EARTHQUAKES
The surface of the Earth is changing constantly. Changes occur over vast
periods of time, such as erosion of mountains. Earthquakes are sudden events
but the process that creates earthquakes is relatively slow. Earthquakes are
the vibrations caused by rocks breaking under stress. The underground surface
along which the rock breaks and moves is called a fault plane.
The outermost shell or crust of the Earth is divided into tectonic plates.
These tectonic plates of rock are moving very, very, very slowly. The tectonic
plates forming the lithosphere (lithos meaning rocky) lie on top of the
asthenosphere (asthenos meaning ‘without strength’) and each plate varies in
thickness. The thick parts are the continents and the thin parts form oceanic
crust.
The rocks of the asthenosphere have a degree of plasticity and are not as rigid
as the layers above it.
The non-rigid rock flow in convection currents within the Earth and rise after
being heated by heat radiating from the Earth’s core. This movement is one of
the main reasons why tectonic plates move.
Each plate has different pressure and movements so they move in different
directions and at different speeds. Two tectonic plates can move away from
each other, can collide with one tectonic plate sliding over the top of another
plate – or can move past each other. When tectonic plates move against each
other they do not slide smoothly. The tectonic plate becomes stuck, even
though they are under pressure to move. The pressure builds up until suddenly
the tectonic plate give way and move, releasing built-up energy. Some of the
energy is released in the form of ‘seismic waves’ (energy that travels in the
form of waves through the Earth). Seismic comes from seismo, meaning
earthquake. These waves radiate in all directions, including towards the Earth’s
surface, where they are felt as shaking or displacement of the ground. We call
these events earthquakes.
Most earthquakes occur on and adjacent to the boundaries of tectonic plates,
but sometimes they are felt in the middle of tectonic plates.