Many occur on plate margins, especially destructive margins (e.g. Nazca and South American Plate) & conservative plate margins (e.g. North American Plate and Pacific Plate)
There is a clustering of earthquakes around the edge of the Pacific Ocean
Cause of Earthquakes
1. Destructive plate margins:
At a destructive plate margin the denser oceanic plate is subducted (pushed under) the less dense continental plate at what is called a subduction zone.
The movement of the plates grinding past one another can create earthquakes. Friction causes a build up of pressure and when one plate eventually slips past the other, seismic energy is released causing an earthquake (90% of earthquakes occur on destructive plate margins).
2. Conservative plate margins:
At a conservative plate margin the plates can either be moving in different directions or in the same direction but at different speeds.
As they move past each other they often get stuck, building up great pressure, until finally they jolt past each other. This sudden movement is what causes earthquakes. Earthquakes can be fairly violent and frequent.
3. Constructive plate margins:
At a constructive plate margin two plates are moving away from each other.
Earthquakes occur at constructive plate margins when tension builds along cracks within the plates as they move away from each other.
Features of Earthquakes
Focus: the point in the Earth’s crust where the earthquake begins
Epicentre: the point on the Earth's surface that is directly above the focus
Shockwaves: the seismic waves generated by an earthquake that pass through the Earth’s crust, causing vibrations on the surface of the land