Earthquake experts gave a tour of four places within the City of Portland Wednesday to explain what would happen to some retrofitted structures in the event of a magnitude nine Cascadia Subduction Zone earthquake.
The tour included an old brick office block, a bridge, the harbor wall, and a fire station and was organized by the Portland Bureau of Emergency Management.
Mayor Sam Adams says the aim was to raise awareness of earthquakes and promote preparedness.
“Our tour today is intended to be both eye-opening and reassuring,” said Adams.
Eye-opening in that it highlights what the dangers are. And reassuring because the tour included structures that have already undergone seismic upgrades.
MercyCorps Office Building
The first stop was Skidmore fountain in front of MercyCorps’ new office building. Like about 1200 unreinforced masonry buildings in Portland, this one was built more than 100 years ago — out of bricks. In a large earthquake, such buildings could well collapse.
Randall Toma, an engineer with ABHT Structural, says that’s why MercyCorps retrofitted the building, with concrete walls and metal beams.
“In some cases the concrete walls encased interior unreinforced masonry walls. In other cases it was placed against the walls. Also we added steel beams and steel columns. It decreased the chance for localized bricks to essentially fall off the building or inside the building,” says Toma.
Toma says the retrofit means people in the building would survive. But it might not be safe afterwards — because of aftershocks.
And aftershocks from a massive Cascadia Subduction Zone earthquake could be substantial.
The subduction zone is a massive fault that runs along the West Coast.
Geologists believe it produces some of the largest earthquakes possible. They say it shakes once every 300 to 600 years and the last one was just over 300 years ago.
The next stop on the tour was Burnside Bridge. Multnomah County, the City of Portland and the state have identified the bridge as a key crossing point over the river. So it’s had a partial seismic upgrade.
Multnomah County engineer, Jon Henrichsen, says the upgrade basically stops the bridge decks from sliding off the bridge columns.
“During an earthquake this could be vibrated so hard that the portion of the bridge that’s on the expansion side, could fall off its seats and then come down, crushing whatever’s underneath it. So the phase one upgrades put these tubes, if you look up — that tube allows the expansion and contraction to happen, but it has a retainer ring on it that prevents it from pulling off the bridge in the event that it’s shaken,” says Henrichsen.
The county is looking at another upgrade to the bridge that would stop its columns from crumbling. With this improvement, the hope is that the bridge would not only remain standing during an earthquake, but it could also be used afterwards.
Portland Harbor Wall
The third structure on the tour was the harbor wall, on which there’s been no seismic study.
City of Portland engineer David O’Longaigh concedes it’s not the most important part of Portland’s infrastructure.
“It’s a remnant of the 1930’s when shipping was more important and the port itself was more important. Now it’s really a recreational asset. It holds up the park and for recreational shipping… So from that regard it’s an important asset when it comes to the mental well being of the city.”
The final stop was the central city’s fire station. As an essential emergency services building, it’s been extensively retrofitted — so it will not only survive an earthquake, but function properly afterwards.
Engineer Devon Lumbard says even though the first station was built in the 50’s, it still needed a lot of work.
Portland Fire Station #1
“Those deficiencies included the doors on the very front of the fire station could jam closed during an earthquake. So the very first thing the fire fighters would have to do would be to cut their way out of the building before they would be able to help anybody,” says Lumbard.
Those problems have now been fixed.
But while Adams and others point to such improvements as evidence progress is being made, the truth is that Portland would suffer terrible damage in the large earthquake.
Yumei Wang of the Oregon Department of Geology and Mineral Industries points to Christchurch in New Zealand as an example of what could happen here. A 6.3 magnitude earthquake struck Christchurch last year. Like Portland, it stands on land that’s susceptible to liquefaction, the process by which shaking soil acts like a liquid.
“About half of downtown Portland has potentially liquifiable soils. In Christchurch their central business district was closed for five months and they’ll never have a full recovery. The area looks like a ghost town,” says Wang.
Geologists estimate there’s a 10% chance of a magnitude 9 earthquake in Portland over the next 50 years — and a 37 percent chance of a magnitude 8 or less.