One of the top priorities in preparing for an earthquake is making sure your home is safe.
Many homeowners in the Pacific Northwest are concerned about how their home will perform in a large earthquake, but they are confused. Some think earthquake insurance is the next step, but haven’t thought much beyond that. Others (wisely) have considered earthquake retrofitting. But that opens the door to all sorts of questions, like:
- How do I verify that the retrofit will actually be effective?
- Does my house go from bad to awesome in terms of earthquake performance, or bad to okay, after the retrofit?
- Is my house okay without any seismic strengthening?
- What else should I do besides the retrofit? What do I need to do myself?
My goal is to provide as much useful, free information as possible, and shed some light on a confusing topic.
Earthquake Vulnerabilities are no Mystery
Although earthquake awareness has increased much in the Pacific Northwest, many people are so overwhelmed by the thought of it that some make statements like this:
“There’s no way we can know what will happen to our house after a 9.0 earthquake”- Typical pessimist’s home earthquake risk assessment
While it’s true that we can’t know for sure what will happen, we can make good estimates based on past earthquake data and engineering principles. Plenty of helpful information is out there, and it’s available to those of us who have searched for it and used it in our work. I’d like it to be more available to the general public, which is why I’m writing this.
I’ve been amazed at the wealth of information available at sources such as FEMA or various earthquake engineers I’ve spoken to in California who have designed earthquake strengthening measures for buildings and then seen them tested with actual earthquakes.
Methodologies to assess earthquake risk have been in development for decades, and are based on actual earthquake damage to various building types.
FEMA’s P-50 (for houses) and P-58 (for various building types) methodologies are very helpful resources for assessing earthquake risk, and in my opinion, their usage needs to be marketed more to home and property owners. I use both methodologies as well as structural engineering principles.
The vulnerabilities that cause damage to homes in earthquakes are well documented, but not easily accessible to the typical homeowner in the Pacific Northwest. So… what are they?
One simple way to categorize the different variables affecting any individual building’s earthquake risk are below-ground and above-ground variables.
Below-Ground Variables
The below-ground variables are the geological site characteristics, such as the distance from the earthquake source and the soil type. Ground shaking will generally increase the closer you are to the earthquake source. This is common sense.
What many don’t know is the effect that soil type can have on ground accelerations. In the 1989 earthquake in San Francisco, for example, ground shaking was five times stronger at the Fisherman’s Wharf area (with soft, saturated soil) compared to the Chinatown area, which is on bedrock and only a half mile away.
In some cases, a site that a house (or any structure) is built on can be so poor that a seismic upgrade is not even worth considering, at least, from an economic perspective. The only reasonable choice for a homeowner in this scenario may be to either move away or simply live with the risk.
Other below-ground hazards include liquefaction and lateral spreading, which tend to occur in sandy, saturated soils in low-lying areas, and landslides in the hills. I also include tsunami risk in this category; although it’s technically not below the ground, it’s a feature unique to the site where a building is located.
With our abundance of water in the Northwest, and the potential for an earthquake shaking 3 to 5 minutes, geological hazards pose a great risk in many areas.
There are helpful free online resources to allow home or building owners to quickly assess their geological hazards. For example, the Oregon Department of Geology and Mineral Industries has an interactive map where all of these different site hazards can be viewed for any location in Oregon (the mapping is on a macro level and does not eliminate the need for a site geotechnical investigation, but is still helpful information). OPB’s “Aftershock” tool combines ground shaking, distance from the Cascadia Subduction Zone and soil type to give you a qualitative explanation of what to expect at your specific address.
These tools, however, are not building-specific, and for this reason, they do not accurately quantify the earthquake risk of your home. They are helpful tools- and I recommend using them- but there will be a huge variability in earthquake damage from one home to another, even in the same neighborhood, because of the differing construction of each home.
Above-Ground Variables
Above the ground, every structure will respond differently in an earthquake. Every home has its unique geometry and construction, which will affect the way it reacts to the forces.
There are plenty of exceptions, but in general, newer homes perform better than older homes.
A building will shake roughly proportional to its weight and height, which means that a smaller one-story house will typically do better than a larger two or three-story house.
Wood-framed houses tend to perform well in earthquakes, if they don’t have any significant vulnerabilities. Wood-framed construction is flexible, which dampens earthquake forces. This is true even with older wood-framed homes, although damage is typically greater. This is one reason why a brick house would likely perform worse than a wood-framed house in the same neighborhood.
