Earthquake

An earthquake is a tremor of the earth that occurs when energy accumulated in rock layers of the earth's crust is suddenly released. This energy propagates in the form of waves (seismic waves) that travel through the earth and cause shaking at the surface.

Most earthquakes occur near the boundaries of tectonic plates—large sections of the earth's crust that move continuously. When two plates collide or slide against each other, stress accumulates until the rock breaks at a location called a fault. Earthquakes can also result from volcanic activity and human activities such as dam filling or deep mining.

How they occur

The point within the earth where the break begins is called the hypocentre (or focus). The point on the surface directly above the hypocentre is called the epicentre. The depth of the hypocentre significantly affects the magnitude of damage: shallow earthquakes at the crust's upper levels generally produce stronger ground shaking than deeper ones, even if the energy released is the same.

Seismic waves

Two types of waves travel through the earth: P waves (primary), which are the fastest and travel through rock, water, and air; and S waves (secondary), which are slower and can only travel through rock. When these waves reach the surface, they generate surface waves that often cause damage to buildings. The time difference between the arrival of P and S waves is used by seismic stations to calculate distance.

Measurement and assessment

The strength of an earthquake is measured by an instrument called a seismograph. Magnitude indicates the energy released and is a single value for each event; the scale currently used in science is moment magnitude. The Richter scale is an older scale still widely used by the media. Intensity, by contrast, measures how shaking is felt at different locations and is assessed using scales such as Mercalli.

Consequences

Earthquakes can destroy buildings, bridges, and roads, and can trigger landslides and fires from ruptured gas and power lines. If an earthquake occurs beneath the ocean and displaces the seabed, it can generate a tsunami. Large earthquakes are usually followed by smaller tremors called aftershocks, which can last for days or months.

Preparedness

Science cannot yet predict accurately when an earthquake will occur. Instead, earthquake hazard is assessed through mapping, building codes that resist earthquakes, public education, and early warning systems that take advantage of the fact that P waves arrive before the damaging waves, providing a brief warning window.