Western metropolitan areas that thought they were at low risk for earthquakes may be in for a shock.

Armed with new technology, geologists have found that more areas in the West are prone to earthquakes, and some are located in areas that are more seismologically active than first suspected.The geologists are using their newfound knowledge to help develop stiffer building codes and improve disaster preparedness. As more people move West, they say, the risk of casualties increases.

"People expect earthquakes in California, but when one of these other major cities has an earthquake, they're really going to be shocked," said Stuart Nishenko of the Federal Emergency Management Agency. "We are living in a dangerous part of the country."

Using satellites, geologists recently determined that a fault near Salt Lake City was more active than once suspected, said Bob Smith, a professor at the University of Utah. Urban areas of Salt Lake City, Ogden and Provo lie on soft lake sediments that geologists are warning could shake violently.

A recent aerial geological survey revealed patterns in the Earth's magnetic field that confirmed the existence of a previously suspected fault running through Portland, Ore. On March 25, 1993, at 5:34 a.m. a magnitude 5.6 earthquake shook the entire region, causing $30 million in damage. It was felt over an area of more than 50,000 square miles.

Reno and Carson City also could be in for a good shaking, warned John Anderson of the University of Nevada.

Many of these areas have buildings that were not built to withstand a major earthquake.

"Everybody's gambling. Reno is going to have a shaking every bit as strong as the North Ridge or Kobe earthquakes," Anderson said.

He said the 1994 Northridge earthquake in California that killed 72 people, injured 9,000 and caused more than $40 billion in damage and economic losses was a "wakeup call" for earthquake preparedness because of the havoc it wreaked.

Although the faults near Reno tend to have an earthquake only every 20,000 years or so, they are grouped in clusters of 20 to 30 and react independently, increasing the possibility of a disaster.

Even Las Vegas, which is not along a major fault line, is not immune. A quake along a major fault line to the west could cause collateral damage and deliver a severe blow, Anderson warned. The Nevada Educational Safety Council recently reviewed emergency procedures to determine the potential impact of a disaster on roads, schools and communications.

Jon Ake of the U.S. Bureau of Reclamation said many communities only now are becoming aware of the risk, and are taking steps to prepare, with help from the Federal Emergency Management Agency.

Ake said some communities have drawn up "scenarios" to determine how to prepare. Public safety agencies are studying the results to see how police and rescue workers should react.

Using a government computer program, communities can predict the violence of a potential earthquake, and estimate the damage to buildings, roads, electrical and water supplies, the number of casualties and the financial impact.

Builders and professional associations are also studying proposed major revisions nationwide to building codes by 2000 that will change the way insurance companies and governments enforce building codes.

The old codes were designed to prevent buildings from collapsing in a major earthquake. The new codes are designed to limit damage.

"Congress is sick and tired of spending billions of dollars on disaster recovery," said Paul Somerville, of the Woodward-Clyde engineering consulting group. "They want to prevent the damage. Everyone's saying, `Let's get smart,' and not only build buildings that won't collapse, but build buildings that can withstand damage from minor earthquakes every 20 years or so."

Before 1994, building codes placed the Puget Sound area of Washington, including Seattle, in a high-hazard zone. After careful study, the higher level was extended to include all parts of Oregon and Washington near potentially serious quakes.

New buildings are now designed to resist earthquakes 50 percent stronger than before. The discoveries also helped persuade public officials to strengthen dams, bridges, water systems, schools and some factories. More than $130 million was invested in seismic upgrades since 1988 in the Pacific Northwest.

The new regulations will apply primarily to new construction, although a few federal buildings and others will be required to retrofit.

That means only about 2 percent of the buildings will be replaced a year as they crumble or are torn down, leaving many buildings and their occupants at risk for the next 50 years.

But the burden will be on the owner, not the government, under the new system, Somerville said.

"The owner of a building will be given a choice. If he wants his building to be operational following a minor earthquake, he should follow the new building code. If he wants to patch it up and go with the other option, that's his choice."