Why earthquakes happen

The mechanism

An earthquake is what happens when two blocks of the Earth suddenly slip past one another, on a surface called the fault. The point at depth where the slip begins is the hypocentre; the point directly above it on the surface is the epicentre.^[1,2]

While the fault is stuck and the surrounding rock keeps moving, the energy that would otherwise slide the blocks past one another accumulates. When the moving blocks overcome the friction of the fault’s jagged edges and it unsticks, that stored energy is released and radiates outward as seismic waves.^[1]

Where they happen

Plate boundaries are made of many faults, and most of the world’s earthquakes occur on them, because the rough edges of plates lock while the rest of the plate keeps moving.^[1,2]

Not all fault motion is the same. At a transform boundary such as the San Andreas, two plates slide horizontally past one another and no crust is created or destroyed; transform faults connect spreading centres or, less often, trenches.^[1,2]

Measuring something you cannot reach

The size of an earthquake depends on the area of fault that slipped and how far it slipped — neither of which can be measured directly, because faults lie kilometres below the surface. Both are inferred from seismograms recorded at the top.^[1]

Magnitude scales are logarithmic: one whole number is a tenfold increase in recorded amplitude. The Richter scale survives only for small local events; for everything else moment magnitude is used, because amplitude-based scales misjudge the largest earthquakes.^[1,2]

An earthquake has one magnitude but many intensities: the shaking varies from place to place with distance, ground material and other factors. The two are different quantities and are frequently confused.^[1]

Afterwards

Mainshocks always have aftershocks, which can continue for weeks, months or years. Foreshocks exist too — but cannot be recognised as foreshocks until the larger earthquake arrives.^[1]

Commonly misunderstood