Earthquakes are caused by the release of built up energy in the Earth's crust, which produces seismic waves.
Seismic waves have different and very distinct properties.
The first waves are called P waves (they are also referred to as primary, pressure, compression or longitudinal waves).
The second waves are called S waves (also referred to as secondary, shear or transverse waves)
surface waves follow - earthquake!
TASK 9: Use the graph to determine how long it took for the S and surface waves to arrive after the P wave. Note that this seismogram recorded a distant earthquake.
P waves push and pull rocks in the direction the wave is travelling. They are waves of compression and will travel through liquids, solids and gases. They are faster and less destructive than S waves...
S waves move at right angles to the direction the wave is travelling (in a shearing motion). S waves do not travel through liquids or gases - when they travel through the Earth's interior, they are deflected by the liquid outer core. S waves are the most destructive waves responsible for earthquake damage.
TASK 10: Try this for yourself. Use a slinky to demonstrate the difference between the two waves. Discuss further activities with your teacher - an example lab experiment can be found here. Remember to write up your observations in your notebook
TASK 11: Create a table that compares P and S waves:
do they pass through solids, liquids or gases?
level of destruction
speed they travel, which arrives first?
direction of particle motion
TASK 12: Use the seismographs opposite to determine how much warning people had before this earthquake.
This is an image of how seismic waves travel through the Earth. Note that only the P waves travel through the liquid outer, and solid, inner core. S waves cannot travel through liquids, and are deflected by the liquid inner core, which is how scientists determined the structure of the Earth's core. You can find out more on the video.
Tsunamis are a direct result of earthquakes. Watch these two videos from National Geographic about the 2004 Boxing Day tsunami that caused devastation around the Indian Ocean and resulted in the death of more than 300,000 people, and the 2011 Japan earthquake and tsunami, which killed more than 16,000 people.
TASK 13: Discuss as a class what you would do if there was an earthquake or tsunami warning. Where would be the safest place to head? Research a plan, considering how much time there might be before each event. You might like to undertake research about earthquakes - do animals detect earthquakes before humans?
Next we'll look at how the interior structure of the Earth is responsible for plate tectonics.