The state is strengthening highway bridges to make them safer in earthquakes, but older brick structures in the Truckee Meadows continue to be unsafe in temblors as large as those that recently hit central California and devastated Iran, officials said.

“No bridge or structure is ever really earthquake-proof,” said Todd Stefonowicz, assistant chief bridge engineer at the Nevada Department of Transportation.

“For the last three years we’ve been working on retrofitting highway overpasses on Interstate 80. We want to keep them standing in moderate quakes and keep the damage to areas that are easily identified and repaired.”

Nevada is the nation’s third most earthquake-prone state, after Alaska and California, scientists said.

On Sept. 12, 1966, a fairly large quake — 6.5 on the Richter scale — centered in the Verdi area shook buildings and triggered minor earth slides, but caused no significant damage or injuries.

One of the last significant earthquakes in the region struck near Portola, Calif., in August 2001, measuring at magnitude 5.4. On Oct. 30, 1998, a 4.9 quake centered near Lake Tahoe’s north shore startled residents awake.

In 1869, an earthquake about the same size “or a little bit bigger” than Monday’s central California temblor hit on the Truckee River near Wadsworth, said Glenn Biasi, a scientist at the University of Nevada, Reno seismology lab.

A stretch of central Nevada from Winnemucca south to Gabbs experienced as many as five strong earthquakes in the 20th century, including a magnitude 7.2 event that shook the Dixie Valley east of Fallon in 1954.

“Earthquake prediction and weather prediction share the same complex math,” Biasi said. “But meteorologists can fly through weather systems and we can’t do that with the ground.”

He said seismologists can predict earthquake probability but can’t forecast temblors.

In 2001, researchers said there was a 34 percent to 98 percent chance that an earthquake of magnitude 6 or greater likely will rock the Reno-Carson City corridor within 50 years. That temblor is sure to cause significant damage on unreinforced masonry buildings.

Unlike highway bridges, older buildings aren’t being reinforced unless the owners decide to do so. Nevada, unlike California, doesn’t require that local officials survey at-risk buildings and adopt laws requiring the structures be reinforced or torn down. In Nevada, older buildings are not covered by newer building codes unless their use changes.

Officials said the current building code, the 1997 version, is adequate to protect newer structures.

In 2001, after a 6.8 earthquake rocked Seattle, Reno officials said they would consider adopting a more stringent building code. But the International Building Code 2000, which requires even greater earthquake-resistant construction, wasn’t adopted.

A new code wouldn’t have affected older brick structures, similar to the historic clock tower in Paso Robles, Calif., which lost its roof in this month’s 6.5 temblor, killing three people. In Paso Robles, building owners have until 2018 to upgrade brick structures to the current building code, according to the California Seismic Safety Commission.

Owners are often given lots of time to upgrade buildings because reinforcing a single old structure can cost more than $300,000.

A 1993 University of Nevada, Reno study identified 114 of the hundreds of at-risk masonry buildings in Northern Nevada built before 1941, but little follow-up has been done, officials said. The 1993 report recommended 58 brick buildings in Carson City, Reno and Sparks for further study. All were constructed of unreinforced masonry and considered highly vulnerable in a significant earthquake.

The study predicted that most Truckee Meadows commercial buildings erected after 1970 would sway and survive a magnitude 6.5 temblor like the one that rocked California’s central coast recently, but many older masonry structures would crumble into history.

A 1996 state study estimated a quake in western Nevada would leave 7,000 people injured, with the greatest risk to people in unreinforced masonry buildings, pre-1950 wood-frame houses, tilt-up concrete buildings and mobile homes.

“Old masonry buildings are a problem anywhere,” said Manos Maragakis, the chairman of the civil engineering department at the University of Nevada, Reno. “They are the first to collapse.”

Maragakis led the study of the earthquake vulnerability of some of the masonry buildings in the Reno-Carson City corridor.

While current building codes likely would minimize structural collapses, older buildings without steel-reinforced walls would shake, rattle and fall during a moderate to major quake, researchers said.

Most of the buildings in the area are built to the newer codes, which require reinforced construction to withstand quakes.

That wasn’t the case with the highway bridges on Interstate 80 and elsewhere. The state determined 250 of the state’s 1,500 bridges might need to be improved to make them less vulnerable to earthquakes.

In 2001 and 2002, Nevada spent $2.4 million to retrofit nine bridges over I-80 through Reno. Crews are working on five overpasses and ramps near the Spaghetti Bowl — the interchange of I-80 and U.S. Highway 395 — and next year are scheduled to begin a $1.3 million project to retrofit five I-80 bridges between Keystone Avenue and Verdi.

Stefonowicz said those overpasses were built in the 1960s before building codes were very strict. As built, they can’t withstand large movements caused by even moderate temblors.

The crews are strengthening the bridge columns, allowing them to move with the earth without completely falling apart, he said. Single columns are encased in a steel sheath, while double-column supports are linked with a wall.

Even with improvements, disruption of transportation routes is an expected result of a magnitude 6 or greater quake in northern Nevada, according to the Nevada Bureau of Mines and Geology.

In its 1996 “Earthquake Scenario for Western Nevada,” the bureau’s researchers forecast that bridge damage, rock slides and other quake effects would close Interstate 80 west of Verdi for three days.