Earthquakes

1. Earthquakes
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
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
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
Seismic 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

2. 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
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.
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 r 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
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 10Each unit increase equals ground motion becomes 10 times larger Intensity of an
earthquake is measured using Marcella Scale Measure of how people feel the earthquake and how much damage it
causes Based on numbers