Earthquakes are measured by special tools called seismographs. These machines detect and record the shaking of the ground. When an earthquake happens, the ground moves in waves. A seismograph has a heavy weight that stays still while the ground shakes around it. A pen attached to the weight draws a wiggly line on a moving roll of paper. This line is called a seismogram. By looking at the seismogram, scientists can learn how strong the earthquake was and where it started. Today, many seismographs use electronics and send data to computers.

This helps scientists study earthquakes more quickly and accurately. A seismograph has a simple design. A drum covered with paper turns slowly. A pen touches the paper. The pen is attached to a heavy weight that hangs from a spring or a wire. When the ground shakes, the drum moves with it, but the weight and pen stay still. This makes the pen draw a wavy line. The size of the waves shows how strong the shaking was. Small waves mean a small earthquake. Big waves mean a large earthquake.

Modern seismometers use magnets and wires to create an electric signal. The signal is recorded and studied by scientists around the world. To measure the size of an earthquake, scientists use scales. The Richter scale was first used in 1935. It measures the height of the waves on a seismogram. Each step up on the scale means the earthquake is ten times stronger. For example, a magnitude 4 earthquake is ten times stronger than a magnitude 3. But the Richter scale is not very good for very large earthquakes. Today, scientists mostly use the moment magnitude scale.

When the ground shakes, the drum moves with it, but the weight and pen stay still.

This scale measures the total energy released by the earthquake. It gives a better idea of the true size, especially for big quakes. Scientists also work out where an earthquake started. This place is called the epicentre. To find it, they use data from at least three different seismograph stations. Each station records the time when the P-wave (fast wave) and S-wave (slower wave) arrive. The time difference between these two waves tells how far away the earthquake is. By drawing circles around each station, the point where they all cross is the epicentre.

This method is called triangulation. It helps scientists quickly find the earthquake's location so they can warn people if needed. Earthquakes can also be described by how much damage they cause. This is called intensity. The Mercalli scale measures intensity from I to XII. A low number means only a few people felt the shaking. A high number means buildings collapsed and the ground cracked. The intensity depends on how far you are from the epicentre and the type of ground you are on. Soft soil shakes more than hard rock. Knowing about all these ways of measuring earthquakes helps us build safer buildings and prepare for future earthquakes. It also helps scientists understand how the Earth works.