Portland is no stranger to the buzz about earthquakes hitting our region and the conversations are eerily common about the “Big One”. Last month, The Great Shake Out hosted their nation-wide earthquake drill, nearly doubling their number of Oregon participants. As our region acknowledges the risks a catastrophic earthquake could have on civilization and structures, we ask ourselves how we can best prepare.
Dr. Charles Clifton, Associate Professor at University of Auckland, visited Portland State University in October to speak about the 2010-2011 earthquakes to hit Christchurch, New Zealand. He spoke on what his research team has learned from the earthquakes, particularly the impacts the quakes had on the structural infrastructure of the central city. Christchurch’s recent earthquake activity was actually six separate, large seismic events, the first taking place in September 2010, and the other five spread out from February to December 2011. While Dr. Clifton’s research is vastly larger than my knowledge, I came away from his talk realizing there are exponentially more variables to consider when looking at earthquake and structural building behavior. I also discovered I had more questions, particularly the question of how our region can benefit from the research and lessons of the Christchurch earthquakes.
The geography of Christchurch is one of the first contributing factors in understanding how this region took the brunt of these earthquakes. The country of New Zealand is sunken; only a small portion of it emerges from the ocean. Christchurch sits on a swamp where soil instability is high, making it not suitable for simple structures. Stretched and rotated by subduction plates close offshore, the region is susceptible to active earthquakes, similar to our Cascadia Subduction Zone. It was noted the quakes caused widespread liquefaction to residential homes in the swampiest areas, and caused major destruction to homes built alongside cliffs where slide activity was prevalent. Dr. Clifton correlated Christchurch’s geography to Portland’s similar housing on the hills surrounding the city, pointing out that the main destruction in the Christchurch neighborhoods was triggered by landslides.
The majority of Christchurch’s buildings were built between 1910 and 1990. Dr. Clifton and his team focused on documenting the effects of these earthquakes on a large variety of architectural history. Residential, timber-framed homes performed well for life safety, and were considered the safest places to be. Steel-framed buildings performed exceptionally well, and steel is the building material of choice as Christchurch reconstructs. All of the steel framed buildings on solid ground, not affected by ground instability, are in service. Some required no repair. Concrete shear wall buildings suffered significant damage. The damages occurred in two ways: walls either had one dominant crack with rebar fracture or the wall showed significant crumbling. Based on these past experiences, New Zealand is rethinking the design of shear walls. Many suggest that rocking walls—a wall designed to dissipate energy by rocking in the foundation—are a viable alternative.
Finally, the behavior of the earthquakes provided interesting data for engineers. In the series of six earthquakes, the first one was not the most damaging. Most likely, it was the succession of quakes and the unzipping of new faults that heightened the severity of this event. Another interesting fact is how the earthquake interacted with the city grid. In New Zealand, the principal axis of most buildings are oriented on a north/south and east/west street grid. In most earthquakes, the principal of the building doesn’t align with the wave paths of the shaking, but with Christchurch it did. This is important because it helped with evaluating building response. Researchers took each individual building response and the recordings from the earthquake, and married the two together, and found that the response was stronger on the east/west direction.
Measured vertical accelerations were unusually higher than the horizontal accelerations in the earthquakes as well. Geologists believe this is the result of hard bedrock below and the soft sedimentary basin on top reacting to each other. Researchers noted multi-level commercial buildings experienced floors deflecting like the imprint of a trampoline, as well as damaged active link connectors. There was less visible damage to the outside of these buildings, yet unfortunately, enough floor diaphragm failure to force these buildings to be destroyed. Large cracks in the floors were wide enough for a human hand to slide through.
The Christchurch earthquakes are a devastation, requiring an estimated reconstruction cost of $50 billion NZ dollars and at least twenty years to fully rebuild the central city. Since 2010, Dr. Clifton and his team have uncovered lessons and experiences to be passed along to future structural engineers and construction professionals. Probably the most notable of these is the reliability of steel-framed buildings, making steel the number one choice for rebuilding. As Portland considers the scenarios for our “Big One”, we should learn what we can, but not place more weight on one component over another. Our region’s geology, age and specific building materials, and the unpredictability of earthquake behavior are just a few considerations in the whole unknown scenario of a Pacific Northwest earthquake. The merit of engaging in conversations and inquiring about Christchurch’s experiences and others like it, can be our first step in adding to our learning curve.