1. How we detect earthquakes
P1h(i) Stable Earth
You will learn about:
How we detect earthquakes
Earthquake waves and how they enable us to determine what is inside the Earth

2. Earthquakes www.PhysicsGCSE.co.uk
REMEMBER: The yellow lines show where the boundaries of tectonic plates are. When these move against one another earthquakes occur due to the massive build up of frictional forces.
Earthquakes happen when rocks below the surface of the Earth move at a fault. This releases shock waves that travel around the Earth as well as through it. When a wave reaches the surface it can cause a lot of damage to the buildings sitting there. Tsunamis occur due to undersea earthquakes. A huge amount of water is shifted upwards when a less dense tectonic plate lifts upon a more dense plate. When the water reaches the land it causes widespread devastation. A tsunami hit Japan in March 2011 and another in the Indian Ocean in December This one had recorded wave heights of over 10m and wavelengths (crest to crest) of just over 500 km. It travelled at speeds reaching 800 km per hour. Nobody was prepared for the devastating effect each would cause. This is because earthquakes are difficult to predict.

3. Seismometers www.PhysicsGCSE.co.uk
A seismometer detects shock waves caused by earthquakes. A heavy weight attached to a pen hangs above a rotating drum. The base is secured to the Earth. When an earthquake occurs the base moves but the pen stays still. As the base moves the pen draws on the paper on the rotating drum. The trace is called a seismograph.
This is an image of a typical seismograph. It shows the vibrations of the Earth due to the seismic waves caused by earthquakes.
This chart on the left shows the number of earthquakes per year detected on Earth.
You can see that very minor earthquakes occur 1,300,000 times a year! But they are so small we do not really feel them. On the right the chart shows how much damage each earthquake can cause.

4. Seismographs www.PhysicsGCSE.co.uk
REMEMBER:
The focus is where the earthquake originated below the Earth’s surface. It is where the focused energy is.
The epicentre is the position on the Earth’s surface above the focus.
Seismographs
Two types of wave are generated by an earthquake: P wave: this is the Primary wave (or Pressure wave). It is Longitudinal and travels fast at between 5 and 8 km/s. It can also travel through solids AND liquids. S wave: this is a Secondary wave (or Shear wave) and travels through the earth slower at 3 to 5.5 km/s because it is a Transverse wave.
Easy way to remember everything about waves:
S wave: Secondary, Shear, tranSverSe or Side to Side, Slow, Solids only.
P wave: Primary, Pressure, Passes quickly
P waves are detected first.
S waves are detected second.
L waves are detected next. These are the relatively slow surface waves that travel from the epicentre. These are the ones that cause the surface of the Earth to shake.
S waves
L waves
P waves

5. Deep into the Earth www.PhysicsGCSE.co.uk S waves can only
Most of us now recognise the image of the structure of the Earth: the solid core, liquid outer core, mantle and lithosphere. But without physically being able to drill down into the Earth how de we know what it is made of?
Deep into the Earth
S waves can only
pass through solids.
As S waves are
not detected on
the other side of the
Earth from the
epicentre, this tells
scientists there must be a liquid layer inside the Earth. A much larger shadow zone exists from 103°
P waves are able to pass through solids AND liquids. The core, due to its high density, causes the P waves to refract. Two shadow zones exist between 103° and 142° on either side of the Earth from where the epicentre is located.
P waves and S waves are BOTH detected at exactly the same point - 103° from the epicentre. Scientists are sure that the liquid part inside the Earth exists around the core and not a layer within the mantle.

6.
Questions
Why does the seismometer weight always point toward the drum?
Adam bolts his seismometer to the side of his house. Adrianne suggests this is not such a good idea. Why not?
Why are S waves slower than P waves?
An earthquake occurs right next to where you are. Your seismograph trace shows three distinct trace patterns. Draw that trace. Your friend on the other side of the Earth also notices his seismometer drawing a trace from the same earthquake. Draw that trace too.

7. Questions www.PhysicsGCSE.co.uk
Why does the seismometer weight always point toward the drum? Gravity pulls it down.
Adam bolts his seismometer to the side of his house. Adrianne suggests this is not such a good idea. Why not? The house may vibrate due to different reasons other than from earthquakes. It should be bolted to the Earth so it can trace seismic vibrations.
Why are S waves slower than P waves? S waves are transverse. This means they travel at right angles to the direction of energy flow. P wave are longitudinal so travel in the SAME direction as energy flow so get to the point first.
An earthquake occurs right next to where you are. Your seismograph trace shows three distinct trace patterns. Draw that trace. Your friend on the other side of the Earth also notices his seismometer drawing a trace from the same earthquake. Draw that trace too.
Your trace draws the P, S and L wave. Notice that the P wave is almost instantly detected because of your close proximity.
Your friend’s trace only detects P waves. S waves will not pass through the liquid outer core. L waves would have transferred their energy before reaching the other side of the Earth.
Notice the long time delay