Sudden movement on a fault releases earthquake energy. Magnitude measures the event's size; shaking and damage vary from place to place.
An earthquake begins when rock slips suddenly along a fault. Tectonic forces deform the rock, but friction can keep the fault locked while stress builds. When the fault gives way, stored energy is released as seismic waves. The rupture starts at the focus, or hypocentre, inside Earth. The epicentre is the surface point directly above it.
Seismic waves carry energy through Earth and along its surface, making the ground move. Seismometers record this motion over time. Scientists use these records to locate the earthquake and estimate its magnitude.
Magnitude describes the earthquake's size and is linked to the energy released. A larger slipping fault area or more slip can produce a larger event. Richter local magnitude, ML, uses recorded wave amplitude with a correction for distance. It suits local events within its useful range, but becomes less reliable for very large earthquakes.
Moment magnitude, Mw, handles large earthquakes more reliably. It is calculated from seismic moment: the rock's rigidity, the area of fault that slips and the amount of slip. Rigidity means how strongly the rock resists deformation. Neither ML nor Mw counts damaged buildings.
One earthquake has a source magnitude, though estimates may change as more data arrive. Shaking intensity differs between places because distance, depth and ground conditions vary. Losses also depend on the people and buildings exposed. Magnitude alone cannot tell you the death toll.
Step by step
Stress builds
A locked fault resists movement while nearby rock deforms.
The fault slips
Sudden movement releases stored energy as seismic waves.
The ground shakes
Seismometers record motion; ground conditions and distance affect local shaking.
Worked example: Same earthquake, different observations
A station on soft sediment may record stronger shaking than a station at a similar distance on firmer rock. These different records do not mean two separate earthquake magnitudes occurred; they show how the same event interacts with local ground.
Watch out for this
The epicentre is the underground point where waves begin.
That is the focus. The epicentre is the surface point directly above it.
Check your understanding
Which statement correctly distinguishes magnitude from local impact?
- Magnitude is simply the number of buildings destroyed.
- Every place must have the same intensity because magnitude is one value.
- One earthquake can have one source magnitude and different intensities at different places.