1. Chapter 8
Earthquakes

2. General Features
Vibration of Earth produced by a sudden release of energy
Associated with movements along faults
Faults – fractures in Earth where movement has occurred.
Focus – the point within the earth where the earthquake starts
Epicenter – location on the surface directly above the focus.

3. Mechanism for Earthquakes
First explained by H. Reid in Early 1900s
Rocks “spring back” – elastic rebound
Vibrations occur as rock elastically returns to its original shape
Often preceded by foreshocks and followed by aftershocks.

4. Elastic Rebound Hypothesis
The springing back of the rock into its original place
The rock behaves much like a stretched rubber band does when it is released.
Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces. When the strength of the rock is exceeded it suddenly breaks, causing vibrations

5. Aftershocks and Foreshocks
The movements that follow a major earthquake often produce smaller earthquakes called aftershocks.
Usually much weaker
Can still cause major damage
Small earthquakes before a major earthquake can also occur. They are called foreshocks

6. Measuring Earthquakes
The study of earthquake waves is called seismology.
Seismographs are instruments that record earthquake waves.
Seismographs amplify and electronically record ground motion producing a trace, called a seismogram

7. There are two kinds of waves produced by an earthquake
Earthquake Waves
The energy from an earthquake spreads outward as waves in all directions from the focus.
There are two kinds of waves produced by an earthquake
Surface waves
Body waves

8. Seismic waves that travel along Earth’s outer layer.
Surface Waves
Seismic waves that travel along Earth’s outer layer.
The motion is complex
Travel along the ground and cause the ground to move.
Like an ocean wave
Move up and down as well as side to side
Destructive

9. Body Waves Travel through the interior
Identified as either P waves or S waves depending on how they travel
P waves are push-pull waves
S waves shake the particles at right angles to their direction of travel

10. Locating an Earthquake
The difference in velocities of P and S waves provides a way to locate the epicenter
The greater the interval the greater the distance to the earthquake source.

11. Earthquake direction
Travel- time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter.
We draw circles on the globe and where the circles intersect is the epicenter

12. About 95% of the major earthquakes occur in a few narrow zone
Earthquake Zones
About 95% of the major earthquakes occur in a few narrow zone

13. Measuring Earthquakes
scientist use two different types of measurements to describe the size of an earthquake – intensity and magnitude
Intensity is a measure of the amount of earthquake shaking at a given location based on the amount of damage
Magnitude measures size of seismic waves

14. Outdated scale for measuring magnitude of earthquakes
Richter Scale
Outdated scale for measuring magnitude of earthquakes
Based on amplitude of the largest wave
Only useful for small, shallow earthquakes within about 500km of the epicenter

15. Moment Magnitude More precise means of measuring earthquakes
Derived from the amount of displacement that occurs along a fault zone
Most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released

16. Damage
The damage to buildings and other structures from earthquake waves depends on several factors
Intensity
Duration of the vibrations
Nature of the material on which the structure is built
Design of the structure

17. Tsunamis Seismic Sea waves
Triggered by an earthquake occurs where a slab of the ocean floor is displaced vertically along a fault.
A tsunami also can occur when the vibration of a quake sets an underwater landslide into motion

18. Other dangers
Landslides
With many earthquakes the greatest damage to structures is from landslides and ground subsidence
Fire