Earthquakes and Volcanoes

1. Earthquakes and Volcanoes Chapter 8

2. Earthquakes Chapter 8 Section 1

3. What causes earthquakes? • When you bend a stick, you notice that is changes shape while you bend it • The stick will spring back if you stop applying force. • But if you don’t stop bending the stick • It changes permanently. • If its elastic limit is passed, the stick may break • As it breaks you can feel vibrations in the stick.

4. What is elastic rebound? • Rocks are like other solid materials • If enough force pulls or pushes on them, they will change shape. • They may even break • After breaking, the ends of the broken pieces may snap back. • This snapping back is called elastic rebound.

5. Elastic Rebound • Inside Earth, pushing and pulling forces cause rocks to change shape slowly over time. • As they are strained, potential energy builds up in them. • This energy is released suddenly when the rocks finally break or move.

6. Elastic Rebound • The breaking and the movement that follows causes vibrations that move through rock. • If they are strong enough, the vibrations are felts as earthquakes • An earthquake is a movement of the ground that occurs when rocks inside Earth pass their elastic limit, break suddenly, and experience elastic rebound.

7. What is a fault? • When part of a rock breaks, rocks on either side move as a result of elastic rebound. • The surface where rocks break and move is called a fault. • Rocks can break in different ways, depending on the forces that cause the break.

8. Three types of faults. 1. Normal Fault 2. Reverse Fault 3. Strike-Slip Fault

9. Normal Fault • Normal faults form where tension forces pull rocks apart • The rock above the fault moves down.

10. Reverse Fault • Reverse faults are caused by compression • Rocks pushed together or compressed • When the two rocks push together, rock above the fault is pushed up.

11. Strike-Slip Fault • Sections of rock move past one another in opposite directions along Earth’s surface. • Also called shearing. • Strike-slip faults are caused by shear forces.

12. Making Waves • Earthquakes release energy causing vibrations • When this energy is released, it moves away from the fault in the form on seismic waves. • The point deep inside the Earth where energy is released causing an earthquake is a focus

13. Making Waves • Some of the energy from the earthquake travels straight up to Earth’s surface where it can be felt. • The epicenter is the point on Earth’s surface directly above the earthquake focus.

14. How do seismic waves travel? • When seismic waves leave the focus of an earthquake, some travel through Earth’s interior, and other travel along the surface. • Three Types of Waves • Primary Waves • Secondary Waves • Surface Waves

15. Primary Waves • Seismic waves that travel fastest through rock material are primary waves or P-waves. • Primary waves cause the material to move from side to side, in the same direction that the wave is moving.

16. Secondary Waves • Other seismic waves that travel through Earth’s interior are called secondary waves. • Secondary waves, or S-waves, do not move as fast as P-waves. • As they move through rock material, they cause the material to vibrate at right angles to the direction of the wave.

17. Surface Waves • Seismic waves that travel along Earth’s surface are called surface waves. • They are the largest and slowest type of seismic wave. • They cause more damage than other types of waves. • Surface Waves move in different ways. • They may move rock and soil in a backward rolling motion. • Like waves of water • Some shake or sway the rock and soil from side to side.

18. How are earthquakes measured? • Scientists who study earthquakes are called seismologist • They use instruments called seismographs to record seismic waves. • One type of seismograph has a drum that holds a roll of paper on a frame

19. How are earthquakes measured? • When seismic waves reach the station, the drum vibrates. • The pen on the pendulum traces a record of the vibration

20. How are earthquakes measured? • The height of the lines traced on the paper measures the magnitude of the earthquake. • Magnitude is the measure of energy released by an earthquake.

21. How is an epicenter located? • The epicenter of an earthquake is the point on the surface of Earth directly above the focus • Far away from the epicenter, the P-waves and S-waves arrive at different times. • But close to the epicenter, the waves arrive at almost the same time.

22. Epicenter • Once scientists know the P-wave and S-wave arrival times for at least three seismograph stations, they can figure out the location of an earthquakes epicenter. • They draw circles on a map. • Each circle shows the distance from the seismograph station to the earthquake. • The point where three or more circles intersect is the location of the epicenter.

23. Epicenter

24. How strong are earthquakes? • Some earthquakes are not felt on the surface of Earth. • People do not even know these small earthquakes are happening. • Larger earthquakes, on the other hand, can cause major damage.

25. What is the Richter scale? • Richter magnitude is based on the measurements of heights of seismic waves as they are recorded on seismographs. • Scientists use this information to determine the Richter magnitude of an earthquake. • Richter magnitude describes how much energy an earthquake releases.

26. Richter Scale • Very weak earthquakes have low magnitudes like 1.0 • Strong earthquakes have high magnitudes in the range of 6 to 7 • For every increase of 1.0 on the Richter scale, an earthquake actually releases 32 times more energy. • This means that an earthquake with a magnitude of 7.5 releases 32 times more energy than an earthquake of 6.5

27. How is earthquake intensity measured? • Another way to measure earthquakes is by the modified Mercalli intensity scale. • This scale measures the intensity of an earthquake. • Intensity is a measure of the amount of damage to structures and to rocks and soil in a specific area. • The amount of damage depends on • How strong the earthquake is • Kinds of structures in an area • Distance from epicenter • Nature of the surface material

28. Mercalli Scale • The Mercalli scale uses Roman numerals I through XII • An earthquake with an intensity of I would be felt by few people • An intensity – VI earthquake would be felt by everyone • An intensity – XII would cause major damage to Earth’s surface and to human-built structures.

29. What are tsunamis? • When an earthquake occurs on the ocean floor, powerful waves are produced • These waves travel outward from the earthquake in all directions. • A powerful seismic sea wave is called a tsunami

30. Tsunamis • Tsunamis traveling in open ocean water are low and fast moving. • But tsunamis change as they approach land. • The speed of the tsunami slows and the height of the wave increases. • Huge tsunami waves can be up to 30 meters high.

31. Giant Sea Waves "In 1992 a mild earthquake, barely noticed, hit San Juan del Sur in Nicaragua. Minutes later the peaceful harbor was drained dry as if someone had pulled a giant bath plug and let the water out. Amazed at the sight, curious people flocked to the harbor to look. As they stared, a giant tsunami rushed in and swept people and buildings far out to sea. This three-part illustration is an example of how the water is drained in a harbor, then builds up height before rushing back to the shore." - Dr. Eldridge M. Moores