1. Earthquakes

2. 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

3. Deformation
Deformation is a general term that refers to all changes in the original form and/or size of a rock body
Most crustal deformation occurs along plate margins
Deformation involves
Stress—Force applied to a given area

4. Deformation How rocks deform
General characteristics of rock deformation
Elastic deformation—The rock returns to nearly its original size and shape when the stress is removed
Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation)

5. Folds
During crustal deformation rocks are often bent into a series of wave-like undulations called folds
Characteristics of folds
Most folds result from compressional stresses which shorten and thicken the crust

6. Folds Common types of folds Anticline—Upfolded or arched rock layers
Syncline—Downfolds or troughs of rock layers
Depending on their orientation, anticlines and synclines can be described as
Symmetrical, asymmetrical, or recumbent (an overturned fold)

7. Anticlines and Synclines
Figure 6.20

8. Faults
Faults are fractures in rocks along which appreciable displacement has taken place
Sudden movements along faults are the cause of most earthquakes
Classified by their relative movement which can be
Horizontal, vertical, or oblique

9. Normal Fault
Figure 6.24 A

10. Reverse Fault
Figure 6.24 B

11. Strike-Slip fault
Figure 6.24 D

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

13. Elastic rebound Mechanism for EQ’s explained by H. Reid
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

14. Elastic rebound
Earthquake mechanism
Slips starts at the weakest point (the focus) occurs
Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound)

16. Elastic Rebound Movie in Mapview

17. Reid elastic rebound cartoon

18. Aftershocks The change in stress that follows
a mainshock creates smaller earthquakes called aftershocks
The aftershocks
“illuminate” the
that ruptured in the mainshock
Red dots show location of
aftershocks formed by 3
earthquakes in Missouri
and Tennessee in 1811/1812

19. Normal Fault Quake - Nevada
Strike Slip Fault Quake - Japan
Strike Slip Fault Quake - California

21. San Andreas: An active earthquake zone
San Andreas is the most studied fault system in the world
Displacement occurs along discrete segments 100 to 200 kilometers long
Most segments slip every years producing large earthquakes
Some portions exhibit slow, gradual displacement known as fault creep

22. Fence offset by the 1906 San Francisco earthquake

26. 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

27. A seismograph designed to record vertical ground motion

28. Motion of a seismograph

29. 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

30. Types of seismic waves 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

32. P-Wave Motion

33. S-Wave Motion

34. 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

35. Earthquake focus and epicenter

36. Locating the epicenter of an earthquake
Three seismographs 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

37. Graph used to find distance to epicenter

39. 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

40. Epicenter located using three seismographs

41. 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

42. Locations of earthquakes from 1980 to 1990

43. Depths of Earthquakes
Earthquakes originate at depths ranging from 5 to nearly 700 kilometers
Definite patterns exist
Shallow focus occur along oceanic ridges
Deep earthquakes occur in western Pacific east of oceanic trenches

44. Earthquake in subduction zones

45. 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

46. 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

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

50. Magnitudes 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

51. 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

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

53. Formation of a tsunami

55. Tsunami 1960, Hilo Hawaii

56. Tsunami Model, Japan Earthquake

57. Tsunami Model, Alaska Quake

58. Can earthquakes be predicted
Short-range predictions
Goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame
Research has concentrated on monitoring possible precursors – phenomena that precede a forthcoming earthquake such as measuring uplift, subsidence, and strain in the rocks

59. Earthquakes cannot be predicted
Short-range predictions
Currently, no method exists for making short-range earthquake predictions
Long-range forecasts
Calculates probability of a certain magnitude earthquake occurring over a given time period