An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth’s crust and cause the shaking that we feel.
What are the tectonic plates?
Earth’s crust is not a single, unbroken layer. Rather, it’s made of sections called tectonic plates that sit on top of the slowly flowing and moving mantle. The tectonic plates are the large, thin, relatively rigid plates that move relative to one another on the outer surface of the Earth.
The tectonic plates sometimes grind against each other, or push into each other to build mountain ranges. In other places where plates are moving towards each other, one plate is forced underneath another plate.
What are seismic waves?
With all this dynamic movement constantly taking place all over the planet, big plates of rock moving around and bumping against each other, it’s no wonder that sometimes it gets a little unstable. As the plates move relative to each other, huge amounts of stress can build up over long periods of time. Eventually there comes a point when all the accumulated stress is suddenly released: rocks break and huge sections of crust are cracked and displaced. These movements result in waves of energy called seismic waves that radiate out in all directions. The waves travel into the interior of the planet and towards Earth’s surface. All this activity takes place deep within Earth, making it difficult to monitor and, therefore, difficult to predict when a future earthquake might occur.
How do we measure earthquakes?
Earthquakes are measured using instruments called seismometers that detect the vibrations caused by seismic waves as they travel through the crust. Seismic waves can be either natural (from earthquakes) or caused by human activity (explosions).
The strength (or ‘magnitude’) of earthquakes used to be determined according to the Richter Scale, which compares the relative strength of different earthquakes. The scale is logarithmic, meaning that an earthquake of magnitude 6 is ten times stronger than one of magnitude 5. A magnitude 7 quake is 10 times stronger than a magnitude 6, and 100 times stronger than a magnitude 5.