How an Earthquake Happens, Step by Step

An earthquake is the shaking caused when two blocks of rock suddenly slip past each other. The crack they slip along is called a fault. Plate boundaries are made up of many faults, which is why most earthquakes happen there.

The plates themselves keep moving slowly and steadily, but the rough edges of a fault stick together. While the fault is stuck, the rock on either side bends a little and stores energy, like a ruler being bent or a spring being squeezed. When the stress finally becomes greater than the friction holding the fault, the rock slips in a sudden jolt and the stored energy is released. Geologists call this idea elastic rebound.

The point underground where the slip begins is the hypocentre, also called the focus. The point on the surface directly above it is the epicentre. News reports usually give the epicentre, because that is where people live.

Most earthquakes start less than 80 km below the surface. The very deepest ones happen only at subduction zones, where cold plate is being pushed far down into the mantle. In 1994, for example, a magnitude 8.3 earthquake under Bolivia started about 636 km down.

The released energy spreads out from the fault in all directions as seismic waves, like ripples on a pond.

Wave	How it moves the ground	Typical speed in the crust
P wave (primary)	pushes and pulls in the direction it travels; passes through solids and liquids	5–7 km/s, the fastest
S wave (secondary)	shakes the ground side to side or up and down; solids only	3–4 km/s
Surface waves	roll and sway along the surface	2.0–4.5 km/s, the slowest

Surface waves arrive last but usually shake the ground hardest, and they cause most of the damage to buildings.

Instruments called seismometers record the shaking as a wiggly line called a seismogram. Because P waves travel faster than S waves, the gap between their arrival times tells scientists how far away the earthquake was, just as the gap between lightning and thunder tells you how far away a storm is.

One station gives only a distance, not a direction. Draw a circle of that radius around three different stations, and the place where the three circles cross is the epicentre. This method is called triangulation.

Every earthquake has a single magnitude, a number for its size. Charles Richter introduced the idea in 1935, but his scale only worked well for nearby, moderate earthquakes in California. Today seismologists use the moment magnitude scale (Mw), which is worked out from the area of fault that slipped, how far it slipped and how strong the rock is.

The scale is logarithmic. Each step of one whole number means the waves on a seismogram are 10 times larger and the earthquake releases about 32 times more energy. So a magnitude 7 releases about 32 × 32, roughly 1,000 times, the energy of a magnitude 5.

How strongly the ground shakes in one place is called the intensity, and it changes from place to place. It depends on the distance from the fault, the depth of the earthquake and the ground underneath: soft mud shakes far more than solid bedrock. After a large earthquake, smaller aftershocks follow in the same area for weeks, months or even years.