Potential Earthquake Damage and Important Decisions.

jill's picture

I plugged in my old address in Altadena, California to this site’s damage estimator, and placed a magnitude 7.5 earthquake (a plausible event) on the San Andreas fault. I placed the epicenter near Palmdale, California. The result I got was unsettling, and frankly made me glad I now live in an area where I only have to worry about ice storms, blizzards, tornadoes and mosquitoes. Here’s what I got:

Your test earthquake produced a simulated peak ground acceleration (PGA) of 19.033%g at your home location. Given your description, the damage factor (DF) for this event is 0.2372. This means on average you would experience $241,000 in damage (assuming a home value of $1,012,500).

If I still lived at this home in southern California, I would certainly be using this tool as a guide to help me figure out whether to buy earthquake insurance, make sure it’s bolted to the foundation, and secure nonstructural components (elements that are not part of the load-bearing system of a building). Or, I might gamble against having to spend a large chunk of my retirement nest egg for repairs to my home if I decided not to buy earthquake insurance. 

So – is 19% g at that location something I should be concerned about? It doesn’t sound that bad. To try to quantify the meaning, I did a quick review of g-forces.

When there is an earthquake, the force caused by the shaking, or acceleration, can be measured as a percentage of gravity, or percent g. The term g-force really refers to acceleration, which corresponds to the force applied to something that causes a change in position or speed. When you’re in a moving car, for example, the g-force is what pushes you around in your seat. Racecar drivers use 3-point harnesses since they can experience continuous forces up to 5 g’s during a race. The space shuttle experiences about 3 g’s on launch and re-entry.  A fighter jet pilot can experience up to 12 g’s during a maximum turn, and has to wear a special pressurized flight suit to avoid blacking out. Fifty g’s is the limit for most humans, who would die or suffer major injuries under such a load.

Earthquake forces vary greatly, and can change rapidly – back and forth, side to side, up and down. The largest earthquake forces measured have been about 1 to 2 g, but most earthquakes have lower forces. Still, even a seemingly small percent g can cause great damage to structures. Because all permanent structures are anchored in some way to the ground, and are designed not to move, it doesn’t take much ground acceleration to cause damage.

Back to my (former) house. From what I learned from my refresher, a percent g-force of 19 probably wouldn’t do much at all to me personally if I were standing outside the house in an open space. But if I were inside the house during the earthquake, I would likely sustain injuries due to breaking windows, flying glass, falling books and wall shelving, or TVs and other appliances rocketing off counter spaces – all of it “nonstructural damage.” In fact, just about any feature of a home that is not load-bearing or actual structural material is considered nonstructural. About 70% of the original cost of most buildings is in the nonstructural components, and most of the value exposed to risk from earthquake damage is nonstructural.

A logical conclusion is that both structural and nonstructural mitigation, coupled with an earthquake insurance policy – at least in the case of this level of risk – are important considerations.


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