Think earthquakes are just the curse of nutty Californians? Think again.

Over the centuries, severe earthquakes have occurred in surprising places throughout the United States–they just happen to have struck before the country was as populous as it is now.

During 1811 and 1812, for example, no less than three major earthquakes struck the little town of New Madrid, Mo., about as close to the “earthquake-free” American heartland as you can get. Seismologists believe these quakes registered 8.6, 8.4, and 8.7 on the Richter scale–as powerful as any Pacific Coast quake of the last 100 years. Another severe earthquake struck Charleston, S.C., in 1886, calling the wisdom of that city’s renowned brick architecture into question, if only briefly.

In fact, about 10 percent of the world’s earthquakes occur outside zones of known seismicity. Ironically, these are the most dangerous quakes of all, since the regions they strike are often totally unprepared to withstand them. Buildings in such places are mainly designed to resist the pull of gravity, which of course acts downward. Unfortunately, earthquake forces act mainly from side to side, producing what engineers call “lateral load.”

Such forces can reduce even the most massive brick building to rubble in seconds during a severe quake.

Woodframe buildings are generally much safer than masonry ones in an earthquake, because they’re lighter and more flexible. Still, many woodframe houses–especially those predating World War II–may still be prone to quake damage, and could benefit from reinforcement.

To be effective, though, earthquake reinforcement must address these three basic types of lateral failure:

  • Shear produces the telltale diagonal or X-shaped cracks you see in plaster, stucco and concrete after an earthquake. Remember your high school physics? A body at rest tends to remain at rest. Under earthquake forces, the foundation of your house begins to oscillate before the roof does, forcing the top and bottom edges of the walls to move laterally past each other, or “shear.” The heavier the building, the greater the shear forces produced, and the greater the potential damage.

    Shearwalls–sections of wall specially reinforced with plywood–are often used to prevent shear failure. However, since any seismic reinforcement is only as strong as its weakest link, the plywood has to be securely nailed to the studs, and the entire shearwall anchored to the rest of the building so that earthquake forces will be transferred to the ground.

  • Sliding occurs when a house isn’t securely attached to its foundation, and the earthquake’s side-to-side motion causes the whole building to literally slide off its foundation (often while remaining otherwise intact).

    Ordinary foundation anchor bolts usually prevent sliding failures.

    Prior to 1936, when building codes were revised in light of California’s disastrous Long Beach earthquake, most houses had either rudimentary foundation bolts or none at all. Sometimes rudimentary bolting is enough, because the quake forces experienced by small houses are usually quite low. Still, it’s wise to have an engineer check the adequacy of the existing anchor bolts, and have more bolts added if necessary.

  • Overturning is the tendency of a section of wall to tear off the foundation at one bottom corner and rotate like a cartwheel. Special anchors called holdowns (not to be confused with anchor bolts) are used to prevent this failure. The forces on holdowns are generally much higher than those on anchor bolts, so they’re much beefier.

    If an engineer determines that you need holdowns, you can install them along with the rest of your seismic retrofit program.

    Whether you’re a Left Coaster, a Right Coaster or smack dab in the Heartland–it’s better to be safe than sorry.


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