1. Table of Contents Forces in Earth’s Crust
Earthquakes and Seismic Waves
Monitoring Earthquakes
Earthquake Safety

2. Forces in the Earth’s Crust
- Forces in Earth’s Crust
Forces in the Earth’s Crust
The movement of Earth’s plates creates enormous forces that squeeze or pull the rock in the crust.
Stress – force that acts on rock to change its shape or volume
Three different kinds of stress can occur in the crust: tension, compression, and shearing.
work over millions of years to change the shape and volume of rock

3. - Forces in Earth’s Crust
Types of Stress
Tension – pulls on the crust, stretching rock so that it becomes thinner in the middle.

4. - Forces in Earth’s Crust
Types of Stress
Compression – stress force that squeezes rock until it folds or breaks.

5. - Forces in Earth’s Crust
Types of Stress
Shearing – stress that pushes a mass of rock in two opposite directions

6. - Forces in Earth’s Crust
Kinds of Faults
Most faults occur along plate boundaries, where the forces of plate motion push or pull the crust so much that the crust breaks.
Fault – a break in the rock of the crust
There are three main types of faults: normal faults, reverse faults, and strike-slip faults.

7. - Forces in Earth’s Crust
Kinds of Faults
Tension in Earth’s crust pulls rock apart, causing normal faults.
Normal fault – hanging wall falls down below foot wall. Ex: Rio Grande rift valley, New Mexico

8. - Forces in Earth’s Crust
Kinds of Faults
A reverse fault has the same structure as a normal fault, but the blocks move in the opposite direction.
Reverse fault – hanging wall move up above foot wall Ex: Northern Rocky Mountains

9. - Forces in Earth’s Crust
Kinds of Faults
In a strike-slip fault, the rocks on either side of the fault slip past each other sideways, with little up and down motion.
Strike-slip fault – two sides slide past each other
Ex: San Andreas Fault

10. Changing Earth’s Surface
- Forces in Earth’s Crust
Changing Earth’s Surface
Over millions of years, the forces of plate movement can change a flat plain into landforms, such as:
Anticlines and synclines, folded mountains, fault-block mountains, and plateaus.

11. Changing Earth’s Surface
- Forces in Earth’s Crust
Changing Earth’s Surface
Over millions of years, the forces of plate movement can change a flat plain into landforms, such as:
Anticlines and synclines, folded mountains, fault-block mountains, and plateaus.

12. - Forces in Earth’s Crust
Folding Earth’s Crust
Anticline – upward fold in rock formed by compression of Earth’s crust.
Syncline – downward fold in rock formed by compression in Earth’s crust.
Folding produced some of the world’s largest mountain ranges
Ex: Himalayas in Asia and Alps in Europe

13. Stretching Earth’s Crust
- Forces in Earth’s Crust
Stretching Earth’s Crust
Fault-Block Mountains – form from two normal faults, running parallel to each other
Two hanging walls fall leaving a raised area in between.

14. Uplifting Earth’s Crust
- Forces in Earth’s Crust
Uplifting Earth’s Crust
Plateau – large area of flat land elevated high above sea level.
Form when forces in Earth’s crust push up a large, flat block of rock
Ex. Colorado Plateau

15. Click the SciLinks button for links on faults.
- Forces in Earth’s Crust
Links on Faults
Click the SciLinks button for links on faults.

16. Earthquakes and Seismic Activity
- Earthquakes and Seismic Waves
Earthquakes and Seismic Activity
Earthquake – shaking that results from the movement of rock beneath Earth’s surface.
Occur all the time, usually too small to notice
Most begin in the lithosphere (100 km below surface)
Focus – point beneath Earth’s surface where rock breaks under stress and causes an earthquake.
Epicenter – point on Earth’s surface directly above an earthquake’s focus.

