How tectonic plates move

A cracked outer shell

The theory of plate tectonics explains that Earth's crust and uppermost mantle form a rigid outer layer, the lithosphere, that is broken into several large plates and many smaller ones. The plates move slowly over a weaker, hotter layer beneath them.

The motion is slow by human standards. Along the Mid-Atlantic Ridge, plates spread apart at an average of about 2.5 centimetres per year, while along the San Andreas fault system in California the Pacific Plate moves past the North American Plate at an average of about 5 centimetres per year. Over millions of years these small movements add up to thousands of kilometres.

Three kinds of plate boundary

At divergent boundaries, plates move apart. New crust forms as molten rock rises, mostly along mid-ocean ridges. Earthquakes there are usually shallow.

At convergent boundaries, plates move towards each other. Where an ocean plate meets another plate, the denser ocean plate can sink beneath it in a process called subduction. Subduction zones produce the world's largest earthquakes and, as the sinking slab descends, earthquakes at intermediate and great depths. Where two continents collide, as in the Himalaya, crust is folded and thickened.

At transform boundaries, plates slide past each other sideways. The San Andreas fault is a well-known example. Earthquakes there are generally shallow.

How stress becomes an earthquake

Plates move steadily, but the rocks along a fault often do not. Friction can lock a fault while the surrounding rock keeps bending, storing strain like a bent stick. When the stress overcomes the friction, the fault slips suddenly and the stored energy radiates outward as seismic waves. This is an earthquake.

After a large earthquake, the surrounding rock adjusts to the sudden change, which is one reason aftershocks follow. The process then begins again, but the timing of the next large earthquake on a fault cannot be predicted precisely.

Earthquakes away from plate boundaries

Most earthquakes occur on or near plate boundaries, but not all of them. Earthquakes also happen within plates, on older faults that are reactivated by stresses transmitted through the plate. These intraplate earthquakes are less frequent, but some regions far from plate boundaries have experienced damaging earthquakes in the past.

This is one reason hazard assessments consider the whole geological history of a region rather than only its distance from a plate boundary.

Seeing plate boundaries in earthquake data

If you open EQRadar's Live Map for the past 30 days, the earthquakes trace long, narrow belts around the Pacific Ocean and through southern Europe and Asia. These belts follow plate boundaries. The 3D Depth Explorer shows the third dimension: beneath subduction zones such as Japan or Chile, earthquakes deepen in an inclined band that follows the sinking plate.

These patterns come from where earthquakes were recorded. EQRadar does not draw plate boundaries or fault surfaces unless a sourced, licensed dataset is configured, because an incorrect line would be misleading.