Earthquakes are caused by the movement of plates (huge slabs of rock) making up the surface of the Earth. The region where two or more plates meet is called a plate boundary. The plates are constantly moving but this plate movement is neither smooth nor continuous, rather the plates often lock together at plate boundaries causing a build-up of energy. When the plates eventually move out of this locked position the energy that is released may be felt as an earthquake.
Much of the world’s earthquake and volcanic activity takes place along plate boundaries, the area where plates meet. Countries located along plate boundaries, such as the Eastern Caribbean islands, Japan, Chile and the USA (California) are likely to experience earthquakes. At these plate boundaries the plates interact with each other in different ways; some of them slide past each other, others spread apart and others move toward each other with crumpling or with one dipping beneath the other. This last type of plate boundary is called a subduction zone which is the main type of plate interaction occurring in the Eastern Caribbean.
A fault is a plane along which rocks are displaced. Faults can vary in size from the smallest that can be recognized at about 2 cm to large plate boundaries hundreds of kilometers long.
A significant magnitude earthquake with which is associated, in time and space, smaller magnitude earthquakes (the lower the magnitudes, the higher the numbers of events), possibly preceding, but certainly following, is described as an earthquake sequence.
As magma makes its way through the crust to the surface of the earth, it breaks apart surrounding rock thereby generating volcanic earthquakes. Volcanic earthquakes are one of the main signs that a volcano is restless. Tectonic earthquakes are caused by the movement of plates when energy accumulated within plate boundary zones is released. Tectonic earthquakes are usually larger than volcanic earthquakes.
The terms foreshock and aftershock have no strict scientific definition. They are used to describe the events within an earthquake sequence to distinguish those events that preceded the mainshock from those that followed it.