Earthquake terms, defined

The terms

Epicenter

The point on the surface directly above where an earthquake starts. News maps mark the epicenter, but the earthquake itself begins underground, sometimes hundreds of kilometres down.

Hypocenter

The point underground where the rupture actually begins, also called the focus. The epicenter is the hypocenter's shadow on the surface, and the depth figure on every event page is the distance between the two.

Magnitude

A single number describing the total energy an earthquake released, measured on the moment magnitude scale. Each whole step is about 32 times the energy of the one below it. One earthquake has one magnitude, wherever you felt it from.

Intensity

How strongly the shaking was felt at a particular place, usually on the Modified Mercalli scale. Unlike magnitude it varies with location: one earthquake has one magnitude and many intensities, fading with distance from the fault.

Aftershock

A smaller earthquake following a larger one in the same area, caused by the crust adjusting to the stress change of the mainshock. Sequences decay over weeks to years, and after a large event an aftershock can itself be damaging.

Foreshock

An earthquake later recognised as having preceded a larger one nearby. It can only be identified in hindsight: nothing about an earthquake reveals at the time whether something bigger is coming, which is one reason prediction fails.

Mainshock

The largest earthquake in a sequence. If a later event is bigger, the labels move: the old mainshock is reclassified as a foreshock of the new one.

Fault

A fracture in the crust where the rock on one side has moved relative to the other. Almost all earthquakes happen on existing faults, which is why hazard maps of past activity predict future shaking well.

Seismograph

The instrument that records ground motion. Networks of them are how USGS locates an earthquake: comparing arrival times of the waves at different stations triangulates the source within minutes.

Depth

How far below the surface the rupture began. Shallow earthquakes, above about 70 km, do the most damage because the waves reach the surface before spreading and weakening. The same magnitude at 500 km is often barely felt.

Liquefaction

When strong shaking makes water-saturated sandy ground behave like a liquid. Buildings sink or tilt, and buried tanks float upward. It is why damage concentrates on reclaimed land and old riverbeds.

Tsunami

A series of ocean waves generated when an earthquake moves the sea floor vertically. Large subduction earthquakes are the usual cause; strike-slip events that move the ground sideways rarely produce one.