An earthquake is a sudden shaking of the ground that can cause buildings to tremble and sometimes collapse. It happens when energy stored in the Earth's crust is released quickly. This energy travels in waves, like ripples across a pond, but through solid rock. The place where the earthquake starts deep underground is called the focus. The point on the surface directly above it is the epicentre. Earthquakes can be very small and barely felt, or enormous enough to destroy entire cities. Scientists study earthquakes to understand them better and help keep people safe.
The Earth's outer layer, called the crust, is broken into huge pieces known as tectonic plates. These plates float on the hot, slowly moving rock beneath them. They are always moving, but very slowly — just a few centimetres each year, about as fast as your fingernails grow. Because they move in different directions, they sometimes bump into each other, pull apart, or slide sideways. Most of the time we don't notice this movement, but when the plates get stuck at their edges, stress builds up over many years. When the stress becomes too great, the edges of the plates suddenly slip and snap into a new position.
This releases the stored energy in the form of seismic waves. The place where the slipping occurs is usually along a crack in the crust called a fault. There are different types of faults, such as normal, reverse, and strike-slip faults. The 1906 San Francisco earthquake was caused by movement along the San Andreas Fault, a famous strike-slip fault in California. The sudden release of energy is what makes the ground shake. Seismic waves spread out from the focus in all directions. There are two main types: body waves and surface waves.
Most of the time we don't notice this movement, but when the plates get stuck at their edges, stress builds up over many years.
Body waves include P-waves (primary) and S-waves (secondary). P-waves travel fastest and arrive first; they push and pull the rock in the same direction they move. S-waves are slower and shake the ground side to side or up and down. Surface waves travel along the Earth's surface and are often the most destructive because they cause the ground to roll like ocean waves. These waves can topple buildings and break roads. Earthquakes happen most often in certain parts of the world, especially around the Pacific Ocean in an area called the Ring of Fire.
This region includes countries like Japan, Indonesia, New Zealand, and the west coast of the Americas. Scientists measure earthquake size using the Richter scale or the moment magnitude scale. Each whole number increase means the earthquake is about ten times stronger. An earthquake of magnitude 2 is usually not felt, while a magnitude 7 or more can cause serious damage. Thousands of quakes occur every day, but most are too small to notice. Understanding how earthquakes work helps us prepare for them. Engineers design buildings to flex and sway without falling.
Communities hold drills so people know what to do during a quake: drop, cover, and hold on. Early warning systems can detect the first, fast P-waves and send alerts before the stronger S-waves arrive. In Australia, earthquakes are less common but still happen, especially near the centre of the country. By learning about plate movements and faults, we can reduce the risks and save lives.