Home Earthquake Vulnerabilities: Hillside Homes and Geological Concerns

The view from hillside homes can be amazing, but this usually comes with higher earthquake risk.

“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.

A hillside neighborhood in northwest Portland.

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.

Some hillside homes seem to compete with each other over which one can defy gravity the most. I’m concerned that gravity may defy some of these houses when the big earthquake shakes for 3 to 5 minutes.

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.

The remnants of a hillside home after the 1994 Northridge earthquake.

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.

A landslide that occurred in an Alaska neighborhood during the Great Alaska Earthquake (M9.2) of 1964.

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.

A snapshot of Portland on the “SLIDO” landslide hazard map by DOGAMI. Brown and red areas indicate past landslides. Notice that entire neighborhoods have been built on some of these areas.

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.

South Napa v.s. Cascadia- and our need for seismic upgrades in the Northwest

On August 24, 2014, a magnitude 6.0 earthquake struck near the California city of Napa. It was subsequently named the South Napa Earthquake. One person died and 200 were injured as a result of the quake. Damage was in the range of $300 million to $1 billion- not an insignificant amount.

Much of the damage associated with structures occurred in brittle buildings like those constructed with URM (brick) or with stone-clad veneer. But there was a good deal of damage to homes and other wood-framed structures, also.


Collapsed chimney from the South Napa Earthquake.  (See more pictures here)

I read an article recently revisiting damage from this earthquake, and I couldn’t help but notice some basic statistics and compare them to our Cascadia threat looming off the coast.

Consider just two data points: Ground accelerations and duration of shaking.

The recorded peak ground accelerations during the South Napa earthquake were .61g (61% of gravity).  The significant shaking lasted for less than 10 seconds.

Compare this to a Cascadia Subduction Zone earthquake:

  • Ground accelerations in the Portland area are expected to be around .75g. The shaking will be greater in areas with soft soil, which comprise a good portion of the metro area.  Areas near the rivers- the Columbia, Willamette, Tualatin, etc are also prone to liquefaction, which will further increase damage. Ground accelerations will also generally increase as you move further west.
  • Duration of shaking will be measured in minutes, not seconds. If the full subduction zone ruptures, the shaking could last as long as five minutes.

What does this simple comparison tell us? It should be a sobering reminder of our need to strengthen our infrastructure in the Pacific Northwest. Consider these points also:

  • California has had multiple earthquakes to help weed out the weaker buildings, so to speak- through damage, repairing, and rebuilding over time. We haven’t even had a “South Napa” (i.e. magnitude 6.0) in the Portland area in recorded history. As a result, we have an excessive amount of weak structures still hanging around.
  • Liquefaction will likely be a huge source of damage during the Cascadia quake. Liquefaction damage was limited in the South Napa earthquake due to drought conditions, but it was a significant source of damage during the 1989 Loma Prieta (magnitude 7.0) earthquake and the 2001 Nisqually (magnitude 6.8) earthquake near Olympia, Washington.
  • The need for retrofitting of homes by strengthening cripple walls, providing foundation anchorage, and using blocking and framing connectors to create an adequate load path is very much needed in the Pacific Northwest. Every significant California earthquake produces this type of damage.
  • 1800 URM (brick) buildings in Portland alone will all likely have significant damage unless they are strengthened. This has been known for at least 20 years, but only a small percentage… I believe it is less than 10%… have been adequately retrofitted.

Cascadia residents pay attention: the Ring of Fire is alive and active

A couple of large earthquakes struck the Ring of Fire in the last 2 weeks: a magnitude 7.8 in New Zealand on November 14th and a 6.9 (according to USGS) on November 21st off the coast of Japan.

I’m fascinated by earthquakes, particularly since I’ve made a decision to focus my career on earthquake resilience.  But even if you’re not as into them as I am, the awesome power of earthquakes was undeniable last week in New Zealand.  And the Japan earthquake is another reminder of the need for preparedness in the Pacific Northwest.

Landslides, open fissures, stranded cows, and the seafloor lifting up out of the ocean are the images that struck me the most.

This drone video of a ground fault rupture in New Zealand looks like Lord of the Rings style special effects. I can easily imagine an army of orcs falling into the ground as it opened up:

And here’s a good article from CNN about the seafloor being raised.  The coast has been permanently changed in a dramatic way (unless an earthquake reverses it):

http://www.cnn.com/2016/11/18/asia/nz-earthquake-pics/

Regarding Monday’s Japan earthquake, the Japan Weather Agency is calling it an aftershock from the 2011 magnitude 9.0 megaquake.  It caused a small tsunami and seems to have shaken people up a bit (bad pun intended) but caused little damage.

It’s been estimated that we have a 37% chance of experiencing an 8.0 or higher Cascadia Subduction Zone earthquake in the next 50 years.  This is a helpful statistic and I intend to bring it up often.  The main application for us in the Northwest is that we have a high risk of this event occurring if we intend to live in this region for long.  It makes sense to prepare for it, but once you start thinking of the implications, it quickly becomes overwhelming from the individual level up to the state agencies.  The bottom line is that an adjustment in lifestyle is needed for all of us living in this region.  More to come on this topic…

How will a Cascadia megaquake compare with these recent earthquakes?  The magnitude will be greater; between an 8.0 and 9.0 and possibly even higher.  Ground fault rupture as shown in the video above won’t likely be an issue, at least not a primary one, as the fault is off the coast between the continental and oceanic plates.  Vertical displacement will likely occur, as the coast is expected to drop on the order of 6 feet relative to sea level.  This, along with the accompanying tsunami, has drastic implications for the low lying coastal areas. Strong ground shaking over a huge region will damage older infrastructure like brick buildings and 100 year old homes not attached well to their foundations.  Landslides are to be expected throughout the region.  Soil will liquify in saturated sandy soils such as near rivers.  This has terrible implications for industrial areas like northwest Portland and just south of downtown Seattle near the Duwamish Waterway.

My personal mission is to help inspire as many individuals and families as possible to be resilient when this event occurs.  If you’ve started preparing or making lifestyle changes, I’m interested in hearing from you as I’m sure I’m not the first person to do so.  I also want to hear from those of you who want to do something but are not sure what to do.  What are the questions you have and what concerns you the most?

Making ourselves and our region more resilient is a marathon, not a sprint.  So, take a breath and put some thought into this topic.  Don’t lose sleep over it, take action instead!