“Resilience” has become a hot topic in recent years, and rightly so. It’s defined as a region’s ability to rebound after a disaster. We look at cities such as New Orleans after Hurricane Katrina, and now Houston after Hurricane Harvey, and recognize cities that were not resilient to a known disaster coming at some point.
A Cascadia Megaquake is our unprecedented disaster, at least, the one that we are methodically ticking closer to on the geological clock.
Our city and region have a long way to go to become resilient. If you want to be more convinced of this, please read the Oregon Resilience Plan Executive Summary. It’s been estimated that perhaps 80 percent of our buildings in Oregon do not comply with the current seismic code requirements (this does not mean most of them would fall down, but some of them would)! For most of Portland’s history, buildings have gone up, and remained, with little regard to earthquake forces or effects.
When I think of dangerous buildings to be in during an earthquake, URM’s (unreinforced masonry or brick), hillside homes, soft-story buildings, and old “tilt-up” buildings come to mind.
Yes, hillside homes can be among the most dangerous places to be in an earthquake, and this post is about the seismic hazards unique to this category of buildings.
The basic seismic retrofit that involves strengthening measures implemented in a crawl space or a basement is becoming familiar. But Hillside homes are often not in the conversation, and they need to be.
Hillside homes are common in Portland and other west coast cities. Many of them went up in the 1960’s, when earthquake risk was considered low. They have great views and character. Unfortunately, they can have catastrophic damage in earthquakes.
Hillside homes are by far the most dangerous demographic of single-family residential structures, as measured in recent California earthquake fatalities.
If you live in a hillside home, you are not necessarily in danger during an earthquake. Your structure is just more likely than other homes to be dangerous. I encourage you to take in the information in this post and get a sense of what the risks of your particular home are, so you can take appropriate action.
FEMA’s P-50-1 document gives us the following statistics from the 1994 Northridge earthquake (magnitude 6.7) in the Los Angeles area:
- 114 hillside dwellings were significantly damaged.
- 15 hillside dwellings collapsed or were so severely damaged that they had to be immediately demolished.
- Another 15 hillside dwellings were close to collapse.
- At least four people died in these homes.
Other earthquakes, such as the 1989 Loma Prieta earthquake near San Francisco, have also resulted in hillside home collapses and fatalities.
Geology Concerns
We have unique geological risks in the Pacific Northwest with hillside homes. The soil in the hills around here often consists of a top layer of clayey or sandy silt, somewhere on the order of 30 feet deep, underlain with bedrock. Earthquakes can trigger landslides, landslides are more likely in saturated soils, and saturated soils are a common condition in the rain-soaked northwest. This soft layer of soil can slip away under the right conditions.
Remember the winter of 2017? The west hills of Portland had numerous landslides earlier this year. Landslides happen during earthquakes even in dry conditions; imagine what would happen if the big earthquake strikes at the end of a soggy winter?
Landslide risk is not only a concern at the exact site of a house or directly below it; an unstable slope above could be equally damaging. Even a landslide just down the street could destroy the road that accesses the home and cause severe injury or death of neighbors.
I’m not suggesting that most hillside homes will collapse and slide down the hill. But landslide risk is important to know about if you live in the hills, and some houses are in high-risk areas.
The Oregon Department of Geology is expecting tens of thousands of landslides to occur during a full rupture of the Cascadia Subduction Zone. The most at-risk areas have been mapped for the entire state of Oregon on a macro level in an online interactive map called “SLIDO“; they include areas where past landslides have been documented and steep slopes with soil characteristics prone to landslides. “A Homeowner’s Guide to Landslides” by the Washington Geological Survey is another helpful tool homeowners can use to qualitatively assess landslide risk.
I’m concerned that the seismic risk to hillside homes in our region may be worse than California, just from landslide risk alone.
What this all boils down to is that an adequate seismic risk assessment or retrofit of a hillside home will often need the input of a geotechnical engineer as well as a structural engineer.
If the soil appears sound and landslide risk appears to be low, at the very least a structural engineer that is attentive to slope stability and geological risks is needed. Sometimes a conservative design with the foundation (such as a continuous footing with significant reinforcing) can make up for limited soil information. I’ll discuss this more in my next post.
I’ve become a proponent of FEMA’s “simplified” seismic assessments and perform them regularly on houses. I highly recommend this as a starting point for those concerned about the seismic risk of a hillside home. They are affordable and take into account both structural and geological seismic vulnerabilities. This methodology makes a relatively thorough, first-pass assessment and helps quantify the benefit of a retrofit and the likely costs involved.
For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.
The next post will discuss common structural earthquake vulnerabilities with hillside homes.
