Home insurance

Could your house withstand an earthquake?

Thursday, May 11, 2017by  Jon Osterberg

excavator demolished damaged house after an earthquakeThe Kitsap Peninsula, near Seattle, has trembled more than a dozen times since May 3 from small earthquakes, including two this morning.

Though the largest of these had a magnitude of just 3.6, it's enough to make you wonder: Could my house withstand a major earthquake?

There are steps you can take to improve your odds. Though pricey, you can buy earthquake insurance to help you rebuild, if it comes to that. You also can seismically retrofit your home to make it less vulnerable when a shaker comes. That's especially relevant if your home was built before 1980, when seismic building standards were upgraded.

I hired seismic retrofitters to strengthen my home in 2003. That might have been about the time I learned the shallow Seattle Fault runs east-west under Vasa Park, just 6 miles south of my Redmond home. I had read that most houses ruined in earthquakes don't collapse to the ground. Rather, lateral and vertical seismic movement knocks houses off their foundations. Once that happens, a house is commonly deemed a total loss.

Especially at risk are older multi-story homes built atop "cripple walls" (also called pony walls) – the walls in crawl spaces that typically are unsheathed aside from exterior wood siding, making them weak. My house, built in 1987, had unsheathed cripple walls in my crawl space.

I figured that at $2,400 (about $3,200 in 2017 dollars), a retrofit was a good investment.

foundation plates for earthquake retrofit in a crawl spaceWorkmen anchored my sill plate to the house's concrete foundation on all perimeter walls with long, stout bolts and strong metal clips and plates. Then they cut and fit thick plywood over the cripple-wall studs, sheathing them entirely while taking care to retain exterior ventilation. That strengthened my home's Achilles' heel.

Theoretically, with the foundation and wood structure now better connected – what's called a continuous load path – my home can better withstand shaking and remain fastened to its base.

cripple wall is sheathed in a crawl space for earthquake retrofitThe only way I'll know for sure is to endure an earthquake, something bigger than the 6.8 Nisqually quake that my house survived in 2001.

And if scientists are correct in predicting a big Cascadia Subduction Zone shaker could reach magnitude 9.0 … well, that would not be a good day. For me, or anyone in the Northwest.

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