An earthquake is a shock wave that radiates to the Earth's surface from underground. Causing a range of effects from unnoticeable, mild tremors to violent, prolonged shaking, an earthquake is a natural phenomenon that occurs frequently only in certain areas of the world. The place where an earthquake begins underground is called the hypocenter, and the area on the Earth's surface directly above the hypocenter is called the epicenter and receives the most powerful shock waves.
Movement in the Earth's crust causes an earthquake. The Earth is made of an inner core, an outer core and a mantle, and the final layer is a thin crust covering the mantle, which is the surface of the Earth including all the oceans and continents. The crust is made of separate rocky portions called tectonic plates, which lie on the mantle like pieces of a jigsaw puzzle. But the jigsaw puzzle is mobile, and the plates move around. Some slide past each other horizontally, some push together and force the ground upward, some slide beneath another plate and some pull apart. Whenever a tectonic plate moves suddenly, this causes an earthquake.
The sudden release of friction and pressure between tectonic plates causes an earthquake. Tectonic plates are made of rough rock and cannot slide past each other smoothly. Friction prevents movement at the plate edges while the rest of the plates continue to move, causing a buildup in pressure. When the pressure overcomes the friction, the plates move suddenly, and shock waves from this sudden movement radiate through rock, soil, buildings and water. Usually, small foreshocks occur at first, followed by one big mainshock. Aftershocks follow and can continue for weeks, months or even years.
People living near fault lines are likely to experience an earthquake. Fault lines are the areas where two or more tectonic plates join, and it's in these areas most earthquakes occur. Well-studied fault lines include the San Andreas Fault that runs down the West Coast of North America and lines between Australia and Papua New Guinea, as well as in New Zealand, Tonga, Japan and Taiwan. Earthquakes can also rarely occur in the middle of tectonic plates. Scientists aren't yet able to predict earthquakes, but people living near fault lines can help protect themselves by living in earthquake-protected housing and practicing earthquake drills.
An earthquake damages buildings and land, causes tsunamis and has many other disastrous effects. Violent shaking from an earthquake collapses buildings, which causes the most deaths and casualties, and destroys power lines and ruptures natural gas supply lines, causing fires. Land can also collapse or pull apart, causing more buildings to fall. Tsunamis occur after an earthquake on the ocean floor. The water shock wave travels through the ocean until it dissipates or meets land. If the wave meets land, the water piles up, creating a single wave or a series of large waves that sweep inland, causing death and destruction.