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Earthquakes are the shaking of the Earth’s crust. This happens when two plates move past each other suddenly. Normally, plate motion is smooth. But, sometimes plates get jammed together. When they finally become free, they slide past each other quickly. This sudden motion causes an earthquake.
A description of Earthquakes.

Different types of plate boundaries. At each, plate motion can stop and start suddenly, creating an earthquake.

When Plates Get Stuck

The theory of plate tectonics tells us that the crust is made up of moving plates. These plates move past, away from, and into each other all the time at faults. If you think about it, that’s astounding. Plates are made of huge, craggy rocks but can somehow slide by each other!
Of course, we should still expect that their roughness causes them to snare. When that happens, both plates are slowed down. That doesn’t mean they’re not trying to move. Instead, they keep pushing against whatever has stopped them. This generates what we call friction.

Friction and Plate Movement

Friction is important to understanding why earthquakes happen. If you don’t know what it is, that’s okay. You can see it for yourself. Here’s how: Take your hands and slide them past each other with your palms inward. Notice that the motion is smooth. Now, close your hands into fists. Slide your knuckles past each other. This motion isn’t smooth at all! Instead, your hands stop and start moving as you keep pushing them.
What stops them from moving smoothly is friction. Friction is a natural force, like gravity. It tries to stop things from moving against one another. Friction is strongest when those things have rough surfaces. That’s why it’s slower to move your knuckles past each other than your palms.
You’ve seen that friction works on something as small as human hands. So, think about how powerful it is when whole, rocky plates are moving past each other! Not much happens when your knuckles shift. But, when plates do, everything around them is affected. Their sudden motion is so powerful it causes an earthquake.

When Plates Get Unstuck

As soon as two plates slide past each other suddenly, an earthquake is set in motion. Wherever they were stuck together becomes what we call the focus of the earthquake. From there, a lot of energy is let off. This is because plates store energy as they try to overcome friction. After they finally do, they release that energy in the form of seismic waves.
Seismic waves sound like a complicated concept. But, they are just a form of energy that pushes things around. They’re generated at the focus and then spread outwards. This causes quaking in the earth. Most of this happens underground, but some seismic waves will reach the earth’s surface. Then, we start to feel them. That’s also when earthquakes become dangerous.
Diagram of seismic activity.

A picture showing both an earthquake’s focus and its epicenter. Notice that the focus is located at a fault. This is a place where plates move past each other. It’s also where there might be a lot of friction.


Usually, when we talk about earthquakes, you’ll hear the word epicenter. This is the point above the focus on the surface of the Earth. It’s also where the most damage occurs. Seismic waves from the focus reach the epicenter first but then spread out around it. This shakes the ground for a while, and that shaking is the part of the earthquake that we can feel.

Measuring Quakes

Not every earthquake is equally strong. Some barely move a picture frame while others cause a lot of damage. To compare them, scientists have developed two related tools.

The Seismograph

A seismograph is a tool which measures seismic waves. It shows both the intensity of the waves and for how long they were sent out. The longer and taller their signal is, the stronger the earthquake is.
The reading of an earthquake by a seismograph machine.

How data from a seismograph looks.

Intensity Scales

After scientists have recorded the intensity of an earthquake, they need a way to talk about it. So, they have developed scales for the intensity of earthquakes. There are many kinds of intensity scales. The most well-known is the Richter scale. If you’re interested, you can learn more about the Richter scale in the resources below.

Other Great Resources:

How a Seismograph Works: