1. Earthquakes Last time we considered faults.
Today we discuss the consequence of a fault’s movement
Slinky
Beaker, Wet Sand, Weight
Ball Point Pen
A seismogram

2. What is an earthquake?
An earthquake is the vibration of Earth produced by the rapid release of energy
Rock subjected to elastic deformation snaps back and/or breaks
Energy radiates in all directions from the break’s location, AKA source, the “focus”
Energy moves like waves as moving rocks push on their neighbors
Seismographs record the event

3. Anatomy of Earthquakes

4. Earthquakes and faults
Earthquakes are associated with faults
Motion along faults can be explained by plate tectonics

5. Causes of earthquakes Sudden release of accumulated strain energy
Creation of fracture zones at faults by rupturing rocks
Creation of new faults by rupturing rocks
Shifting of rocks at preexisting faults
Deformed rock straightens back to original shape, but offset from matching beds on the other side of the fault.

6. Elastic rebound 1 Mechanism for Earthquakes
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

7. Elastic rebound 2
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.

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9. Aftershocks The change in stress that follows
a main shock creates smaller earthquakes called aftershocks
The aftershocks
“illuminate” the fault
that ruptured in the main
shock
Red dots show location of
aftershocks formed by 3
earthquakes in Missouri
and Tennessee in 1811/1812
Symbols: Reelfoot Fault is a Reverse Fault, teeth point to the upthrown side, so to the hanging wall. Cottonwood Grove a Transform Fault, note the arrows pointing in the directions of movement

10. Normal Fault Quake - Nevada Reverse Fault Quake - Japan
Divergent
HW Down
Convergent
HW Up
Transform
Strike Slip Fault Quake - California

11. San Andreas: An active earthquake zone
San Andreas is the most studied strike-slip (transform) fault system in the world
Displacement occurs along discrete segments 100 to 200 kilometers long
Most segments slip every years producing large earthquakes
However, some portions exhibit slow, gradual displacement known as fault creep. The rocks have low strength minerals that cannot store strain, they just crumble and smear continuously as the plates move.

12. Fence offset by the 1906 San Francisco earthquake
San Andres Fault

13. Landscape Shifting, Wallace Creek
San Andres Fault

14. San Andreas Fault

15. Earthquake Hazards Four major hazards occur during earthquakes
One is well known: the collapse of buildings crushes people
Three more are less well known
Fire
Liquifaction
Tsunami

16. Fires caused by 1906 San Francisco Earthquake
Gas mains break, fires shaken out of furnaces and fireplaces. Water mains break.
Debris blocks streets. Fire Fighters cannot drive to the fire.

17. Liquefaction
Crystals from dredge muds are arranged like pick-up-sticks.
Pressure waves from the earthquake force the crystals apart. Now your house is being supported by water.
Makes “quick clay”

18. Before we consider Tsunami
We need some background in seismology

19. Seismometers - instruments that record seismic waves
Seismology
Seismometers - instruments that record seismic waves
Formerly: Recorded the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
Today: use motion sensors similar to those in your smart phone

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

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

22. Types of seismic waves Surface waves Complex motion, great destruction
High amplitude and low velocity
Longest periods (interval between crests)
Termed long, or L waves

23. Two Types of Surface Waves
Most of the destruction
Larger amplitude than body waves

24. 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

25. P and S waves
Smaller amplitude than surface (L) waves, but faster, P arrives first

26. Locating the source of earthquakes
Focus - the place within Earth where earthquake waves originate
Epicenter – location on the surface directly above the focus
Epicenter is located using the difference in velocities of P and S waves

27. Earthquake focus and epicenter

28. Delay between P and S arrivals gives distance to epicenter
Note how much bigger the surface waves (aka L waves) are.
Body Waves
P to S delay

29. Graph to find distance to epicenter
Average S wave speed is 3800 km in 12 minutes
Average P wave speed is 3800 km in 7 minutes

30. Locating the epicenter of an earthquake
Three seismographs from different observatories needed to locate an epicenter
Each station determines the time interval between the arrival of the first P wave and the first S wave at their location
A travel-time graph then determines each station’s distance to the epicenter

