Lesson video

Hello, my name's Mrs. Harking.

The title of today's lesson is Earthquakes.

It's taken from the unit, Tectonic Hazards: Why is Earth Restless? In today's lesson, we're going to be looking at where and why earthquakes happen, including having a look at whether we get earthquakes in the UK.

I hope you enjoyed today's lesson.

Let's get started.

Our outcome today is to describe where earthquakes occur and to explain how they happen.

Here are some keywords useful to know before our lesson today.

If any of them are unfamiliar to you, it's a good idea to pause the video and write them down so that you can refer to them.

First of all, we have tectonic plates.

These are the huge pieces the Earth's surface is broken into.

Plate boundary.

This is where two plates meet.

Fault lines.

These are fractures in the Earth's surface.

We'll look into a little bit more detail about plate boundaries and fault lines and how they differ during the lesson today, and then focus points.

This is the point inside Earth's crust, where the earthquake starts.

Again, there'll be a diagram during the lesson today to explain that one.

Today's lesson is split into two parts.

Firstly, where do earthquakes occur, and then how do earthquakes happen? We're going to start with where do earthquakes occur? Earthquakes are sudden violent movements of the ground.

When the ground shakes, buildings that have stood for many years collapse.

You can see in the image here, this is an area that has experienced an earthquake, that this is the sort of image you might see after an earthquake has hit.

The movement of Earth's tectonic plates is the main cause of earthquakes.

These rigid sections of the Earth's lithosphere are constantly on the move.

Now, lithosphere just means the outer layer of our planet earth, so it's the surface of planet earth.

So Lucas is saying here, this is why there is an earthquake somewhere around the world every 30 seconds.

Can you believe that it's true? Every 30 seconds on average, there is an earthquake somewhere around the world.

Earthquakes that occur around the world vary in size.

So not all of those earthquakes are significant earthquakes that cause buildings to collapse like we saw in that first photograph.

Here we can see a world map of earthquakes by magnitude in 2017.

Magnitude is the measure of energy released when the ground shakes.

So how significant, how big, how bad the earthquake is.

You can see down here where I've circled that we've got our magnitude scale.

The larger the circle, the greater the magnitude, and also the colour changes as well to help us identify that.

Time for a check for understanding, most of the seven plus magnitude earthquakes occur along the boundary of which plate? So we've got three plates labelled onto our world map.

We've got the Pacific Plate, which is on the left and is labelled A.

Now the Pacific plate is also on the right.

Can you see that over on the right? Because our world is a globe and the Pacific Plate is from the perspective we're looking at it at the moment, wrapped around the back of our globe.

We've got the African Plate B and the Indian Plate C.

So around which plates are there most of the 7+ magnitude earthquakes? Let's have a look at the key.

So when we go down to the key, we can see that the 7 to 7.

9 magnitude earthquakes are an orange colour, and then the 8+ magnitude earthquakes are darker orange and they're slightly bigger.

So we're looking for the largest two circles that are orange or dark orange.

Well done.

Yes, it's the Pacific Plate isn't it? So do remember that the Pacific Plate is both on the left of this map and on the right hand side.

So a lot of those 7+ magnitude earthquakes can be seen around the Pacific Plates on the right hand side of our map.

On a tectonic map of the world, each plate boundary looks like a nice neat line that separates one plate from another.

So if we have a look at these red lines on this world map here, it looks like somebody has taken a knife and carefully cut out these jigsaw type shapes.

Is this really what Earth is like? What do you think? In reality when you zoom in, it is clear that there are many fault lines or fractures in the crust that make up one plate boundary.

So what I've done here is zoomed in to the North American Plates and the Pacific Plates and taken a look at the boundary between the two.

But we can see here in California, actually the fault lines are numerous.

There's so many smaller fault lines that make up that one plate boundary.

So can you see on the map on the left that we have got fault lines in red, and these are all of the little, we can almost think of them as cracks in the Earth's surface.

And together all of those fault lines make up the plate boundary that separates the Pacific Plate from the North American plates.

Time for a check for understanding now.

A red line on this map shows is it A, the plate boundary, B, a fault line, or C, an earthquake? Well done.

