Homes would go dark. Water supplies would be cut. Buildings would topple. And help might not arrive for a long time.

A massive earthquake centered on the South Coast would have many scary outcomes, including being cut off from the rest of the world by road.

The rivers and sloughs that offer fishing holes, avenues for excursions and picturesque views also would prevent outside help from getting in if bridges fail.

Making bridges strong enough to withstand a huge quake is an expensive proposition, and state officials aren’t sure where to start.

They estimate they’ll need $3 billion to prepare Oregon’s bridges to withstand a major earthquake along the coast, far more money than they have. Without such repairs, a 9.0 temblor would leave U.S. Highway 101 impassable and state highways 38 and 42 in disrepair.

ODOT has long known an earthquake would cause catastrophic damage, said spokesman Dave Thompson. A study released in October, prepared by ODOT and Portland State University, quantifies what the damage would be.

“This report is confirmation that a major quake will do major damage,” he said.

The state’s next step will be to analyze what bridges are most in need of repairs and deserving of the state’s scant resources. The ODOT’s bridge section has established a basic priority list, ranking bridges along interstates 5, 405 and 84 as the top three. State Highway 42 has the fourth highest priority, while U.S. Highway 101 is seventh.

The state’s interest in retrofitting bridges on Highway 42 is because it would serve as a route for emergency vehicles following an earthquake, Thompson said. Highway 101, on the other hand, would be much more complicated.

“The problem with 101 is its length and the risk of slides,” he said. “Not only will the bridges be impassable, but the roads away from the bridges will be covered in slides.”

Even if slides prevent quick re-entry into the area, preventing bridges from collapsing or suffering extensive damage would be beneficial. Replacing a severely damaged bridge costs an average of $3 million, the report said.

State Highway 38 has the third lowest priority of highways under consideration, in part because it has some of the safest bridges in the state. ODOT replaced several bridges along the route connecting Reedsport with the I-5 corridor last year, but some require upgrades or replacement, including the Scottsburg Bridge.

People in Coos Bay and North Bend conceivably could be trapped if the area’s major bridges failed. Coos Bay firefighters would have limited access to Eastside and would possibly rely on Millington volunteers.

“Bridges are a concern to us,” said Battalion Chief Mark Anderson, with Coos Bay Fire. “I don’t think you can get to Coos Bay-North Bend without crossing a bridge somewhere.”

But bridges aren’t the only problem, said Jared Castle, an ODOT spokesman in Roseburg.

“Liquefaction is one of the issues,” he said. “An earthquake of the Cascadia Subduction Zone would have dramatic impacts to roads and to the approaches to bridges as much as the bridges themselves.”

They’d sink and buckle.

No one knows for sure when an earthquake will hit, but engineers have some idea how buildings would respond.

Researchers used a computer program that simulates damage to bridges in an earthquake. It can predict ground motions caused by an earthquake, as well as the resulting bridge damage and its cost.

In one scenario, an 8.3 magnitude earthquake centered on the South Coast, two bridges collapsed and 23 suffered enough damage that would require replacement. State officials anticipated such an earthquake would require $363 million simply to repair the bridges.

At greatest risk to damage are bridges built before 1974, when new design standards were established in the aftermath of an earthquake in San Fernando, Calif. While older bridges wouldn’t necessarily fall down, they’d likely suffer enough damage to knock them out of service. The list of susceptible bridges includes McCullough Bridge, Umpqua River Bridge in Reedsport and the Patterson Memorial Bridge in Gold Beach.

“The older bridges, as beautiful as they are, and as strong as they have been built, all of them would become impassable,” he said.

The McCullough Bridge is receiving cathodic protection, a process that was completed several years ago on Patterson, but it won’t strengthen the bridge for an earthquake.

There are two tiers of repairs the state considers useful for bridges. The first tier are less expensive, but may leave bridge’s susceptible to damage in a major quake.

Thompson said ODOT will hold meetings to discuss funding priorities. He also said there wasn’t a deadline to have the next round of analysis completed.