17. - Earthquakes and Seismic Waves
Types of Seismic Waves
Seismic waves – carry energy from an earthquake away from the focus, throughout the Earth

18. - Earthquakes and Seismic Waves
Types of Seismic Waves
There are 3 main categories of seismic waves: P waves, S waves, and surface waves.
P waves - seismic waves that compress and expand the ground like an accordion.

19. - Earthquakes and Seismic Waves
Types of Seismic Waves
S waves – seismic waves that vibrate from side to side as well as up and down.

20. - Earthquakes and Seismic Waves
Types of Seismic Waves
Surface waves - move more slowly than P waves and S waves, but they produce the most severe ground movements.

21. Seismic Waves Activity
- Earthquakes and Seismic Waves
Seismic Waves Activity
Click the Active Art button to open a browser window and access Active Art about seismic waves.

22. Measuring Earthquakes
- Earthquakes and Seismic Waves
Measuring Earthquakes
There are three commonly used scales of measuring earthquakes:
Mercalli scale, Richter scale, and the moment magnitude scale.

23. Measuring Earthquakes
- Earthquakes and Seismic Waves
Measuring Earthquakes
Mercalli scale – developed to rate earthquakes according to the amount of damage at a given place.

24. Measuring Earthquakes
- Earthquakes and Seismic Waves
Measuring Earthquakes
Richter scale – rates an earthquake’s magnitude based on the size of its seismic waves.
Magnitude – measure of an earthquake’s strength based on seismic waves and movement along faults.
Seismograph – device that records ground movements caused by seismic waves

25. Measuring Earthquakes
- Earthquakes and Seismic Waves
Measuring Earthquakes
Moment magnitude scale – rates earthquakes by estimating the total energy released by an earthquake.
Determined by studying data from seismographs
Each one-point increase in magnitude represents the release of roughly 32 times more energy.

26. Measuring Earthquakes
- Earthquakes and Seismic Waves
Measuring Earthquakes
Magnitude below 3 = Scarcely noticed
Between 3-5 = little damage
Between 5-6 = Moderate damage
Above 6 = Great damage
Largest Earthquakes ever to be recorded had a magnitude measuring above 9
Chile 1960
Alaska 1964

27. It’s Your Turn!!
Create an Earthquake!!

28. - Earthquakes and Seismic Waves
Seismic Wave Speeds
Seismographs at five observation stations recorded the arrival times of the P and S waves produced by an earthquake. These data are shown in the graph.

29. Seismic Wave Speeds Reading Graphs:
- Earthquakes and Seismic Waves
Seismic Wave Speeds
Reading Graphs:
What variable is shown on the x-axis of the graph? The y-axis?
X-axis––distance from the epicenter; y-axis––arrival time.

30. Seismic Wave Speeds Reading Graphs:
- Earthquakes and Seismic Waves
Seismic Wave Speeds
Reading Graphs:
How long did it take the S waves to travel 2,000 km?
7 minutes

31. Seismic Wave Speeds Estimating:
- Earthquakes and Seismic Waves
Seismic Wave Speeds
Estimating:
How long did it take the P waves to travel 2,000 km?
4 minutes

32. Seismic Wave Speeds Calculating:
- Earthquakes and Seismic Waves
Seismic Wave Speeds
Calculating:
What is the difference in the arrival times of the P waves and the S waves at 2,000 km? At 4,000 km?
2,000 = 3.5 minutes
4,000 = 4.5 minutes

33. Locating the Epicenter
- Earthquakes and Seismic Waves
Locating the Epicenter
Geologists use seismic waves to locate an earthquake’s epicenter.

34. Locating the Epicenter
Seismic waves travel at different speeds.
P waves travel faster than S waves
To tell how far the epicenter is from the seismograph, scientists measure the difference between the arrival times of the P waves and S waves.
The longer the time difference, the further the epicenter is from the seismograph station.

