What Causes an Earthquake?

Earthquakes — Elementary

What is an earthquake?

An earthquake is when two blocks of Earth slip past each other all at once.

The place where they slip is called a fault.

It is also called a fault plane.

The ground is like a puzzle

The top of Earth is like a thin skin.

The skin is not in one piece.

It is made of many pieces, like a puzzle.

We call the pieces tectonic plates.

The plates move very, very slowly.

They slide past each other.

They bump into each other, too.

The edges of the plates are rough.

The edges are made of many faults.

Most earthquakes happen on those faults.

Stick, build, and slip

The edge of a plate can get stuck.

The rest of the plate keeps moving.

So energy builds up and is stored.

At last, the plate has moved far enough.

The stuck edge comes unstuck.

The stored energy rushes out.

That sudden slip is the earthquake.

Waves through the ground

The energy moves out in all ways.

It moves as waves. The waves are called seismic waves.

They are like ripples on a pond.

The waves shake the ground as they move through it.

Where does it start?

An earthquake starts under the ground.

That starting place is called the hypocenter.

The spot on the ground right above it is called the epicenter.

Small shakes before and after

Sometimes small earthquakes come first, in the same place.

They are called foreshocks.

No one can tell a shake is a foreshock until the big one comes.

The biggest shake is called the mainshock.

Small shakes can come after it, in the same place.

They are called aftershocks.

Aftershocks can go on for weeks, months, or even years.

How do scientists record it?

A tool called a seismograph records an earthquake.

The record it makes is called a seismogram.

Part of the tool sits firm on the ground. That part shakes.

A heavy weight hangs free. The weight stays still.

The gap between the shaking part and the still part is the record.

A short line that wiggles a little means a small earthquake.

A long line that wiggles a lot means a large earthquake.

How big, and how strong?

The size of an earthquake is called its magnitude.

There is one magnitude for each earthquake.

Intensity is how strong the shaking is.

The shaking can feel stronger in one place and weaker in another place.

So intensity is not the same in every place.

Two kinds of waves

One kind of wave is called a P wave.

The other kind is called an S wave.

P waves move faster than S waves.

Think of lightning and thunder.

Light moves faster than sound.

So you see lightning first. Then you hear thunder.

P waves are like the lightning. They get there first.

S waves are like the thunder. They come next.

If you are close, the two waves come one right after the other.

If you are far away, there is more time between them.

Finding where it happened

Scientists look at the time between the P wave and the S wave.

That time tells how far away the earthquake was.

But it does not tell which way it was.

So scientists draw a circle on a map.

They do this for three seismographs.

Where the three circles meet is the epicenter.

Using three tools like this is called triangulation.

Can scientists predict earthquakes?

No.

USGS says it is unlikely they will ever be able to predict them.

Scientists have tried many ways. None of them have worked.

They know a fault will have another earthquake some day.

But they have no way to tell when it will happen.

Earthquakes — Middle

What is an earthquake, in one line?

USGS defines an earthquake as what happens when two blocks of the Earth suddenly slip past one another along a surface called the fault, or fault plane.

The interesting question is why the blocks were ready to slip in the first place.

Earth’s layers and the moving skin

Earth has four major layers: the inner core, the outer core, the mantle, and the crust.

The crust together with the top of the mantle forms a thin skin on the surface of the planet.

That skin is not one solid sheet. USGS compares it to a puzzle: it is broken into many pieces that keep slowly moving around, sliding past one another and bumping into each other.

Those puzzle pieces are the tectonic plates, and their edges are called plate boundaries.

Plate boundaries are made up of many faults, and USGS says most of the earthquakes around the world occur on these faults.

Why the slip is sudden: stick, store, release

The edges of the plates are rough, so they can get stuck while the rest of the plate keeps moving.

While the edges of a fault are stuck together and the rest of the block is still moving, energy is being stored up instead of causing the blocks to slide.

Eventually the force of the moving blocks overcomes the friction of the jagged edges of the fault.

When the fault unsticks, all that stored-up energy is released at once.

That release is the earthquake itself — not a slow creep, but a sudden slip after a long, slow build-up.

Hypocenter and epicenter are not the same point

The location below Earth’s surface where the earthquake starts is the hypocenter.

The epicenter is a different point: it is the location directly above the hypocenter, on the surface of the Earth.

So the hypocenter names the starting place underground, and the epicenter names the matching spot on the map above it.

Foreshocks, the mainshock, and aftershocks

USGS sorts shakes around the largest event by timing. Smaller earthquakes that happen in the same place before the larger earthquake are foreshocks.

There is a catch: scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens.

The largest, main earthquake is the mainshock.

Mainshocks always have aftershocks that follow — smaller earthquakes that occur afterwards in the same place as the mainshock.

Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock.

How the energy travels: seismic waves

The released energy radiates outward from the fault in all directions in the form of seismic waves.

USGS uses a calm comparison for this: the waves spread like ripples on a pond.

The seismic waves shake the Earth as they move through it, and when the waves reach Earth’s surface, they shake the ground.

How a seismograph turns shaking into a record

Earthquakes are recorded by instruments called seismographs, and the recording they make is called a seismogram.

A seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free.

When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not.

The spring or string that the weight hangs from absorbs the movement.

What gets recorded is the difference in position between the shaking part of the seismograph and the motionless part.

Magnitude versus intensity

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but faults are many kilometers deep, so scientists cannot measure them directly with a measuring tape.

Instead they use the seismogram recordings made at the surface. A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake.

The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

The size of the earthquake is called its magnitude, and there is one magnitude for each earthquake.

Intensity is different. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

So magnitude describes the earthquake itself, while intensity describes the shaking at a particular place.

P waves, S waves, and the lightning-and-thunder comparison

Seismograms also help scientists locate an earthquake, and being able to see the P wave and the S wave is important.

P waves and S waves each shake the ground in different ways as they travel through it, and P waves are faster than S waves.

USGS explains the timing with a weather comparison. Light travels faster than sound, so during a thunderstorm you will first see the lightning and then you will hear the thunder.

If you are close to the lightning, the thunder will boom right after the lightning. If you are far away, you can count several seconds before you hear the thunder. The further you are from the storm, the longer it will take between the lightning and the thunder.

P waves are like the lightning, and S waves are like the thunder. The P waves travel faster and shake the ground where you are first, then the S waves follow and shake the ground also.

If you are close to the earthquake, the P and S waves will come one right after the other, but if you are far away, there will be more time between the two.

Locating a quake: one distance, then three circles

By looking at the amount of time between the P wave and the S wave on a seismogram, scientists can tell how far away the earthquake was from that seismograph.

One station alone cannot give the direction, only the distance. If scientists draw a circle on a map around the station, where the radius of the circle is the determined distance to the earthquake, they know the earthquake lies somewhere on the circle.

To pin down where, scientists use a method called triangulation. It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake.

If you draw a circle on a map around three different seismographs, where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter.

Can scientists predict earthquakes?

No, and it is unlikely they will ever be able to predict them.

That is the flat answer USGS gives. Scientists have tried many different ways of predicting earthquakes, but none have been successful.

On any particular fault, scientists know there will be another earthquake sometime in the future, but they have no way of telling when it will happen.