1. What is an earthquake?
An earthquake is the vibration of Earth produced by the rapid release of energy
Energy radiates in all directions from its source, the focus
Energy moves like waves
Seismographs record the event
Slinky, Rubber Band SEISMOGRAM
Beaker, Wet Sand, Weight
Cardboard Fault models
Chewing Gum
Wood meter stick or plastic ruler
pencil

2. Anatomy of Earthquakes
Earthquakes are associated with faults

3. Earthquakes are caused by sudden release of accumulated strain energy along Faults
Rocks on sides of fault are deformed by tectonic forces
Rocks bend and store elastic energy
Frictional resistance holding the rocks together is overcome by tectonic forces
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Hands Demo

4. Earthquake mechanism
Slip starts at the weakest point (the focus)
Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound)
The motion moves neighboring rocks
And so on.
DEMO – elastic rebound w/ ruler

5. Relationship Between Stress and Strain
Demo: Rubber Band
Strain can be a change in shape (a deformation) due to an applied stress

6. Relationship Between Stress and Strain at low Temps and Pressure or Sudden Stress
Demo: Pencil

7. Relationship Between Stress and Strain under High Temps or Pressure
Demo: gum

8. Strike and Dip
Strike is long line, dip is short line
Note the angle of dip given 45o
Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface

9. Vertical Movement along Dip-Slip Faults
Divergent
Convergent

10. Horizontal Movement Along Strike-Slip Fault

11. Normal Fault Quake - Nevada Reverse Fault Quake - Japan
DEMO – Types of faults
Strike Slip Fault Quake - California

12. Fence offset by the 1906 San Francisco earthquake
San Andreas is the most studied transform fault system in the world
discrete segments 100 to 200 kilometers long
slip every years producing large earthquakes
Some portions exhibit slow, gradual displacement known as fault creep

14. Fires caused by 1906 San Francisco Earthquake
Gas mains break, fires shaken out of furnaces. Water mains break, cannot fight fires. Debris in streets, Fire department cannot reach fires.

15. Landscape Shifting, Wallace Creek
San Andreas Fault, a Transform Margin

16. Liquefaction
Demo: Liquifaction

17. Seismometers - instruments that record seismic waves
Seismology
Seismometers - instruments that record seismic waves
Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape

18. A seismograph designed to record vertical ground motion
The heavy mass doesn’t move much
The drum moves

19. Lateral Movement Detector
In reality, copper wire coils move around magnets, generating current which is recorded.

20. Seismic Waves 1: Surface waves
Complex motion, great destruction
High amplitude and low velocity
Longest periods (interval between crests)
Termed long, or L waves

21. Types of seismic waves (continued)
Body waves
Travel through Earth’s interior
Two types based on mode of travel
Primary (P) waves
Push-pull motion
Travel thru solids, liquids & gases
Secondary (S) waves
Moves at right angles to their direction of travel
Travels only through solids

22. P and S waves
Demo: P and S waves
Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L

23. Earthquake focus and epicenter

24. Note how much bigger the surface waves are

25. Graph to find distance to epicenter

26. Locating Earthquake Epicenter

27. Epicenter located using three seismographs

28. 95% of energy released by earthquakes originates in narrow zones that wind around the Earth These zones mark of edges of tectonic plates
Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates

29. Earthquake Depth and Plate Tectonic Setting
Subduction Zones discovered by Benioff

30. Earthquake in subduction zones

31. Earthquakes at Divergent Boundaries - Iceland
Crust pulling apart – normal faults

32. Measuring the size of earthquakes
Two measurements describe the size of an earthquake
Intensity – a measure of earthquake shaking at a given location based on amount of damage
Magnitude – estimates the amount of energy released by the earthquake

33. Intensity scales
Modified Mercalli Intensity Scale was developed using California buildings as its standard
Drawback is that destruction may not be true measure of earthquakes actual severity

34. Magnitude scales
Richter magnitude - concept introduced by Charles Richter in 1935
Richter scale
Based on amplitude of largest seismic wave recorded
LOG10 SCALE
Each unit of Richter magnitude corresponds to 10X increase in wave amplitude and 32X increase in Energy

35. Magnitude scales
Moment magnitude was developed because Richter magnitude does not closely estimate the size of very large earthquakes
Derived from the amount of displacement that occurs along a fault and the area of the fault that slips

36. Tsunamis, or seismic sea waves
Destructive waves called “tidal waves”
Result from “push” of underwater fault or undersea landslide
In open ocean height is > 1 meter
In shallow coast water wave can be > 30 meters
Very destructive

37. Formation of a tsunami
Tsunamis are actually huge, extending from the fault on the sea floor up to the surface, but they don’t stick up more than a meter or so in the deep ocean. However, when they reach shallow water they must rear up and slow down. Discussion: Kinetic vs. potential energy

38. Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere

39. Tsunami 1960, Hilo Hawaii

40. Tsunami Model, Alaska Quake

41. Earthquake prediction
Long-range forecasts
Calculates probability of a certain magnitude earthquake occurring over a given time period
Short-range predictions
Ongoing research, presently not much success

42. Long Term Predictions
Seismic Gaps

43. Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE

44. Seismic Gap along Himalayas
2005

45. Dilatancy of Highly Stressed Rocks
Short-Term Earthquake Prediction
Dilatancy of Highly Stressed Rocks

46. Investigating Earth’s Interior
Seismology helps us understand Earth’s Interior Structure. We use:
Speed changes in different materials
due changes rigidity, density, elasticity
Reflections from layers with different properties
Attenuation of Shear Waves in fluids
Direction changes (Refraction)

47. Investigating Earth’s Interior

48. Surface Components magnified!

49. Seismic-wave velocities are faster in the upper mantle
Velocity increases w depth, waves bend back to surface.
Waves that travel via mantle arrive sooner at far destinations
Mohorovičić discontinuity

50. Wave Velocities
Upper Mantle Fast
Asthenosphere
Slow
Lower Mantle Fast

51. The S-Wave Shadow Zone
Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a larger shadow for S waves covering everything past 103 degrees away from the source.

52. The P-Wave Shadow Zone
P-waves through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole
HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid.
Inge Lehmann
Behavior of waves through center reveal Earth’s Interior