The following common above-ground vulnerabilities tend to generate earthquake damage:
- Brick Chimneys. Chimneys are heavy, tall, skinny, and brittle. This is a dangerous recipe. Even in moderate earthquakes, chimney damage is common and can result in injury or death.
- Weak Cripple Wall. A “cripple wall” is the wood-framed wall between the home’s foundation and its first floor. A house with an elevated porch often has a cripple wall, particularly if there is no basement. This is a common weakness in older homes, and failure to strengthen a cripple wall can result in the house suddenly dropping and shifting laterally a few feet during an earthquake. This usually results in a complete economic loss of the home.
- Inadequate Foundation Anchorage. In hindsight, it’s amazing that builders didn’t think it was necessary to attach wood-framed houses to the concrete basement walls or foundations way back when, but that’s how they commonly built homes. It’s also amazing that many relatively new homes sometimes have inadequate anchorage, even homes built after the building codes required it. The code began catching up to our knowledge of a potential large subduction earthquake about 20 years ago, but I sometimes see homes built as late as the early 2000’s with missing nuts and plate washers on many of the anchor bolts. Inadequate anchorage is a common failure mechanism in earthquakes which results typically in total economic loss as the house slides off the foundation during strong shaking.
- Deteriorating concrete or brick basement walls and foundations. This is a common structural problem with homes around 100 years old in Portland. It’s a hazard that should be addressed regardless of earthquake risk. It’s also important to not attempt a textbook retrofit that attaches to poor concrete or brick without an expert’s input.
- Soft or Weak Story homes. A practical definition of a “soft story” is an exterior wall line that has very few wall segments (i.e. it is mostly composed of windows or openings). A common example of this is a garage door with a living space above it and very little wall width each side of the garage door. The narrower the walls each side of the garage door, the greater the likelihood of severe damage. Another similar issue with older homes is that after a century of different owners, the current floor plan is open with more windows and less walls than it originally had. If enough wall segments are removed, very little lateral strength remains. A weak story combined with liquefaction-prone soil is particularly dangerous in earthquakes.
- Hillside Homes. By far the most dangerous demographic, these homes can suffer severe damage during an earthquake. Not only is the structure often weak and top-heavy, as in the case of homes on “stilts”, but they can have catastrophic landslide risk. They also often have other structural problems such as torsional weakness and lack of ductility with bracing or shear walls.
- Split Level Homes, Complex Floor Plans and Roof Lines. Complexities to homes add character, but sometimes they are problematic for an earthquake load path. The more discontinuities in roof, floor, or wall lines, the more likely separations will occur.
- Elevated Porches and Decks. These types of “add-ons” to a house sometimes detach from the house during an earthquake and collapse without adequate bracing.
In the past decade or two, a number of contractors have established a niche for residential earthquake retrofitting. Typically, an earthquake retrofit contractor will provide services primarily relating to weak cripple walls and inadequate foundation anchorage. Rightly so, because these vulnerabilities are common and relatively inexpensive to fix compared to say, a home on stilts or with a severe soft story problem. But as I’ve established, there are many variables of earthquake risk both with the site of a home and the structure itself, and these risks aren’t always communicated or addressed.
Bracing For Cascadia
Many people have latched onto phrases like, “everything west of I-5 is toast” (a quote made somewhat infamous after the 2015 New Yorker article, “The Really Big One”), and they envision a post-earthquake Northwest where all or most buildings are destroyed. Some suppose the tsunami will enter the Willamette Valley and Portland. Neither of these ideas are true whatsoever (and that’s not what the quote meant). I expect most buildings to remain standing after our big earthquake. I expect most homes to do even better than other buildings overall, as they have done in past earthquakes.
That’s not to say the earthquake won’t be a major disaster. It will certainly be. Power outages for 1 to 3 months in the Portland area, which is what the state expects, is a disaster.
As far as home preparedness goes, we need a realistic view of our earthquake risks. We need to make sure we don’t have a home that is prone to damage. We want to ride through the earthquake uninjured if possible, so we can help others. And as most of us know, there are numerous other tasks we need to do to prepare for the earthquake, so let’s make sure our homes are safe to the best of our ability.
For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.
