1. Chapter 9: Earthquakes 9.1: Earthquakes occur along faults
9.2: Earthquakes release energy
9.3: Earthquake damage can be reduced

2. 9.2 Earthquakes release energy
Terms:
faults,
plate boundaries
stress
earthquake
All earthquakes occur along ______
The force exerted when an object pushes, pulls, or presses against another object is called _______
Most faults are located along _______

3. Earthquakes release energy
Energy from earthquakes travel through Earth
Ripple of rock in pond, but in all directions
Energy travels as seismic waves: vibrations caused by earthquakes
Earthquakes start beneath Earth’s surface
Focus: point underground where rocks first begin to move
Seismic waves travel outward from the focus
Epicenter: point on Earth’s surface directly above the focus
If equal strength earthquakes occur, the more shallow the focus results in greater damage
Depth is related to the direction in which the plate move
Pulling apart: shallow: new crust that forms is thin
Subduction zones: wide range of depths, anywhere along sinking plate
CA video
Spring activity - energy travels
Focus and epicenter images

4. Waves and Energy
All waves (sound, seismic, light) carry energy from place to place
As a wave moves through a material, particles of the material move out of position temporarily, causing the particles next to them to move:
Energy moves through the material, matter does not (*light is different)
October 17th earthquake in San Francisco: shook the stadium around for 15 seconds
20 minutes later the seismic waves reached the other side of the Earth: detect only by sensitive scientific instruments

5. Three types of waves Each type moves through material differently
Can reflect, or bounce, off boundaries between different layers
Can bend as pass from one layer to another
Scientists learn about Earth’s layers by studying the paths and speeds of seismic waves traveling through Earth
waves video

6. Three types of waves Primary (P waves)
the fastest seismic waves First to reach any particular location after an earthquake
Travel through Earth’s crust at an average speed of 5 km/s (3 mi/s)
Can travel through solids, liquids, gases
As they travel through a material the particles are slightly pushed together and pulled apart
Buildings experience this push and pull as p waves pass through the ground

7. Three types of waves Secondary waves (S waves)
the second waves to reach a location after an earthquake
Originate at the same time as P waves, but travel at half the speed
As they pass through a material, the material’s particles are shaken up and down or side to side
This rocks buildings back and forth as they pass
can travel through rock but not liquids or gases
Primary waves alter the material density and volume slightly
Particles are pushed and pulled in the direction the waves travel
Secondary waves alter the material’s shape
Liquids and gases have no definite shape
Particles move at a right angel to the direction the waves travel
When scientist learned S waves cannot pass through the earth’s outer core they realized it was not solid!

8. Three types of waves Surface waves
Move along Earth’s surface, not through the interior
Make the ground roll up and down or shake from side to side
Cause the largest ground movement and most damage
Slowest type of seismic wave
As depth increases, motion of the particles decreases

10. Types of waves Name Primary Secondary Surface Speed Fastest Medium
Slowest
Location
Earth’s interior
Earth’s surface
Type of material
All
Solids
_____
Type of movement
Push/pull
Up/down; side/side
Damage
Some
Most

11. Seismic waves can be measured
Seismograph: an instrument that constantly records ground movements
Separate ones are needed to record side-to-side movements and up-and-down movements
Side-to-side: uses a heavy weight attached to a wire, which remains still as the ground moves beneath it
Up-and-down: uses a heavy weight hanging from a spring, which remains almost still as the spring absorbs the movement by getting longer or shorter
Can detect movements as little as one hundred-millionth of a centimeter ( cm)

13. Locating an Earthquake
If you know the speed of S and P waves, and you know the time it takes for their arrival, you can determine distance
Speed = distance/time
While the speeds vary, the ratio of the speeds do not! Just multiply the S-minus-P (S-P) time, in seconds, by the factor 8 km/s to get the approximate distance in kilometers.
Need seismographs from at least three stations
Scientists find the difference between the arrival times of the primary and secondary waves at each of the three stations
The time difference is used to determine the distance of the epicenter from each station (the greater time, the father away)
A circle is drawn around each station, with a radius corresponding to the epicenter’s distance from that station. Where the three circles meet is the epicenter

14. Locating an Earthquake
Seismographs can also be used to locate the focus of an earthquake
Can study waves that have reflected off boundaries inside Earth
Can help determine the earthquake’s depth
Record the time when
the first primary wave arrives, by a direct path
and also the first reflected primary wave arriving, which first reflects off the surface before reaching the station
The difference in arrival time indicates the depth of the focus
Also used to determine earthquakes’ magnitude (strengths)
More energy an earthquake releases, the greater the ground movement recorded