31. Locating Earthquake Epicenter

32. Locating the epicenter of an earthquake
A circle with radius equal to distance to the epicenter is drawn around each station
The point where all three circles intersect is the earthquake epicenter

33. Epicenter located using three seismographs

34. Earthquake Belts
95% of energy released by earthquakes originates in narrow zones that wind around the Earth
These zones mark of edges of tectonic plates

35. Locations of earthquakes from 1980 to 1990
80% of seismic energy around Pacific Rim
Broad bands are subduction zone earthquakes, narrow are MOR

36. Depths of Earthquakes
Earthquakes originate at depths ranging from 5 to nearly 700 kilometers
Definite patterns exist
Shallow focus occur between mid-ocean ridges
Deep earthquakes occur in Pacific landward of oceanic trenches
Central continent (intraplate) earthquakes are of various causes. Some causes still uncertain.
Devastating earthquakes occur less than 60 kilometers because cold rock is more elastic, and transmits waves better than warmer rocks below

37. Earthquake Depth and Plate Tectonic Setting
Weakest are the divergent zone earthquakes
Strongest Here
Strongest, with worst Tsunamis, at entrance to subduction zones
Subduction Zones discovered by Benioff

38. Earthquakes in subduction zones
Recent example, 9.0 Christmas 2004 Earthquake and Tsunami, Sumatra

39. Earthquakes at Divergent Boundaries - Iceland
A new graben, down dropped hanging wall block - Normal Fault – divergent zone MOR

40. 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 to buildings.
Magnitude – estimates the amount of energy released by the earthquake.

41. 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 energy

42. Earthquake destruction
Amount of structural damage depends on
Intensity and duration of vibrations
Nature of the material upon which the structure rests (hard rock good, soft bad)
Design of the structure

43. 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

45. 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

46. Tsunamis, or seismic sea waves
Incorrectly called “tidal waves”
Result from “push” of underwater fault or undersea landslide
In open ocean wave height is < 1 meter
In shallow coast water wave can be > 30 meters (more than about 98 feet)
Very destructive

47. Formation of a tsunami 3. Deep water
Wave 1 meter above surface, very fast
2. Wave as deep as water
1.Water pushed up
4. In shallows wave rears up,
and slows down.
SNAP

48. The Hawaiian Islands vulnerable
The Hawaiian Islands vulnerable. Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere

49. Tsunami 1960, Hilo Hawaii

50. Tsunami Model, Japan Earthquake

51. Tsunami Model, Alaska Quake

52. 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

53. Long Term Predictions
Seismic Gaps

54. Long Term Predictions
Strain Energy - accumulates uniformly - release irregularly
Some locked by friction “Seismic gaps”
Prime candidates for major earthquake
Some release energy continuously- creep
No major earthquakes there

55. Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE

56. Seismic Gap along Himalayas
USGS web page on October magnitude 7.6 in Pakistan

57. Can earthquakes be predicted? Short Term, Not very well
Short-range predictions
Goal: provide warning location & magnitude
within a narrow time frame
Research on precursors due breaking prior slip. Breaking of rock lets the fault slip
Breaks called fracture zones. The thickness of the fracture zone is proportional to the length of the fault.
Breaking causes volume increase (dilation) and uplift in the rocks.
Dilatancy causes many measurable changes

58. Dilatancy of Highly Stressed Rocks
Short-Term Earthquake Prediction
Dilatancy of Highly Stressed Rocks
BOX OF ROCKS

59. Investigating Earth’s Interior
Earthquakes help 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)

60. Result: 3 Major Layers of Earth

61. Shallow Components of Earth!

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

63. Wave Velocities Upper Mantle Fast Asthenosphere Slow Lower Mantle Fast
Crust slow
Mohorovičić discontinuity
Upper Mantle Fast
Asthenosphere
Slow
Lower Mantle Fast

64. Mineralogy of Earth’s Layers
This slide for graduate students.

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

66. The P-Wave Shadow Zone Discovery of the solid inner core
P-waves pass ing 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 reveals Earth’s Interior

67. End of Earthquakes