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An earthquake is a natural geologic phenomenon caused by the sudden and rapid movement of a large volume of rock. Most earthquakes are caused by slippage along fractures in Earth’s crust called faults. The place along the fault surface where slippage begins is the focus (foci = a point) or hypocenter. Located at the surface directly above the focus is the epicenter (Figure 3).

During large earthquakes, a massive amount of energy is released which travels in all directions in the form of seismic waves. These waves are analogous to those produced when a stone is dropped into a calm pond. Just as the impact of the stone sets water waves in motion, an earthquake generates waves that radiate from the focus. Even though the energy dissipates rapidly with increasing distance from the focus, sensitive instruments located throughout the world record the event.

More than 30,000 earthquakes that are strong enough to be felt occur worldwide annually. Fortunately, most are minor tremors and do very little damage. Only about 75 significant earthquakes take place each year, and many of these occur in remote regions. Occasionally, however, a large earthquake occurs near a large population center. Under

FIGURE 3 The focus of an earthquake is located at depth. The surface location directly above it is called the epicenter.

these conditions, an earthquake is among the most destructive natural forces on Earth (Figure 4).

The shaking of the ground, coupled with the liquefaction of some soils, wreaks havoc on buildings and other structures.
In addition, when a quake occurs in a populated area, power and gas lines are often ruptured, causing numerous fires.

FIGURE 4 Some of the destructive earthquakes recorded since 1960.