Wiggle Test: Why Some Buildings Stand Strong in an Earthquake

Imagine the ground starts to shake and wiggle like jelly. Some buildings sway back and forth and stay standing. Other buildings crack and fall down. What makes the difference? It's all about how the building is built.

- What an earthquake is and why the ground shakes - The difference between a flexible building and a stiff building - Why triangles and wide bottoms make buildings stronger - How builders test buildings before an earthquake ever happens

Deep underground, giant slabs of rock called tectonic plates are always slowly moving — about as fast as your fingernails grow. Sometimes two plates get stuck and push against each other. When they finally slip loose, all that stored-up energy shakes the ground. That shaking is an earthquake, and it can last anywhere from a few seconds to a few minutes.

You might think the strongest building is the stiffest one, like a tall stack of blocks. But in an earthquake, stiff buildings can crack because they cannot move with the shaking ground. Flexible buildings, built with materials that bend a little (like steel), sway back and forth instead of snapping. Some very tall buildings even have a giant weight near the top, called a damper, that swings the opposite way to cancel out the shaking.

Look closely at bridges, cranes, and earthquake-strong buildings and you will spot triangles everywhere. A square can squish into a slanted shape if you push it, but a triangle cannot change shape without breaking one of its sides. That is why builders add triangle-shaped braces to walls to help buildings survive shaking.