Earthquakes occur by the thousands around the world every day, most so small that we don’t feel them. When we do feel an earthquake, it can be alarming. Earthquakes are caused by sudden movement of Earth’s surface, called a fault. This sudden movement can cause ground shaking, liquefaction (land shaking that changes the shape of land), landslides and tsunamis.

Think of the Earth’s crust as a giant puzzle. The rough edges of these pieces, called tectonic plates, are continually sliding past and bumping into each other. Over time, the friction and pressure build up until eventually one of the plates slips sideways along a fault. This sudden release of energy generates seismic waves, including the destructive shear and compression waves. Earthquakes that happen at convergent faults are more destructive than those that occur at divergent or transform faults.

How big an earthquake is depends on how much the fault slips. This is difficult to measure at the Earth’s surface, but scientists can use recordings from seismographs at observatories around the world to estimate how large an earthquake is. These records are arranged in “waveforms” that show how fast the ground shakes, and how long the shaking lasts. If a waveform is short and doesn’t wiggle very much, the earthquake was small. A long, wiggly line indicates a larger earthquake.

If you live in an area prone to earthquakes, prepare for them by storing supplies and building strong homes. When an earthquake occurs, move to a safe place away from buildings, streetlights and power lines. Drop, cover and hold on to something sturdy during the shaking.