As Oregon comes to terms with growing evidence that an extremely large earthquake is likely to strike in coming decades, transportation authorities in the state said on Monday that half the state's bridges could collapse when "the big one" strikes.

In a worst-case scenario, a magnitude 9.0 quake would destroy dozens of bridges and overpasses on U.S. Highway 101, the major north-south road along Oregon's coast, as well as those on every significant road connecting isolated coastal communities with inland areas, Don Hamilton, a spokesman for the Oregon Department of Transportation, told Reuters.

"How do we get relief supplies through to the Oregon Coast? We will lose significant supply routes," Hamilton said.

Up through the early 1980s, the risk of a very large earthquake to the region was considered relatively low, and most bridges in Oregon were built before current seismic construction standards were enacted, Hamilton said.

However, researchers now believe that an 8.0 to 9.0 magnitude temblor has shaken the region roughly every 230 years, with the last striking 315 years ago, according to the U.S. Geological Survey.

In a review of the state's bridges, the transportation department found that a major Interstate 5 bridge that connects Oregon and Washington, and was built nearly a century ago, would also almost certainly collapse as the infill on which its foundations rest liquefied.

Other interstates bridges might survive, but see their on-ramps collapse.

In the assessment of bridge health released earlier this month, transportation officials estimated that maintaining bridges at current standards would cost $240 million per year, but they did not estimate the cost of upgrading all bridges to withstand a massive quake.

"Every new project that we work on includes seismic upgrades," Hamilton said. "But this is an enormous issue, and obviously upgrading all the bridges would be very, very expensive. This is a challenge that consumes us all the time."

(Reporting by Courtney Sherwood in Portland, Oregon; Editing by Dan Whitcomb and Sandra Maler)