Earthquakes

An earthquake is a sudden shaking of the ground caused by the rapid release of energy stored within the Earth's outer layers. Most earthquakes occur because the Earth's rigid surface is broken into large tectonic plates that move slowly relative to one another. Along the boundaries between plates, friction can lock the rock in place while stress builds. When the rock finally fractures or slips along a fault, the stored energy is released and radiates outward as seismic waves that shake the surface.

Seismic waves travel through and across the Earth in several forms. Body waves move through the planet's interior: primary waves, which compress and expand the rock and arrive first, and slower secondary waves, which shake it from side to side. Surface waves travel along the ground and usually cause the strongest shaking and most of the damage. Instruments called seismometers record these vibrations, and the resulting seismograms let scientists locate an earthquake's origin and estimate its size.

The point underground where rupture begins is the focus, or hypocentre, and the spot on the surface directly above it is the epicentre. Earthquake size is commonly described by magnitude, a measure of the energy released, while intensity describes how strongly shaking is felt at a given place. Because plate boundaries are where most stress accumulates, global seismicity is concentrated along belts such as the rim of the Pacific Ocean. Earthquakes can also occur away from plate edges and may be associated with volcanic activity or movement on faults within a plate.

Frequently asked questions

What causes most earthquakes?

Most are caused by the movement of tectonic plates. Stress builds where plates lock together, and when rock suddenly slips along a fault the released energy shakes the ground.

What is the difference between the focus and the epicentre?

The focus, or hypocentre, is the point underground where the rupture starts. The epicentre is the point on the surface directly above it.

How are earthquakes detected and measured?

Seismometers record ground vibrations as seismograms. From these, scientists locate the earthquake and calculate its magnitude, a measure of the energy released.