It only took the rupture of a 1-inch natural gas pipe to start the domino effect that would lead to last week's blast in Northwest Portland. When it was over, eight people were injured, one building was destroyed and several others were significantly damaged.

So what happens when a big earthquake, the kind the Pacific Northwest is overdue to experience, causes dozens, if not hundreds, of ruptures to natural gas lines all over the metropolitan area?

NW Natural, the natural gas utility that serves more than 700,000 customers in and around Portland, says it's taken all the necessary steps to avoid such a catastrophe, installing flexible pipes and automatic shut-off valves.

But some experts say the region's infrastructure still faces significant challenges.

Those challenges include large areas of the city prone to liquefaction, where underground pipes could be easily compromised by shifting soil, and prolonged outages for gas service that could be out for days, weeks or longer if the worst-case scenario comes to pass.

Not a matter of if, but when

A significant quake striking the Cascadia Subduction Zone, the roughly 600-mile fault line that runs off the West Coast from northern Vancouver Island to Cape Mendocino, California, is not a matter of if, but when.

Estimates of how often the fault line shakes were recently revised upward, with experts now saying that a massive seismic event is predicted to hit along the section of the fault that runs from Newport to Astoria every 350 years.

The last time the subduction zone saw such a quake where the entire zone ruptures, which scientists have been able to pin down because it caused a tsunami that traveled across the Pacific Ocean to Japan, was in 1700, or 316 years ago.

And while the coastal areas of Oregon certainly face an enormous risk of a tsunami after a large earthquake, those who live inland will deal with other hazards.

One of those hazards is liquefaction -- which happens when loose, sandy soil is shaken then acts more like a liquid than a solid. Anything on even a slight slope can slide downhill, and any pipes in that once-solid ground will likely rupture.

Much of Portland is prone to liquefaction. Huge swathes of land along the Columbia River, including where the Portland International Airport sits, are at high risk as are most of the Willamette River's banks. Much of inner southeast Portland is considered to be at a lower risk for liquefaction but could still become unstable in a large quake.

"In order to amass more developable land, fill materials were brought in to cover lakes, swamps, and added to river banks. In areas of loose natural soils or fills, liquefaction and settlement are likely," Yumei Wang, a geotechnical engineer with the Oregon Department of Geology and Mineral Industries, wrote in a 2009 study.

A map of the areas prone to liquefaction in and around the Portland area in the event of a large earthquake.

The risk posed by liquefaction was more than evident in San Francisco's Marina District, much of which was built on landfill, after the 1989 Loma Prieta earthquake. Entire apartment buildings dropped onto their foundations and, at Beach and Divisadero Streets, a ruptured natural gas line fed a massive fire that burned for hours after the shaking stopped.

"If you look at history, fire is often an equal threat to structural damage in the aftermath of an earthquake," said Chris Goldfinger, a leading expert on tectonic activity and professor of earth sciences at Oregon State.

The Loma Pieta quake, known for hitting during the Bay Area World Series between the Oakland Athletics and the San Francisco Giants, was a magnitude-6.8 and lasted between eight and 15 seconds. A worst-case scenario for the Pacific Northwest, in which the entire Cascadia Subduction Zone ruptures, could produce an earthquake in the neighborhood of magnitude-9 where the shaking could last for minutes.

Preparation key to avoiding catastrophe

NW Natural began taking preemptive steps to avoid widespread natural gas leaks after an earthquake more than 30 years ago, said spokeswoman Daphne Mathew, by working to eliminate rigid cast-iron gas transmission pipes with more flexible materials.

"We began installing polyethylene pipe in the early '80s. Our formal cast-iron system replacement began in the mid-1980s," Mathew said. "By 2000, NW Natural was one of the first companies in the country to eliminate cast iron pipes. All known bare steel was removed by the close of 2015."

But Bill Steele, communications director for the Pacific Northwest Seismic Network, said that even the more flexible materials could be insufficient if the big one hits and there is widespread land displacement.

"Pipeline systems are a concern for a few reasons," Steele said, namely because they are widely distributed and are sure to be affected by earth movement. "Modern pipes are more (flexible), but with some types of earth movement, like lateral spreading, the soil column can move tens of meters, and that is a big problem."

In the event that the more flexible pipes did suffer a rupture, Mathew said, the utility has deployed more than 200,000 shut-off valves throughout the system installed at individual residences. Some stop the flow of gas automatically when they detect a change in pressure caused by damage to a pipe, others are controlled remotely, and some need to be turned by hand.

There was no such valve in place at the site of the explosion on Northwest 23rd Avenue, Mathew said.

NW Natural maintains a round-the-clock staff to run their control center and has a "seismically protected" auxiliary control center in Sherwood that can be used if the Portland facility is knocked offline.

Beyond that, Mathew said, the utility has put its staff through training to deal with the types of issues that will no doubt spring up in the aftermath of a disaster, including leak detection and emergency shut-offs. NW Natural also has mutual assistance agreements with other utilities across the West in case the post-earthquake problems prove too large and reinforcements are needed.

The responsibility of the utility only goes as far as the gas meter, however. Pipes inside homes and businesses are the responsibility of the building owner who will need to do their own preparation, Mathew said. That includes knowing where the shut-off valve is, having the proper tools to shut it off and knowing when it is appropriate to do so.

That also includes preparing your own gas appliances inside your home by replacing rigid pipes with flexible ones and strapping hot water heaters in place so they don't topple over in a quake.

Mathew said the safeguards put in place should be capable of handling whatever Mother Nature throws at it.

"We're very confident in our system," Mathew said.

More than just natural gas at risk

Wang, the geotechnical engineer at the Oregon Department of Geology and Mineral Industries who has studied earthquake risk to infrastructure in the Willamette Valley extensively, said the problems would not be limited to natural gas.

A 2013 report, on which Wang was a co-author, found that the entire energy sector in Oregon would likely be stymied after a large-scale earthquake.

"We looked at natural gas, electricity and liquid gas and determined that all three sectors would have significant problems," Wang said. "These systems were built a long time ago, before we understood that we needed to build to withstand these larger earthquakes."

Wang's report did not look specifically at the risk of fire and explosions, like the one on 23rd, caused by natural gas leaks after a quake, but she said "it is something we should expect to happen."

Beyond the potential for leaks and fires, Wang said the public should be prepared for long-term outages of some vital services from utilities, many of which are dependent on each other.

"A lot of these systems work together and take a long time to get back up and running," she said. "Natural gas systems aren't designed to be shut down, but they will likely be inoperable for an extended period of time."

-- Kale Williams

kwilliams@oregonian.com

503-294-4048

@sfkale