1. Earthquakes
Chapter 8

2. Forces Within Earth Stress & Strain
Most earthquakes are the result of movement of Earth’s crust produced by plate tectonics
Tectonic plates tend to move gradually
Over time, stress builds up
Stress: the total force acting on crustal rocks per unit area
Movement occurs when stress overcomes strength
Strain: the deformation of material in response to stress

3. Types of Stress Compression Tension Shear - Stress towards each other
- Stress in opposite directions (object becomes long and skinny)
Shear
- Top and bottom move in different directions
- Causes a material to twist

4. Earthquakes
Shaking of the ground caused by the sudden release of energy stored in the rocks beneath Earth’s surface
Rocks can be twisted, squeezed and stretched. They fracture when stress and strain reach a critical point.

5. Why do earthquakes occur?
Elastic rebound theory:
Sudden release of progressively stored strain in rocks causes movement along faults
Stage 1: Stress acts on a rock
Stage 2: Stress causes strain in rock
Stage 3: Rock breaks suddenly, releasing energy, and rock moves along fault
*Similar to the process of pulling a rubber band and it snapping

6. Faults
Fault: any fracture or system of fractures along which Earth moves
A. Dip-Slip Faults
Movement up or down parallel to the fault plane
Hanging wall and footwall
B. Strike-slip faults
Movement is horizontal and parallel to the fault surface

7. A. Dip-slip Fault Hanging Wall: sits above footwall
Footwall: below the fault line

8. A. Dip-Slip Faults 1. Normal Fault 2. Reverse Fault
Hanging wall moves down
Tensional Stress
2. Reverse Fault
Hanging wall moves up
Compressional stress

9. B. Strike-Slip Faults
1. Right Lateral 2. Left Lateral * Use movement of feature to determine left vs. right lateral

10. Describing Earthquakes
Location
Focus- center of movement of the earthquakes (occurs underground)
Epicenter- point on Earth’s surface directly above the focus (can be a city)

11. Describing Earthquakes
Seismic Waves: waves of energy
A. Body waves- travel through Earth’s interior
Move through the rocks in the subsurface
B. Surface waves- travel along Earth’s surface

12. A. Body Waves 1. P waves: compressional waves
Motion parallel to direction
P stands for primary – first to arrive (because they travel the fastest)
Speed: 4-7 km/sec
Travel through solids or liquids

13. A. Body Waves 2. S waves: shearing waves
Motion transverse to direction
S stands for secondary – arrive after P waves
Speed: 2-5 km/sec
Travel through solids only

14. B. Surface Waves Surface waves: travel on the surface
Cause LOTS of damage
Love waves: side to side
2. Rayleigh waves: up and down (similar to the way waves move on water)

15. Measuring Earthquakes
Seismometer: recording device that produces a record of earthquake motion
The first seismic wave detected by a seismometer is a P wave
Seismogram: output of the seismometer
Can calculate strength and location of earthquake
Now digitized

16. Seismogram

17. Locating Earthquakes
Each station calculates a distance radius to earthquake
3 stations are required for an unique epicenter solution
Epicenter Location: where all three distances overlap

18. Locating Earthquakes
Depth of focus beneath Earth’s surface can also be determined
Shallow focus: 0-70 km
Intermediate focus: km
Deep focus: km

19. Earthquake Distribution
Along Plate Boundaries
Divergent: Shallow, low magnitude
Convergent (Subduction Zones): Shallow to Deep focus, low to high magnitude
Transform: Shallow, along faults, low - medium magnitude
Most earthquakes are shallow focus (<70km)
The deepest earthquakes are associated with subduction convergent plate boundaries (Caused from friction on the plate as its subducted)

21. Describing “size” of Earthquake
Mercalli Index:
Earthquake intensity measured by damage (close to epicenter, the intensity is higher)
Values I to XII
Varies with distance from quake and type of geologic material underlying buildings

22. Mercalli Index

23. Describing “size” of an Earthquake
Richter Scale:
Measure of earthquake magnitude = energy releases
How much energy came away from the site of the earthquake
Height of “wiggles” on seismogram
Values 1 to 10: Logarithmic scale
Logarithmic scale- when you move up one value, the energy increase by 10x

24. Earthquake Prediction
Earthquakes cannot be predicted to an exact time or place unless the earthquake has already occurred
They MAY be predicted in a time frame and general area
Based on change in rock properties
Small tremors
Water pressure
Surface elevation or tilt

25. Earthquake Prediction
Based on patterns of earthquake in space and time
Seismic gaps: a segment of an active fault known to produce significant earthquakes that has not slipped in an unusually long time
Calculate average time between earthquake and estimate probability of another occurring
Historical record analysis

26. Effects of Earthquakes
Ground motion
Displacement of land surface
Breaks windows, cracks walls
Topples buildings
Most injuries or fatalities are from falling debris
Building design
Building location

27. Effects of Earthquakes
Fire – broken gas and water mains
Landslides – triggered by ground shaking
Liquefaction – water saturated soil turns from solid to liquid when shaken

28. Tsunami Tsunami: A wave of ocean water generated by an earthquake
Seismic sea waves
Caused by sudden upward or downward movement of the sea floor
Can also be caused by underwater landslides or volcanic eruptions
Generally produced by magnitude 8+ earthquake
Wavelength: 160 km
Speed: up to 700 km/hr (430 mi/hr)
Can cross the entire Pacific Ocean in less than a day
Waves: ft high

29. In the deep ocean, tsunami waves may appear only a foot or so high
As they approach shoreline and enter shallower water, they slow down and begin to grow in energy and height
A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors
A warning sign of a tsunami

31. 26 December 2004
Earthquake in the Indian Ocean off the coast of Sumatra, Indonesia
Magnitude 9.3
Waves up to 30 meters (100 ft) high
Over 230,000 causalities
One of the deadliest natural disasters in recorded history
Earthquake was caused when the Indian Plate was subducted by the Burma Plate