Earthquakes occur when two tectonic plates of Earth’s crust slide past each other along a fault. Earth’s plates are always moving, which causes a build-up of friction and tension. When that energy releases suddenly, an earthquake occurs. The shaking you feel during a quake is caused by seismic waves passing through the lithosphere, which is the rigid layer of the planet composed of the crust and upper mantle. Tens of thousands of measurable earthquakes occur each year.

Scientists use a tool called a seismograph to measure earthquakes. A seismograph has a heavy base fixed to the ground and a weight with a pen that hangs on a string or spring. During an earthquake, the base shakes with the ground while the weighted pen remains still. Paper on a rolling drum records the motion creating a record of the earthquake called a seismogram. Today, many seismographs are electronic and record the swinging of the pendulum by changes in electric voltages generated during the quake.

The size of an earthquake is its moment magnitude, a quantitative measure tied to an event’s seismic moment (the function of the earthquake's area, average distance of the fault's slip, and a constant determined by local rock type) as opposed to the amplitudes of seismic waves of a seismograph. This method of classifying earthquakes was developed in the 1970s by Hiroo Kanamori and Thomas C. Hanks, and reliably measures the largest earthquakes with a magnitude greater than eight.

Worldwide, earthquakes are measured by a series of seismographs, which are part of the Global Seismographic Network. Scientists use three seismographs to record one event. This is a technique called triangulation; it more precisely measures an earthquake’s epicenter.

You can help scientists too! If you experience an earthquake you can use the Did You Feel It? website and let experts know. This helps them map the area impacted.

Earthquakes have a wide variety of effects on Earth’s surface and its inhabitants. An earthquake can cause the ground to rise, sink, or separate. It can trigger tsunamis, landslides, or liquify sandy ground. Human-built landscapes also see significant damage. Earthquakes can also topple buildings or bridges, crumble roads, bend railways, or snap pipelines spilling their contents. Sometimes an earthquake’s damage is just an inconvenience, while others result in loss of life. Many municipalities in areas where earthquakes are common have adopted building policies designed to prevent disastrous impacts during this natural disaster.

Do you know what to do to prepare for an earthquake?

1. Practice drop, cover, and hold on.
2. Create an emergency plan (and don’t forget your pets).
3. Prep your home by securing heavy and large furniture and fragile items.

This map layer displays the location of major (magnitude of 7.0 or higher) global earthquakes between 1950-2020 filtered from the United States Geological Survey (USGS) Earthquake Catalog. Explore any particular quake by clicking on one of the points. This will open a popup showing you the date it occurred, its depth in kilometers, and its magnitude.