Yes, it's a fault line, isn't it? And together all of those fault lines make up the plate boundary that separates the two plates.

Another check for understanding.

Can you complete the sentence? A, what is made up of many fault lines? Is it A, a plate boundary, B, a fault, or C, a plate? Well done.

Yes, it's a plate boundary, isn't it? Well done.

"So do earthquakes occur away from these plate boundaries, for example, in the UK?" this is what Jacob's asking.

Let's have a look and see whether he's right.

So we've got a map here showing our closest plate boundaries and then the UK.

So the UK is really quite far from any plate boundaries.

However, locations far from plate boundaries like the UK, can experience very small earthquakes, but they are much smaller magnitude than those that occur at boundaries.

So on the map here, we would expect to see larger earthquakes at the plate boundary that's labelled compared to any that are experienced in the UK.

Have you ever felt an earthquake shake the ground in the UK? Time for check for understanding.

True or false.

The UK experiences lots of major earthquakes.

That is false.

You're right.

Justify your answer for me.

Is it A, the UK doesn't experience earthquakes, or B, the UK experiences low magnitude earthquakes? Well done.

The UK experiences low magnitude earthquakes.

This map shows some very minor earthquakes detected in the UK over a 60 day period, each earthquake is represented by a yellow circle.

So in 60 days, there's actually many earthquakes in the UK.

However, it's unlikely that all of these were felt.

Most earthquakes in the UK are never felt by people at least as we normally only feel the earthquakes of magnitude three and above.

So if we have a look at this scale here showing us the magnitudes of earthquakes, it's only the three and above that we would expect to feel.

Most of the UK's earthquakes are magnitude one or two, and they're not felt by people.

So how do we know they happen? Seismometers are actually used to detect earthquakes of all sizes, meaning we know that these earthquakes have happened even though the humans can't feel them.

Time for a check for understanding.

Which statement best describes the location of earthquakes? Is it A, earthquakes occur in tectonic plates around the world that are on the move? B, most earthquakes occur at plate boundaries with some minor quakes occurring away from these boundaries.

Or C, earthquakes are only located at plate boundaries or very close to them as plate movement is needed.

Well done.

Yes, it's B, most earthquakes occur at plate boundaries with some minor quakes occurring away from these boundaries.

Time for our task A now.

I would like you to do two things.

First of all, I'd like you to describe the distribution of earthquakes.

I've put a world map here with the earthquakes and their magnitudes just to help you out.

Then we've got the second part of our task.

Can you correct the following paragraph? So this paragraph has a number of mistakes and these need to be corrected.

There are four mistakes to hunt for.

Good luck.

Pause the video now to give yourself time to do the task.

Well done everyone.

Let's take a look at the answers now.

So for our first question, describe the distribution of earthquakes, the model answer looks like this.

The majority of earthquakes occur along plate boundaries.

Locations far from plate boundaries, for example the UK, can experience very small earthquakes.

The second question was to correct the paragraph.

Your answer may look like this, the UK does, not doesn't, but does experience earthquakes.

Most earthquakes in the UK are never felt by people as we normally only feel earthquakes of magnitude three and above.

Seismometers are used to detect earthquakes of all sizes, meaning we know that earthquakes have happened even though humans can't feel them.

So there were four errors there.

Did you spot all four? I've changed the fonts of the words that have been corrected.

So hopefully you can pick them out and here's one from each sentence.

So the first sentence was the does, second sentence, the never.

Then it was three and then seismometers.

Just double check you've got all of those.

I'm sure you did.

Well done, great effort.

Let's have a look at the second section of our lesson now.

How do earthquakes happen? There are many different causes of earthquakes, volcanic eruptions, landslides and explosions because all three can cause shaking of the ground.

However, the majority of earthquakes are caused by plate movements, not by these three.

Can you do a quick check for understanding for me now? Which of the following could cause earthquakes? Is it A, tectonic plate movement, B, volcanic eruptions, C, hurricanes or D, landslides? Well done.

So it's three of those, isn't it? Tectonic plate movement, A, B, volcanic eruptions and D, landslides.

