Understanding the Cascadia Subduction Zone and Our Work to Prevent DamageJul 29 2015 · 0 comments · NISHKIAN DEAN, Seismic ·0
A recent article in The New Yorker entitled “The Really Big One: An earthquake will destroy a sizable portion of the coastal Northwest. The question is when.” has caused a media storm with outlets across the country now talking about, what was for many, a previously little-known fault line, the Cascadia Subduction Zone, and its anticipated impact on the Pacific Northwest.
The Cascadia Subduction Zone refers to a fault line just off the Oregon/California/Washington coastlines, paralleling a series of volcanic mountains called the Cascade Range, where the North American and Juan De Fuca tectonic plates meet in the Pacific Ocean. These tectonic plates are so tightly wedged against one another and the pressure is so intense that when they eventually slip along its length, scientists are anticipating a 9.0, or higher, magnitude earthquake accompanied by a potentially 45-foot tall tsunami that will batter the north Pacific coastline from California to Canada. And, according to those same scientists, we are 315 years into a 243-year recurrence cycle.
According to The New Yorker article, “the Pacific Northwest has experienced forty-one subduction-zone earthquakes in the past ten thousand years. If you divide ten thousand by forty-one, you get two hundred and forty-three, which is Cascadia’s recurrence interval: the average amount of time that elapses between earthquakes.” The last known quake took place on January 27, 1700.
Despite the fact that just a few decades ago, nobody even knew it existed, this earthquake is a very real threat to our region and, as structural engineers, our team is well aware of the type of damage a natural event of this scale could cause to local structures. Much of the Portland metro, where the Nishkian Dean offices are located, consists of older buildings that were constructed long before seismic design was a concern and most have not received the proper seismic strengthening needed to offset a serious quake.
According to the article, “FEMA calculates that, across the region, something on the order of a million buildings – more than three thousand of them schools – will collapse or be compromised in an earthquake. So will half of all highway bridges, fifteen of seventeen bridges spanning Portland’s two-rivers, and two-thirds of railways and airports; also, one-third of all fire stations, half of all police stations, and two-thirds of all hospitals.” However, several property brokers, business owners, investors, and residents have recognized the need for this strengthening and worked with our team to make the necessary strengthening.
Nishkian Dean has been the lead consultant on a variety of projects to seismically strengthen fire stations in the Portland area. Our team recently served as the lead consultant for the design of seismic strengthening projects in the cities of Gresham and Gladstone. These projects, funded by FEMA and SRGP state grants, ensure that the buildings are designed to meet current code requirements for an Essential Facility intended to remain functional after a seismic event. These are considered “critical life safety buildings” so it is essential that they remain operational, particularly in the event of a natural disaster.
Another structural seismic strengthening our team has supported is the Smith’s Block building in downtown Portland. This four-story, 25,000 square-foot historic building was constructed circa 1870. As an unreinforced masonry building, it was highly susceptible to major damage or collapse in the event of a sizeable earthquake. The building underwent a major tenant improvement to provide all the function and sustainability of a modern office building while maintaining the character of a historic structure. Structural challenges included the strategic placement of reinforced shotcrete walls to maintain the building’s historic appearance and allow proper office function, design of new foundations to avoid disturbance to existing masonry, leveling of the existing floors, and anchoring the masonry walls to the floors.
Projects such as these may help structures survive the event when it does occur, and with the recent passing of Senate Bill 85, which we reported on in our most recent newsletter (read more in the Daily Journal of Commerce – Oregon article), and the seismic strengthening that may now be financially achievable, we are hoping many more Portland area business owners and residents will be preparing for the worst while hoping for the best.
Additionally, our team has decades of experience in what it takes to strengthen homes and there are relatively cost efficient steps that homeowners can take to reinforce their own homes to increase its structural integrity, making it stronger and safer in the case of an earthquake. Older homes are typical victims to earthquake induced damaged due to insufficient code requirements at time of construction. However, even “newer” homes built within the last 30 years may be susceptible to damage as seismic codes and construction practices have improved. Bolting down the wall framing to the concrete foundation is one of the most efficient steps that can be taken to strengthen a home. A home with cripple walls can be braced by attaching plywood to these short walls connecting the foundation to the first floor framing. Metal clips and straps can be added to the framing from within a crawlspace to tie the floors above to the cripple walls or foundation. While these measures alone do not bring an existing building up to current code, they can greatly reduce the risk of earthquake-induced damage.
To learn more about Nishkian Dean’s work in the Portland metro or if you have questions about seismic strengthening, please contact our offices at (503) 274-1843. And if you would like to receive our Monthly Newsletter featuring topical and industry trending information, email Rachael Watson at firstname.lastname@example.org.