Month: July 2017

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.

Seismic Shake-Up In Real Estate Transactions

The state of Oregon is scrambling to streamline its public policy in such a way that it encourages (or possibly in some cases, mandates) seismic upgrades to our old infrastructure which will not perform well in a large earthquake.

The latest is House Bill 2140, which requires homeowners to disclose whether their house was built before 1973 and whether or not the home was bolted to its foundation.  The bill was signed into law on June 21, 2017 and becomes effective on January 1, 2018.

The bill passed by a large majority, but there are some who suggest it will be a burden to homeowners.

Frankly, the risk of a megaquake in our region is a burden to everybody, but here we are. It’s hard to complain about a bill that requires two more tidbits of information when selling a home.

Will this affect some home sales?  Of course, it will.  There is a percentage of potential home buyers that will see this disclosure and think differently about a house they are on the fence about buying.  Some will ask the seller to have a seismic upgrade done to the house.  Some buyers may want to pay for a retrofit themselves, or split the cost with the seller.  Some buyers may not care at all.  The point of the disclosure is to pass on important information.

I believe the greatest need for homeowners in the Pacific Northwest regarding earthquake risk is accurate information- so I’m in favor of a mandatory seismic disclosure.  I’m doing my best to be a reliable source of this information by providing affordable seismic risk assessments.

Mandatory upgrades to buildings are a different topic, which I think is worth debating.  San Francisco and Los Angeles have both required mandatory upgrades to “soft story” buildings in the last five years, and the City of Portland has been discussing mandatory upgrades to URM (brick) buildings.  Expect more legislation of some sort in the near future.

House Bill 2140 clearly is aimed at encouraging the textbook seismic retrofit (i.e. attaching an older home to its foundation). Overall, this is good, since there are a high percentage of old homes in the Pacific Northwest that need this work done, and it’s relatively affordable.

Home seismic risk, however, is more complicated than foundation bolts.  The text of the bill, as far as I can see, didn’t mention weak cripple walls, which probably are a more common cause of structural damage in earthquakes. There are actually numerous potential points of earthquake weakness near a home’s foundation that can result in significant damage.  I wonder if some homeowners will sneak through this disclosure with a weak cripple wall that is bolted to its foundation.  Or, perhaps, they will add a few inadequate bolts themselves and call it good on the disclosure.  In these scenarios, a buyer could be essentially given a false sense of security about the home’s earthquake risk. I hope this bill doesn’t backfire in terms of its effect.

My biggest concern is hillside homes (not mentioned in the bill), which are the most dangerous demographic in terms of single family residences.  Some of these types of homes were obliterated in the 1989 Loma Prieta and 1994 Northridge earthquakes in California, and people died.  If you own one of these homes, I strongly encourage an assessment by a structural engineer (it doesn’t have to be me, but I’d be happy to help).

Whether there is legislation or not, the public’s awareness of earthquake risk is increasing.  This will inevitably affect property values.  Watch over the next ten years or so: structures with lower earthquake risk will increase in value while high risk properties will lose value.

If you’re interested in a home seismic risk assessment, please visit the Cascadia Risk Solutions website.