Earthquakes and Plate Tectonics

An earthquake is a sudden shaking of the ground caused by the slipping of huge slabs of rock deep inside the Earth. These slabs are called tectonic plates, and the whole crust of our planet is broken up into a few dozen of them. They are slowly drifting around at speeds of just a few centimetres per year (about as fast as your fingernails grow), but the energy released when they finally slip past each other can flatten cities in seconds. Roughly 500,000 earthquakes are detected around the world each year, but most are far too small for anyone to feel.

  • Major tectonic plates7Plus 8 minor ones
  • Average plate speed~ a few cm/yearAbout as fast as your fingernails grow
  • Earthquakes per yearApprox. 500,000 detectedMost are too small to feel
  • Strongest ever recorded9.5 magnitudeValdivia, Chile, 1960
  • Famous faultSan Andreas FaultRuns 1,200 km through California
  • Ring of FireApprox. 75% of all earthquakesHappen around this Pacific ring

What causes earthquakes

Most earthquakes happen at the boundaries between two tectonic plates. The plates are constantly being pushed around by hot currents in the mantle below, but their edges are rough and locked together. Pressure builds up for years, decades or even centuries, until the rock finally cannot take it any more and slips suddenly. That sudden slip releases an enormous amount of energy, which travels outward as seismic waves through the Earth.

The Richter and moment magnitude scales

Earthquakes are usually rated for size on the Richter scale (or the more modern moment magnitude scale, used today by scientists). The scale is logarithmic: each whole-number step is about 32 times more energy.

  • Magnitude 2: barely noticeable.
  • Magnitude 4: clearly felt, no damage.
  • Magnitude 6: significant damage in populated areas.
  • Magnitude 7: serious damage over a wide area.
  • Magnitude 8+: enormous earthquake, devastating.
  • Magnitude 9+: very rare. Only the most extreme events.
Fact The Ring of Fire is a horseshoe-shaped zone around the edges of the Pacific Ocean where most of Earth's earthquakes and volcanic eruptions happen. About 75% of all earthquakes and 90% of the world's active volcanoes are along it. The reason is that the Pacific Plate (the biggest tectonic plate) is constantly grinding against many other plates around its edges, releasing huge amounts of geological energy.

Pick a topic below to find out more.

What Is an Earthquake?A sudden shaking of the ground caused when blocks of the Earth's crust slip past each other.
Tectonic PlatesThe huge pieces of crust the Earth's surface is broken into. They drift slowly on the hot rock beneath, carrying the continents with them.
Fault LinesThe cracks where two plates meet and grind against each other. Most earthquakes happen along fault lines.
The Richter ScaleThe most famous way of measuring earthquake size. Each step up the scale means a quake 10 times stronger.
TsunamisGiant ocean waves often caused by undersea earthquakes. They can travel across whole oceans at the speed of a jet plane.
The San Andreas FaultA famous fault line running through California, where two huge tectonic plates slide past each other.
Continental DriftThe slow movement of the continents over time. Africa and South America were once joined; the Atlantic widens by a few cm every year.