35. Locating the Epicenter
For scientists to figure out the location of the epicenter they need P and S wave information from at least three different locations
Once the distance is calculated for each station a circle is drawn on a map (with a radius = the distance calculated) around each of the three seismograph stations
The place where the three circles intercept should be the earthquake’s epicenter

37. Seismic Waves in the Earth
- Earthquakes and Seismic Waves
Seismic Waves in the Earth
Click the Video button to watch a movie about seismic waves in the earth.

38. The Modern Seismograph
- Monitoring Earthquakes
The Modern Seismograph
Seismic waves cause the seismograph’s drum to vibrate. But the suspended weight with the pen attached moves very little. Therefore, the pen stays in place and records the drum’s vibrations.

39. Reading a Seismogram
Seismogram – record of an earthquake’s seismic waves produced by a seismograph.
The height of the jagged lines are greater for a more severe earthquake.

40. Instruments That Monitor Faults
- Monitoring Earthquakes
Instruments That Monitor Faults
Geologists have developed instruments to measure:
changes in elevation, tilting of the land surface, and ground movements along faults.

41. Instruments That Monitor Faults
Tiltmeter – measures tilting or raising of the ground
Creep meter – uses a wire stretched across a fault to measure horizontal movement of the ground
Laser-ranging device – uses a laser beam to detect horizontal fault movements
GPS – (Global Positioning System) measure tiny movements of markers set up on the opposite sides of a fault.

42. Using Seismograph Data
Seismographs and fault-monitoring devices provide data used to map faults and detect changes along faults.
Geologists are also trying to use these data to develop a method of predicting earthquakes.

43. Monitoring Changes Along Faults
How rocks move along a fault depends on how much friction there is between the sides of the fault
Friction – force that opposes the motion of one surface as it moves across another surface.
Ex: San Andreas Fault, California

44. Monitoring Changes Along Faults
- Monitoring Earthquakes
Monitoring Changes Along Faults
The map shows the probability of a strong earthquake along the San Andreas fault. A high percent probability means that a quake is more likely to occur.

45. Links on Earthquake Measurement
- Monitoring Earthquakes
Links on Earthquake Measurement
Click the SciLinks button for links on earthquake measurement.

46. - Earthquake Safety
Earthquake Risk
Geologists can determine earthquake risk by locating where faults are active and where past earthquakes have occurred.

47. How Earthquakes Cause Damage
Causes of earthquake damage include:
shaking, liquefaction, aftershocks, and tsunamis.
Shaking - triggers landslides and avalanches
Destroys buildings and bridges, topples utility poles, and fractures gas and water mains
Liquefaction – violent movements suddenly turn loose soil into liquid mud.

48. How Earthquakes Cause Damage
Aftershock – earthquake that occurs after a larger earthquake in the same area.
Can sometimes cause the most damage
Tsunami – large wave produced by an earthquake on the ocean floor.
Ex: December 26, 2004 – earthquake on the seafloor of the Indian Ocean caused a tsunami to hit the shores of Indonesia killing 200,000 people

49. How Earthquakes Cause Damage
- Earthquake Safety
How Earthquakes Cause Damage
Tsunamis spread out from an earthquake's epicenter and speeds across the ocean.
The waves are amplified as they approach the shore.

50. Steps to Earthquake Safety
The best way to protect yourself during an earthquake is to drop, cover, and hold.
Before an earthquake occurs be prepared
Keep a supply of canned food, bottled water, flashlights, batteries and a portable radio

51. Designing Safer Buildings
- Earthquake Safety
Designing Safer Buildings
To reduce earthquake damage, new buildings must be made stronger and more flexible.

52. Designing Safer Buildings
Base-isolated building – buildings mounted on bearings designed to absorb the energy of an earthquake.
rests on shock-absorbing rubber pads or springs
Flexible joints can be installed in gas and water lines to keep them from breaking.

53. Click the Video button to watch a movie about earthquake damage.
- Earthquake Safety
Earthquake Damage
Click the Video button to watch a movie about earthquake damage.

54. More on Earthquake Risk
- Earthquake Safety
More on Earthquake Risk
Click the PHSchool.com button for an activity about earthquake risk.