1. Earthquakes

2. Definition
Earthquakes – the shaking of the ground caused by the release of energy waves as rocks in the earth’s crust move suddenly
Seismology – The study of earthquakes
Seismologist – Scientists who study earthquakes

3. Where Do Earthquakes Occur?
Near edges of tectonic plates
Tectonic plates can move toward each other, away from each other or slide past each other
These movements create faults in the Earth’s crust
A fault is the break in the Earth’s crust along which blocks of the earth's crust slide
Earthquakes occur along these fault lines

4. What Causes Earthquakes?
As tectonic plates move along faults, stress builds up as the rocks get stuck together
This causes the rocks along the fault to change their shape or deform
Elastic deformation leads to earthquakes
Rocks can stretch but eventually they will unstick and move suddenly into a new position
When the rocks unstick they release energy in the form of seismic waves. These waves of energy cause earthquakes

5. Seismic Waves Waves of energy that are released during an earthquake.
These waves are able to travel through the Earth’s interior and along the Earth’s Surface
It is these waves that cause damage during an earthquake

6. P and S Waves P Waves are the fastest seismic waves
Also known as Primary Waves
They are able to travel through both liquids and solids so they can reach across the entire planet
S Waves travel at half the speed of P Waves
Also known as Secondary Waves
They can only travel through solids so they can not travel through the Earth’s outer core

7. Locating an Earthquake
Seismologists use seismographs. These instruments can detect and record seismic waves
Based on the arrival times of P and S waves to the seismograph, seismologists can determine the location of the earthquake.

8. Epicenter and Focus
Epicenter is the point on the Earth’s surface directly above an earthquakes starting point
Damage done during an earthquake will usually be the most severe at and around the epicenter
Focus is the point inside the Earth where an earthquake begins
Seismographs and the timing of the S and P waves can help to locate the epicenter and the focus of an earthquake

9. Measuring Strength and Intensity
Strength of an earthquake is measured using Richter Scale
Strength also known as magnitude
Measure of ground motion
Based on a scale of 1 to 10
Each unit increase equals ground motion becomes 10 times larger
Intensity of an earthquake is measured using Mercalli Scale
Measure of how people feel the earthquake and how much damage it causes
Based on numbers I to XII
I is not felt at all, XII is total damage to area