The Upland earthquake, the strongest in Southern California in more than two years, has focused new attention on the potential danger that a major quake poses to the massive interchange of the Santa Monica and San Diego freeways.
One of the busiest highway connections in the nation, it ranks at the top of the state's list of Los Angeles-area freeway segments considered most vulnerable to damage in a serious earthquake.
Three of the five overpasses where Interstates 405 and 10 come together have been assigned Caltrans' highest rating of seismic risk, a rating based on the volume of traffic, the age of the structures and their proximity to the Newport-Inglewood Fault. The fault, which runs less than two miles east of the interchange, is considered the most dangerous in the urbanized portion of Southern California.
An estimated 526,000 vehicles a day used the junction in 1988, Caltrans spokeswoman Lisa Covington said.
A $4.8-million program to tie freeway bridge decks together with steel cables is scheduled to begin this summer, but the effort is only the first phase in strengthening the 25-year-old interchange.
Concerned about the potential risk to motorists, Assemblyman Tom Hayden (D-Santa Monica) is introducing legislation that would direct Caltrans to establish a priority system for retrofit projects based on risk rating, potential danger to the public and probability of an earthquake nearby.
Hayden aides said the measure is intended to ensure that the San Diego-Santa Monica interchange is one of the first to be upgraded with state-of-the-art seismic safety engineering.
After the devastating San Francisco Bay Area quake last October, Hayden called on Caltrans Director Robert K. Best to make improvements to the interchange--the linchpin of the Westside's entire transportation network--a top priority.
"Over half a million cars pass through that interchange each day," Hayden said in a letter to Best. "Any structural weaknesses in this interchange presents an enormous risk to human life. For this reason alone, it is imperative that the steps required for seismic safety be taken as quickly as possible."
Caltrans spokesman Jim Drago said the concrete and steel columns that support nearly 400 freeway bridges statewide have been identified as needing reinforcement, including many along the San Diego and Santa Monica freeways.
Bridges were assigned a risk rating based on a scale from 0.00 to 1.00, with the highest rating representing the most vulnerable structure. Three of the freeway connectors at the Santa Monica-San Diego Freeway junction were assigned 1.0 ratings--higher than any other interchange in Los Angeles County. Two other connectors in the same interchange were rated 0.92 and 0.95.
California freeways underwent a fundamental redesign after the 6.4 magnitude Sylmar earthquake in 1971 caused extensive damage to overpasses in the northern San Fernando Valley.
Single-column structures built in the 1950s and 1960s, before the Sylmar quake, are considered most at risk. The San Diego-Santa Monica Freeway interchange was completed in August, 1964, at a cost of $25 million, and it predated those design changes.
Court Burrell, Caltrans deputy district director, said Thursday that a "rather substantial project" to tie the freeway bridge decks together at the interchange will go out to bid in May. Work is expected to begin during the summer and last for six months, he said.
Burrell said the Phase I retrofitting "reduces the risk there tremendously," but, he said, "there will still be some risk left."
Phase II of the retrofitting program will involve improvements to single-column structures that support the roadway. The concrete columns will be wrapped in a steel jacket to enhance their ability to resist a major quake.
Drago said the second phase of the retrofitting program will cost at least $100 million. Financed out of highway funds and proceeds from an emergency increase in the state sales tax, the program will take until the end of 1991 to complete, he said.
After the 5.9-magnitude Whittier earthquake in October, 1987, Caltrans engineers predicted that a major quake along the Newport-Inglewood Fault would subject at least a dozen major freeways or interchanges in Los Angeles to strong shaking. They predicted that as many as 100 fatalities could be attributed to bridge collapses along the San Diego, Long Beach and San Gabriel River freeways.
After the San Francisco earthquake collapsed a double-deck section of the Nimitz Freeway, killing 42 people, engineers retracted the predictions, saying they had no "specific knowledge" that 100 deaths could occur.
James H. Gates, a structural mechanics engineer for Caltrans, said last fall that the Newport-Inglewood Fault, which runs nearly parallel to the San Diego Freeway, poses a major risk. The 1933 temblor estimated at 6.3 magnitude that devastated much of Long Beach was along that fault. (Wednesday's Upland quake, by comparison, measured 5.5.)
"Everybody talks about the San Andreas, but we think the Newport-Inglewood is the critical one," Gates said. "If you get a 7 on the Newport-Inglewood, you're going to kill three times as many people as you would on the San Andreas."