Historic masonry buildings and earthquakes don’t mix very well, and this is especially true in Oregon with regards to a potential Cascadia subduction zone earthquake. The “Really Big One”, as a New Yorker article called it, is a Cascadia subduction zone earthquake that will produce earthquakes of magnitude 8-9, with shaking that will last three to five minutes. (To put these numbers in perspective the 1994 Northridge earthquake was a magnitude 6.7 with shaking lasting 10 to 20 seconds. Damages from the Northridge earthquake were estimated at $49 billion.) The greatest shaking will be along the coast and it will be sufficiently powerful to cause significant damage in the Willamette Valley and even east of the Cascade mountains. Even more troubling, is its likelihood. A full rupture of the subduction zone happens, on average, every five hundred years with the smallest interval between events being about 175 years. Smaller ruptures along the southern portion of the subduction zone happen every 240 to 320 years. The last earthquake in the Cascadia subduction zone took place on January 24th, 1700 (317 years ago), so the statistical chances of a rupture in the near future are quite high. Of course, the built environment of this region has been radically altered in the last three centuries so the buildings, bridges and other infrastructure have never been exposed to this kind of event.
Unlike our neighbors California and Washington, Oregon hasn’t experienced many earthquakes and those we have had have been relatively small. This means we have a large stock of unreinforced masonry buildings that haven’t been winnowed by large earthquakes or been seismically upgraded. Oregon has between 5,000 and 10,000 unreinforced masonry buildings, of which approximately 2,000 have been identified as historic. Only a small percentage have been upgraded to current code, adopted in 1994, that considers the potential of a Cascadia earthquake.
In general, masonry buildings are quite durable. However, they are not very resistant to ground shaking, which causes failure at the mortar joints, breaking apart the walls that then fall away from the building, causing fatalities and injuries at the perimeter of the buildings. The minimum solution for seismically upgrading these buildings is to tie parapets and walls back to the roof and floors, which typically need to be made stiffer. Further upgrades involve creating shear elements, which are typically made of steel or concrete. It is only recently that concrete has been determined to be the better solution since its stiffness more closely matches that of the masonry. For a higher level of performance, base isolation can be used, which separates the building from the ground and reduces the transfer of ground shaking to the building structure. While effective, the retrofit costs for this kind of upgrade are prohibitive. Currently, the Pioneer Courthouse, an 1869 masonry building located in Portland and owned by the Federal Government, is the only historic building that is base isolated in Oregon. Experience has shown that these solutions do protect masonry buildings during smaller earthquakes and save lives during larger earthquakes.
Even though the statistical risk from a Cascadia earthquake is relatively high, it still is a rare event. The economics of seismic retrofits provide a financial return-on-investment over the long term. However, the expense of seismic retrofits makes it difficult for building owners in the short term. Oregon is trying to encourage building owners to retrofit by enacting legislation that allows local communities to create loan or loan guarantee programs for seismic retrofits and to give special property exemptions to encourage private building owners to seismically upgrade their properties. Portland is also moving closer to requiring mandatory upgrade for unreinforced masonry buildings, which helps level the playing field for building owners who do upgrade their buildings.
The need for regulation is created, in part, by the fact that there are few market incentives supporting retrofits. One effort to create market incentives is the introduction of a building rating system by the US Resiliency Council, which will certify a building’s seismic performance. The rating system looks at safety, repair costs, and time to regain function over a range of performance levels, where code level is considered average. Buildings are identified with plaques to inform the public of the performance level. The hope is that the market will create financial benefits for higher rated buildings. This approach actually exists in Japan, where base isolated buildings are common and command a 25% increase of sale and lease rates.
Historic buildings need to be seen within their economic role, in addition to their cultural and social roles. A Special Report by Restore Oregon, Resilient Masonry Buildings: Saving Lives, Livelihoods, and the Livability of Oregon’s Historic Downtowns pointed out the importance of historic masonry buildings to the economies of Oregon’s communities, especially the historic downtowns. Renovation of historic masonry buildings can act as a catalyst for economic revitalization of downtowns and become part of the branding for marketing of the businesses and communities. The example of the 2011 Christchurch, New Zealand earthquake provides a cautionary tale in this regard, which had a large stock of heritage buildings. The example is especially relevant to Portland, which is a similarly sized community with similar characteristics. Many of Christchurch’s heritage buildings collapsed or were heavily damaged during the 2011 earthquake. It has proven difficult to restore damaged buildings and many have been torn down since the earthquake. What is also sobering, is that buildings meeting New Zealand’s building code, whether historic or not, often were torn down since their codes, like ours, are geared towards life/safety and do not ensure that buildings can be reused. Over 70% of the buildings in Christchurch’s downtown performed as designed, yet were torn down since the costs of repairs exceeded replacement cost.
If a goal of historic preservation is to preserve historic buildings for future generations to enjoy, then consideration of the seismic performance level of historic buildings located in earthquake prone areas is essential. It is possible to retrofit historic buildings to increase safety, reduce the cost of future repairs, and speed the eventual reuse these buildings. Measures can also be implemented to provide for stabilizing buildings following an earthquake. This can prevent further damage from aftershocks, and procedures can be put in place with local building departments to prevent demolition of historic buildings without proper review. This effort will require looking at the risk to historic masonry buildings and their resilience within a wider context of the community as a whole. Restore Oregon’s recommendations for resilient masonry buildings were incorporated into the Oregon Resilience Plan, which has served as a model for other community resilience efforts, such as NIST’s Community Resilience Planning Guide.