Earthquake – A natural event that occurs when huge pieces of Earth’s crust, called tectonic plates, interact with each other. Earthquakes occur along faults, or weak spots in Earth’s crust created by shifting plates. There is no way to stop an earthquake from occurring.
Tectonic plates – Giant interlocking slabs of rock that make up Earth’s rocky outer layer, or crust. As they move, the plates pull apart from, collide into, slide against, or slide over or under one another. Sometimes pressure builds up, causing pieces of the plates to shift, releasing energy that can cause the ground to shake violently. Plate movement can cause earthquakes as well as volcanic eruptions.
Epicenter – The point on Earth’s surface that is directly above where an earthquake started underground (called the focus or hypocenter). The epicenter of the recent Japanese earthquake was off the northeast coast of the Japanese island of Honshu. In general, areas closest to the epicenter of an earthquake sustain the most damage.
Tsunami – A series of giant powerful waves of seawater often caused by an earthquake under the seafloor. Earthquakes deep in the ocean cause most tsunamis. When the seafloor snaps up, it lifts a column of water above it. Gravity pulls the water back down, fanning waves outward. As the waves approach the shore, they grow in size. In Japan, a 23-foot wall of water swept onto the eastern coast, killing thousands of residents, flooding roadways, carrying away homes, cars, and boats.
Ring of Fire – The area along the edge of the Pacific Ocean where many tectonic plates meet. The islands of Japan are located in the Ring of Fire. Many of the world’s largest earthquakes and volcanic eruptions occur in this area.
Magnitude – A measure of an earthquake’s strength. The intensity of an earthquake is measured on a scale from 0 to 10. Earthquakes that have a magnitude of 7.0 or higher are considered major quakes. The quake in Japan registered a magnitude of 9.0.
Aftershocks – Earthquakes, usually small ones, that follow major quakes. Aftershocks can continue for weeks, months, or years after the main quake. In the first few days after the Japanese earthquake, more than 100 aftershocks—ranging from 5.0 to 7.1 in magnitude—rocked the country.
Seismologist – A scientist who studies earthquakes.