1. What are earthquakes?
Stresses in the crust cause rock to move suddenly
The sudden movement causes vibrations that travel as waves inside the Earth and along its surface.
The vibrations caused by an earthquake are called seismic waves
Seismic waves cause motion in the rock/soil that they move through

2. Wave types and origins Body waves Surface waves
Originate at the focus of the earthquake
within the crust (within the body of the Earth)
Travel through the body of the Earth
Surface waves
Originate at the epicenter of the earthquake
the point on the surface of the Earth above the focus
Travel along the surface of the Earth

4. Comparing earthquakes
How big is an earthquake?
It depends on how you measure it…
Two ways to measure earthquake “size”
Magnitude
A measure of the amount of energy released
Related to the height/amplitude of the largest P-wave
Measured using the Richter Scale
Intensity
A measure of the amount of damage done
Related to the height/amplitude of surface waves
Measured using the Modified Mercalli Scale

5. What causes earthquakes?
Forces in the Earth’s crust due to plate tectonics cause rocks in the crust to be
Pulled apart
Pushed together
Twisted
Stress – the force per unit area acting on a material
Tension – pulled apart
Compression – pushed together
Shear - twisted

9. Where do earthquakes occur?

10. Earthquake hazards
Most earthquakes occur along the boundaries of the tectonic plates that make up the crust.
Earthquakes are more likely in these areas than in other places.
Similarly, past seismic activity in any region is a reliable indicator of the potential for future earthquakes.
Thus, maps of historic earthquakes can be used to make maps of “seismic hazards”

11. Earthquakes in North Carolina
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12. Earthquake hazards in NC
Peak accelerations with 2% probability of being exceeded within 50 yrs
(Red = % g, Blues = 12-16% g)
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