As an earthquake retrofitting contractor, I often have prospective clients tell me how their home doesn't need a retrofit because it has survived three "big" earthquakes. The Nisqually earthquake was ten years ago, on February 28, 2001.
In general, earthquakes are good for the retrofitting business. They provide a wake-up call, and get homeowners to take action on retrofitting, after years of thinking about doing the work.
The Nisqually earthquake probably did us all a disservice by creating a false sense of security. Sure it was a magnitude 6.8 earthquake - no small tremor - but it was centered 32 miles deep in the earth's crust and some 50 miles from Seattle.
That variety of earthquake, a deep slab event, occurs relatively frequently in this area - every 30 to 60 years. But it is typically much less destructive than the other two, less frequent, types of shakers.
A subduction zone mega quake, like the 2010 event in Chile, could be as large as magnitude 9. That type of earthquake is likely to be centered maybe 100 miles West of Seattle, so by the time the shaking reaches Seattle, it may be similar in intensity to Nisqually. The difference is that it could shake for two to four minutes or more!
So homes that survived 40 seconds of Nisqually would be put to the test. Retrofitting would provide these homes a big advantage in this type of earthquake.
We know that the last subduction zone earthquake occurred in January of the year 1700 (tsunami records in Japan show this). Geologists say we can expect one of these mega quakes every 300-600 years, so we are due for the next one.
Probably our most serious earthquake threat is a shallow crustal earthquake. The Seattle Fault Zone runs from Issaquah to Bremerton, right under I-90 and downtown Seattle.
This network of faults is only a mile or two below the surface. Due to the shallow depth and the proximity to urban areas, a shallow crustal earthquake could be much, much more damaging than Nisqually was.
Richter scale magnitude is not the best measure of how destructive an earthquake is likely to be to homes and other structures (like bridges and roads). A better metric is lateral acceleration. The quicker the earth's crust jerks sideways, the more likely that homes will be jolted from their foundations.
February's quake in Christchurch, New Zealand, was an example of a shallow crustal earthquake. Its shallow depth and proximity to the city allowed that magnitude 6.3 earthquake to cause significant damage and casualties.
Fortunately, earthquakes centered on the Seattle Fault Zone are only predicted to occur every 1,000 years or so. Unfortunately, geologists tell us that the last one was about 1,100 years ago.
CREW (Cascadia Region Earthquake Workshop) is a group of regional experts in various fields. They imagined what it might look like when (not if) a Seattle Fault Zone earthquake strikes. Their scenario is grim.
In the CREW scenario, a 6.7 shallow earthquake strikes on the Seattle Fault Zone. They project 1,660 deaths, 24,000 injured, 9,700 buildings destroyed, all major highways substantially damaged, collapsed bridges, many utility outages, and on and on.
That type of earthquake WILL happen, it is just a matter of when. We are overdue. We could continue to be overdue for hundreds of years, or it could happen tomorrow. Unfortunely, and unlike many other types of natural disasters, we will have no warning.
Rather than wait until it is too late, this blogger hopes that Nisqually's 10 year anniversary will serve as a warning.
Geologists, structural engineers, and building officials have studied how homes fail in earthquakes, and they have learned how they can be protected: seismic retrofitting.
Homeowners insurance companies have spent a lot of time studying the risks involved in earthquake insurance. Their underwriters have determined that one thing mitigates the risk of insuring older homes against earthquake damage: seismic retrofitting.
When the earth shakes, REALLY shakes; one thing may allow you to stay in your home instead of trying to find room in an emergency shelter: seismic retrofitting.
Seismic retrofitting works.
Edit: Seattle Times science reporter Sandi Doughton does a much better job than I did at explaining the very real risk of devastating earthquakes. Read her article HERE.
Edit: Seattle Times science reporter Sandi Doughton does a much better job than I did at explaining the very real risk of devastating earthquakes. Read her article HERE.
