building Sciences, about half of the nation's states and territories -- over 109 million people and 4.3 million businesses -- are "exposed to risks from seismic hazards."

The average direct costs of damages from earthquakes in the U.S. runs about $1 billion annually, while indirect business losses are thought to top $2 billion each year.

Beyond the immediate human costs and property damage, some fear that another major earthquake could spark a major economic setback to the affected region, or even nationally.

California has long been America's test lab when it comes to earthquake preparedness and mitigation. State and local researchers have pioneered a lot of the innovations in structural technology, as well as the regulations, that have helped to reduce the loss of both life and property during its many earthquakes.

In 1933, after a deadly earthquake devastated the Long Beach region, the state passed one of the nation's first comprehensive building safety codes, requiring more earthquake-resistant construction standards for schools. And in 1994, following the magnitude 6.8 earthquake centered under Northridge that rocked much of greater Los Angeles, the state required most of California's hospitals to be retrofitted against earthquake damage.

Surprisingly, however, "[California] doesn't really have a mandatory seismic upgrade ordinance covering any kind of building," said Thomas Sabol, a professor at UCLA''s Civil and Environmental Engineering Department. "That's usually left to the discretion of cities and counties."

And many of those municipalities, Sabol notes, have created ordinances to deal with un-reinforced masonry, which can rain down from older buildings during earthquakes and is often a leading cause of deaths and injuries.

Along with retrofitting vulnerable older buildings with steel framing and other earthquake-resistant materials, more advanced methods are being developed to strengthen old brick and masonry buildings, including use of a material called fiber reinforced polymer.

"It's essentially a very sophisticated wallpaper," Sabol explained. "It's similar to the composite materials that are used in the [Boeing] 787 Dreamliner. They're applied to the brick masonry with an epoxy."

Safeguarding older buildings against earthquakes can be expensive, and those costs usually have to be considered on a case-by-case basis

"The rule of thumb is that if it costs more than 50 percent of the replacement value of the building, there is no economic value in upgrading the building," Sabol said.

"Many owners would even... say that once you get above 30 to 40 percent it's not even worth it. So the effective range is going to be in the 10 to 20 percent of the replacement value. And it depends on what kind of building you're looking at."

For all the innovations, regulations and updated preparations, a lot of the U.S. still remains economically vulnerable to the long-term impacts of a major earthquake.

The cost of earthquake insurance has risen dramatically in some quake-prone regions. And a 2008 scenario, created by the U.S. Geological Survey, predicted a major earthquake along California's San Andreas fault would kill 1800 people, injure 50,000, cause $200 billion in damage "and have long-lasting social and economic consequences."

In 1977, Congress set up the National Earthquake Hazards Reduction Program (NEHRP), with the goal of having the federal government coordinate efforts to reduce the deaths, injuries and property loss caused by earthquakes.

NEHRP also has its own recommended "seismic provisions" for new buildings and othe structures. But the Federal Emergency Management Agency recently noted that "adoption of the model codes is uneven across and within states, even in areas with high levels of seismic hazard."