Hurricanes do not cause earthquakes.

They're a weather hazard, which is separate to this.

Where two tectonic plates meet at a plate boundary, they become stuck or locked together due to friction, yet they may be moving in different directions or at different speeds.

Stress builds up over time until the rock fractures or breaks releasing energy.

We can model this using our hands.

One hand represents one plate, the other hand represents the other plate.

I'm interlocking my knuckles together, so I'm going to try and move one hand up, one hand down.

But the friction is keeping the plates locked together.

Then the pressure builds up, until that pressure is released when the rock fractures, which causes an earthquake.

Waves of energy called seismic waves or shock waves travel to the surface, meaning the ground shakes.

This is an earthquake.

You can see the seismic waves on the diagram there.

The point deep underground where the rock fractures releasing energy is called the focus point.

You can see that labelled on the diagram here.

The point on the surface directly above that focus point is called the epicentre, labelled on the diagram here.

So the focus is where the earthquake starts under the ground and the epicentre is the point on the surface, which has the most shaking directly above that focus point.

It is where the earthquake is first felt.

Time for a check for understanding now.

True or false, where two tectonic plates meet, they slide past each other smoothly.

Think about the justification for your answer as well.

You can have a read below.

Well done.

It's false.

And which justification did you pick? Well done, so the rough edges of the plate become locked together.

That's caused by friction, isn't it? Another check for understanding.

Which is the correct label for X? Is it A, epicentre, B, focus point or C, seismic waves? Well done.

It's the focus points.

Minor earthquakes far from plate boundaries can be caused by ancient fault lines that were once active.

For example, The Great Glen Fault in Scotland, there could be stress from plate movement causing small isolated cracks or faults to shift, and then volcanic eruptions, landslides or explosions like we mentioned before, can also cause earthquakes.

So if any of those occur away from plate boundaries, that could also trigger a minor earthquake.

True or false? Plates movement can cause minor earthquakes far from plate boundaries.

Well done.

That's true.

Can you justify your answer? Well done.

Both of these justifications actually are correct.

So if you picked one and not the other, maybe take a look at the other one as well and familiarise yourself with that.

So human activities can trigger earthquakes far from plate boundaries.

For example, explosions, but also ancient fault lines may move within tectonic plates causing minor earthquakes.

So our task B now.

Can you label the diagram to show focus points, epicentre and seismic waves? Our second part of the task is to create a flow diagram to explain how plate movement leads to earthquakes at plate boundaries.

Use the following words in your answer.

Tectonic plates, stress, fracture, seismic waves, focus points.

Pause the video now to give yourself time to do the task.

Well done everyone.

Let's take a look at the answers.

Here we have a labelled diagram.

Now yours, if you have drawn the diagram yourself, does not need to be 3D and perfect, but as long as it's neat and clear, that's absolutely fine.

We should have the focus underground where the earthquake starts, the epicentre at the surface directly above the focus, and then the seismic wave should be these, represented by these circles emanating from the focus.

Our second part to the task, we have got our explanation.

Now yours may look something like this.

Feel free to edit your answer as we go.

Where two tectonic plates meet at a plate boundary, they can become locked together, yet maybe moving in different directions.

You might have added in there as well they could be at different speeds instead of different directions.

Over time, stress builds until eventually the rock fractures or breaks and the stress is released.

Seismic waves are released from the focus points, meaning at the surface, the ground shakes.

This is an earthquake.

It's unlikely that yours looks identical to mine.

That's absolutely fine.

What I was looking for here is correct explanation and a flow diagram so you can see what happens first, second, third, and also that you are using the key terms that we've learnt today.

Well done everyone, really great effort today.

We're going to go through this summary now as to remind you of the key points from today's lesson.

First of all, earthquakes are sudden violent movements at the ground normally caused by the movement of tectonic plates.

Most earthquakes occur at or near plate boundaries, where two plates meet, they will become locked together resulting in the buildup of stress in the rock.

Eventually, the rock fractures causing shockwaves to radiate out from the focus points.

The epicentre is the place on the surface where the earthquake is first felt.

Thank you very much for learning with me today.

I hope to see you next time.

Goodbye.