1. Earthquakes Chapter 6

2. Elastic Rebound Theory Rocks on each side of the fault are moving slowly If the fault is “locked”, stress increases Rocks fracture and spring back to their original shape Rocks along the fault release energy as seismic waves

4. Focus: area on the fault where the slippage first occurs. Epicenter: point on the earth’s surface directly above the focus

5. Focus Depths Shallow-Focus Earthquakes –Within 70 km of the surface –Causes the most damage Intermediate Focus Earthquakes –Within 70-300 km of the surface Deep-Focus Earthquakes –From 300-650 km of the surface –Occur in subduction zones –Occur farther inland

6. 3 Major Earthquake Zones

7. Pacific Ring of Fire

8. Mid-Ocean Ridges

9. Eurasian-Melanesian Mountain Belt

10. Fault Zones: –G–Groups of interconnected faults –F–Form at plate boundaries because of the intense stress that results San Andreas Fault

12. 3 Types of Seismic Waves

13. P waves Primary waves –Fastest –First recorded by a seismograph –Travel through solids and liquids –Faster through more rigid material –Compression waves Cause rock particles to move together and apart along the direction of the waves

14. P-Wave movement

15. S waves Secondary Waves –Second waves recorded by a seismograph –Only travel through solids –Can’t be detected on the side of the earth opposite the earthquake’s epicenter. –Shear waves Cause rock particles to move at right angles to the direction in which the waves are traveling

17. S-Wave movement

18. Surface Waves Slowest moving wave Last to be recorded by seismograph Travel similar to ocean waves Cause the surface to rise, fall and turn Very destructive: especially in loose earth

19. Surface Waves movements

20. Finding the Epicenter of an Earthquake

21. Analyze difference in arrival times between P waves and S waves. Need info from 3 seismograph stations at different locations

22. Measuring Earthquakes Measuring Earthquakes also called a Seismic Event Three standard measures of the "size" of the earthquake are commonly used: Richter Magnitude Scale (based on energy released as measured by seismometers) less than 3.5 Moment Magnitude Scale (based on total energy released by an EQ or seismic event.) Modified Mercalli Intensity Scale (based on damage caused as assessed by people).

23. Magnitude Measure of the energy released by an earthquakeMeasure of the energy released by an earthquake The amount of ground motionThe amount of ground motion Higher the magnitude the larger the seismic event.Higher the magnitude the larger the seismic event.

24. Moment Magnitude Scale Introduced 1979 to replace Richter Scale. Each step is an increase of 10 x 10… –A 4 is 10000 times a 3 –A 3 is 1000 times a 2 –A 2 is 100X a 1.

25. Moment Magnitude Scale One advantage of this scale is that it does not saturate at the upper end. –For example: with Richter Scale a major EQ lasting 1 minute could be a 9, while the same EQ lasting 5 min is still a 9, in fact still a 9 if it lasted 10 minutes. So, moment magnitude is most often used to estimate large earthquake magnitudes. The USGS uses Richter for EQs smaller than 3.5.USGS

26. Modified Mercalli Scale Modified Mercalli (MM) Scale based on 1909 model of volcano damage. Lower steps on the MM scale deals how the EQ is felt by people. Higher steps are based on observed structural damage.

27. Modified Mercalli Scale I. People do not feel any Earth movement. II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers. VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.

28. Modified Mercalli Scale VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change. IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

29. Magnitude Scales Richter Scale Richter scale no.No. of earthquakes per year Typical effects of this magnitude < 3.4800 000Detected only by seismometers 3.5 - 4.230 000Just about noticeable indoors 4.3 - 4.84 800Most people notice them, windows rattle. 4.9 - 5.41400 Everyone notices them, dishes may break, open doors swing. 5.5 - 6.1500 Slight damage to buildings, plaster cracks, bricks fall. 6.2 6.9100 Much damage to buildings: chimneys fall, houses move on foundations. 7.0 - 7.315 Serious damage: bridges twist, walls fracture, buildings may collapse. 7.4 - 7.94Great damage, most buildings collapse. > 8.0One every 5 to 10 years Total damage, surface waves seen, objects thrown in the air.

30. Destruction to Buildings and Property 1. Most buildings are not designed to withstand shaking from an EQ. 2. Type of ground beneath affects the way buildings respond to EQ. 3. Buildings on loose soil and rock, much more likely to be damaged. 4. Buildings on firm soil or bedrock are much more likely to survive.

31. Tsunamis Form in one of two ways 1. Faulting, causes a sudden drop or rise in ocean floor, displaces a large volume of sea water. This volume of water churns up and down as it adjusts to the change. This sets into motion a series of low, long waves that develop into tsunamis.

32. Tsunamis 2. Underwater landslide causes tons of debris to drop down to sea floor, which causes water to be displaced, then this sets into motion a series of low, long waves that develop into tsunamis.

33. Earthquake Warnings and Predictions Smiley face denotes success of research. 1. Animal research continues to determine if they can tell us something.  2. Using past EQ dates scientists approximate future dates.  3. Sensors placed at faults help to measure strain on rocks.  4. SEISMIC GAPS_ a place where fault is locked & does not move lets strain build up surrounding it. These are believed to be future EQ 5. Slight tilting of ground before EQ. 6. Magnetic & electrical properties of rocks change 7. Increase in natural gas seepage from rocks 8. Decrease in P wave speed, longer the decrease stronger the EQ