Answer Geek: Earthquake-Safe Homes

<br> -- Q U E S T I O N: I survived the Seattle earthquake! Now there's a lot of talk about earthquake retrofitting. I know it essentially has to do with making buildings more earthquake-proof, but if I want to retrofit my home, what is involved?

— Ian C.

A N S W E R: After spending the last month looking at the mysteries of the power grid and the technologies behind various sorts of alternative energy sources, it's a bit of a relief to take a break and talk about another topic. Never fear, though; I still have a couple of good power-related questions up my sleeve, and we'll get to some of them in the coming weeks.

Well, Ian, congratulations for coming through the Feb. 28 earthquake intact. Fortunately, and for reasons that are still something of a mystery, everyone survived here. It seems to have been a big surprise to everyone who studies earthquakes that the 6.8-magnitude event did not wreak more destruction than it did.

Enough to Budge a House

Your basic assumption about retrofitting is correct: It is an attempt to re-engineer an existing structure to help it withstand the rocking of a powerful quake. How it is accomplished depends on the type of structure being retrofitted. But since you asked, we'll confine ourselves to houses, which can take a fearful beating.

During the 1994 Northridge earthquake in California, for example, nearly 50,000 homes suffered significant damage. That quake measured 6.7. Damage from the recent quake in Washington state was much lighter, but it's pretty easy to spot toppled chimneys and other evidence that an earthquake can do some serious damage to a house.

The problem: If the ground shakes with enough force and violence, the back-and-forth motion just might nudge a house off its foundation, causing major damage or even complete collapse. This is mostly a problem for older homes where the superstructure of the house isn't connected securely enough to the foundation.

The typical solution: Bolt that baby to the foundation. Normally this is done by drilling a hole through what is called the sill plate — the board that sits directly on the foundation — into the concrete foundation. The generally accepted standard is that the bolt must extend at least four inches into the foundation. Two kinds of bolts are used: expansion bolts, which expand inside the hole, and something called chemical anchors, which use adhesive materials like epoxy or polyurethane to get a good hold.