April 18, 1906, California's San Andreas Fault snaps, shaking San Francisco for nearly 60 terrifying seconds. When the trembling stops, the disaster is only beginning. Gas lines rupture, setting off massive fires, some 700 people die. Most of the city is reduced to ruins. This trembling of the ground caused when masses of rocks suddenly shift below the Earth's surface is called an earthquake. Hundreds of little earthquakes shake the planet every day, but most pass unnoticed. They usually occur along the boundaries of the thin plates that cover the Earth like an egg shell.

Driven by the heat deep within the Earth's core, the plates grind against each other along lines called faults. When a plate's motion is blocked, stress builds up. Finally the fault gives way. The released energy raises through the Earth in the form of seismic waves. Scientists record these waves on a device called a seismograph. These zigzag lines show the strength of various seismic waves. Using the line, scientists grade the earthquakes on the Richter scale. For a quake to measure one number higher on the Richter scale, it must release about 30 times as much energy as the number below it.

Every year about 100,000 earthquakes rumble through the ground hard enough for people to feel them. Of these, only about 1,000 are strong enough to damage property. But a powerful earthquake can be devastating. On average, about 10,000 people die each year as a result of earthquakes. The greatest recorded earthquake ever to hit North America measured a massive 9.2. The tremor struck Alaska on March 28, 1964. A camera onboard a ship docked in Valdez recorded the draining of the entire harbor as a chasm opened up on the seafloor.

There is no stopping the surface of the Earth from changing and moving, so engineers are focusing on ways to create better buildings, highways and bridges, structures that will remain safe and stay in one piece the next time the Earth begins to shake.