What Happens During an Earthquake?

What causes earthquakes, and what types of earthquakes are there? To answer these questions, it is first helpful to have an understanding of Earth's composition.


How Are Earthquakes Measured?

Two different viewpoints underpin the most important measurements related to earthquakes: magnitude and intensity. To scientists, an earthquake is an event inside the earth. To the rest of us, it is an extraordinary movement of the ground. Magnitude measures the former, while intensity measures the latter.


Can Seismologists Predict Earthquakes?

It is not currently possible to predict exactly when and where an earthquake will occur, nor how large it will be. However, seismologists can estimate where earthquakes may be likely to strike by calculating probabilities and forecasts.


How Do Earthquake Early Warning Systems Work?

Earthquake early warning systems don't predict earthquakes. Instead, they detect ground motion as soon as an earthquake begins and quickly send alerts that a tremor is on its way, giving people crucial seconds to prepare.


How Do We Make Buildings More Earthquake Resistant?

Learn how the Community Seismic Network gathers data about how the ground and buildings move to inform safer construction and better damage detection.

Earthquake Preparedness

What Should You Do Before, During, and After an Earthquake?

It is impossible to predict when and where an earthquake will strike. Nonetheless, you can take steps before, during, and after a quake to help yourself stay safe and recover quickly.

Ask a Caltech Expert

Lucy Jones on Earthquakes

Find answers to common questions about earthquakes from seismologist and science communicator Lucy Jones, a visiting associate in geophysics at Caltech and the founder of the Dr. Lucy Jones Center for Science and Society.


Terms to Know

A smaller earthquake that occurs after the largest event of an earthquake sequence (mainshock). Aftershocks can occur for days, months, or years after the mainshock, but reduce in frequency over time.

Fault where the two sides have moved vertically relative to each other.

When significant earthquakes begin, these systems distribute electronic alerts to warn people before shaking reaches them, providing potentially crucial seconds of preparation time.

The probability of an earthquake of a certain size happening within a certain period of time. This is different from an earthquake "prediction," which requires a specific date and time, location, and magnitude. It is not yet possible to predict earthquakes.

The point on the surface of the earth directly above where an earthquake rupture begins.

A fracture in earth's crust. On either side of a fault, blocks of crust have moved relative to each other. Earthquakes occur when rock on one side of the fault suddenly slips.

An earthquake that comes before a larger earthquake in the same sequence. Foreshocks do not show any identifying characteristics when they occur; they are identified when a larger event follows.

For a given earthquake, the location within Earth's interior where the earthquake rupture starts

The severity of an earthquake in terms of what people and structures experience. A single earthquake has different intensities in different places.

The overall size of an earthquake, based on measurements of seismic waves recorded by seismographs. One earthquake may be felt at various intensities in different places, but it has only one magnitude.

The largest earthquake in a sequence. Mainshocks are sometimes preceded by foreshocks, and always followed by aftershocks.

A measure of earthquake magnitude based on the area of fault that moved, the amount that it moved, and the friction between the rocks. Developed by Caltech's Hiroo Kanamori and seismologist Thomas C. Hanks, this is the only method of measuring magnitude that is uniformly applicable to all sizes of earthquakes, but it is more difficult to compute than other scales.

A measure of earthquake magnitude based on seismic wave amplitudes that was introduced in 1935 by Caltech seismologists Charles Richter and Beno Gutenberg. The term is used colloquially to reference magnitude of any kind despite the fact that other magnitude scales, such as moment magnitude, are more commonly used today.

The squiggle-shaped record of ground motion collected by a seismometer.

A precise motion sensor used to record vibrations in the ground, such as seismic waves from earthquakes. Also known as a seismograph.

Fault where the two sides have moved horizontally relative to each other.

Large, rigid pieces of the earth's crust that move relative to one another. Places where tectonic plates meet experience more earthquakes.

Faults that occur in areas of Earth's crust where one slab of rock compresses against another, sliding up and over it during an earthquake. Thrust faults have been the sites of some of the world's largest quakes.

A series of earthquakes with no distinct mainshock occurring in a single area in a short period of time.

Dive Deeper

Epicenter of an earthquake
U.S. Geological Survey Earthquake Education
Image from the Caltech Seismological Laboratory
The Caltech Seismological Laboratory
Jennifer Andrews at the Caltech Seismological Laboratory
What Is it Like to Be a Caltech Seismologist during a Big Quake?