After the devastating 1989 Loma Prieta earthquake, newly released documents show, PG&E identified numerous spots — from San Jose and Milpitas to San Francisco and Brentwood — where its natural gas lines might be damaged by the massive temblor that is all but certain to slam the Bay Area someday.
But despite extensively upgrading its pipes in recent years, deep concerns remain about the vulnerability of PG&E’s gas operations to a major quake, especially given the Sept. 9 San Bruno explosion, which left eight people dead.
The aged pipe that ruptured was found to be riddled with welding flaws. And PG&E, which has miles of antiquated pipe still underground, disclosed in the 225,000 pages of documents it turned over to state regulators last month that its gas lines have been plagued with welding problems for decades. As recently as March last year, one of its reports warned that old welds on line 107 — a major gas transmission pipe through southern Alameda County — might fail in Fremont if the Calaveras and Hayward faults it crosses suddenly lurch.
Such revelations are hardly reassuring from “somebody that’s running natural gas transmission pipes in the San Francisco Bay Area,” said Robert Nickell, a structural engineering consultant who helped a California Public Utilities Commission panel review the San Bruno blast. “You shouldn’t have defective pipe like that in your system.”
Manual shut-off valves
UC Berkeley engineering professor Bob Bea agreed, noting that the 1995 earthquake in Kobe, Japan, ruptured new and old pipe welds, which “resulted in large explosions and subsequent fires.”
Mary Lou Zoback, former chief scientist with the U.S. Geological Survey’s earthquake hazards team in Menlo Park and chairwoman of a San Francisco seismic advisory committee, also is deeply troubled by PG&E’s heavy reliance on manual valves. The San Bruno fire burned for about 90 minutes before PG&E workers could get through traffic and manually shut off the valves feeding gas to the pipe, according to investigators.
When the company told Zoback’s committee about a year ago that the utility’s workers would use manual valves to shut off gas in San Francisco after a quake, her group’s response, she said, was, —‰’What about all of the debris in the street and the likelihood you’re not going to be able to drive?’ We were dumbfounded that was their plan.”
Asked about the impact of a major temblor, PG&E spokesman Brian Swanson replied, “We design our pipelines to be resistant to earthquake damage” and noted that since the 1990s, the company has replaced dozens of miles of older pipe along the Peninsula and in the East Bay. He added that PG&E is checking many older pipes with water-pressure tests that should uncover weaknesses; those tests were ordered by state regulators in the wake of the San Bruno explosion.
Nonetheless, Swanson acknowledged that upgrading its gas lines — portions of which were installed more than a half-century ago — remains a work in progress. On the Peninsula, only about 66 percent of the original pipe on transmission gas line 109 has been replaced, he said, and on line 132 — the pipe that ruptured in San Bruno — only about 29 percent has been redone.
There is some urgency to finish the improvements quickly. Experts with the USGS warn that the Bay Area stands a 62 percent chance of being slammed by a 6.7 magnitude or greater quake within the next 30 years.
After the 6.9 Loma Prieta jolt, 1,094 gas-system leaks were identified, and more than half of those were deemed potentially hazardous to life or property, according to a California Seismic Safety Commission study. It found that a third of the leaks had been aggravated by material failure, construction defects or other weaknesses in the pipes before the quake, and that “natural gas could have been a factor” in a third of the 57 quake-sparked fires.
In its reports after the Loma Prieta quake, PG&E highlighted the vulnerability of its pipes that cross the San Andreas, Hayward and Calaveras faults, all capable of producing a massive quake. In response, the company moved lines 109 and 132 so they no longer cross the San Andreas. However, other lines still traverse major faults, including a pipe from Crystal Springs Reservoir to Half Moon Bay, three gas mains in Fremont and a line in the Richmond area.
Swanson said one of the Fremont segments has been replaced with stronger pipe and a trench that can better “resist deformation and fracture” was put in. The Richmond pipe has undergone similar upgrades, though more improvements are being considered on that line. He added that PG&E now believes the two other Fremont lines can withstand a 7.0 quake.
The reports also identified various places on the Peninsula, East Bay and south of San Jose where PG&E’s pipes were at risk of quake-triggered landslides and liquefaction, in which poorly packed and watery soil is suddenly weakened by shaking.
Swanson said PG&E still considers liquefaction a risk to its three main Peninsula pipes and others near Antioch and Brentwood.
So far, some experts say, PG&E seems to be making a serious effort to prepare for a major shaker.
“My sense is they are taking the right approach to the problem,” said Tom Brocher, director of the USGS’ earthquake science center in Menlo Park.
But Fred Turner, a staff structural engineer with the state’s Seismic Safety Commission, cautioned that minimizing the damage quakes can inflict on natural gas pipes “is a very large problem and it’s going to take decades of concerted investment to appropriately manage the risk.”
Contact Steve Johnson at 408-920-5043.