CSULA - Geology: Faults & Earthquakes

Faults | Earthquakes | Seismology | Earthquake hazards | Links

Faults
- What are Earthquakes?
  - An earthquake is the vibration of Earth produced by the rapid release of energy Earthquakes most often occur along existing
  - faults whenever the frictional forces on the fault surfaces are overcome
- Faults are fractures in the crust along which appreciable displacement has occurred.
  - No movement = a joint
  - Sudden movements along faults = earthquakes
- Types of Faults
  - Dip-Slip
    - Normal
    - Reverse
    - Thrust
  - Strike-Slip
    - Right lateral
    - Left lateral
    - Transform
  - Dip Slip Faults
    - Movement is primarily vertical and parallel to the fault plane
    - May produce long, low cliffs called fault scarps
    - Parts
      - Hanging wall (rock surface above the fault)
      - Footwall (rock surface below the fault)
    - Normal Faults
      - Hanging wall block moves down relative to the footwall block
      - Accommodate lengthening or extension of the crust
      - Most are small with displacements of a meter or so
      - Larger scale normal faults are associated with structures called fault-block mountains
    - Reverse faults
      - Hanging wall block moves up relative to the footwall block
        
        Reverse faults have dips greater than 45°
        
        Thrust faults have dips less then 45°
      - Accommodate shortening of the crust (compression)
  - Strike-Slip Faults
    - Dominant displacement is horizontal and parallel to the strike of the fault
    - Right-lateral
    - Left-lateral
    - Transform fault
- Recognition of faults
  - Earthquakes
  - Geological observation
    - Offset of the ground surface
    - Linear fault scarps
    - Sag ponds, offset drainages
    - Fracture with different rocks on opposite sides
    - Zones of crushed rocks
  - Get professional help . . .the geological kind

Earthquakes

Earthquake Locations

Most earthquakes are generated at plate boundaries
Major earthquake zones include:
- the Circum-Pacific belt (i.e., subduction zones)
- Mediterranean Sea region to the Himalayan complex
- and the oceanic ridge system
- ~95% of the energy released by earthquakes originates in these regions!
A few are intraplate earthquakes
- New Madrid, Missouri, quakes of 1811-12
- Charleston, South Carolina, quake of 1886
Focus depths
- Depths range from 5 to nearly 700 km
- Arbitrarily classified as:
  - shallow (surface to 70 km),
  - intermediate (70 - 300 km),
  - and deep (+300 km)
Measuring Earthquakes
- Two measurements that describe the size of an earthquake:
  - Intensity – a measure of the degree of earthquake shaking at a given locale based on the amount of damage
  - Magnitude – estimates the amount of energy released at the source of the earthquake
- Intensity Scales
  - Modified Mercalli Intensity Scale
    - Developed using California buildings as its standard
    - Rates from I (not felt) to XII (total damage)
    - The destruction caused may not be a true measure of the earthquakes actual severity
  - Magnitude Scales
    - Richter magnitude - concept introduced by Charles Richter in 1935
      - Based on the amplitude of the largest seismic wave recorded
      - Accounts for the decrease in wave amplitude with increased distance
      - Used primarily for local/nearby earthquakes
      - ML
    - Moment magnitude
      - Developed to more adequately estimate the size of very large earthquakes
      - Derived from the amount of displacement that occurs along a fault
      - Often used by seismologists
      - MW

Largest Earthquakes in the WorldSince 1900 (http://wwwneic.cr.usgs.gov/neis/eqlists/10maps_world.html)

Location	Date	Magnitude
Chile	1960	9.5
Prince William Sound Alaska	1964	9.2
Andreanof Islands	1957	9.1
Kamchatka	1952	9.0
Sumatra	2004	9.0
Off the Coast of Ecuador	1906	8.8
Rat Islands	1965	8.7
India-China Border Region	1950	8.6
Kamchatka	1923	8.5
Banda Sea	1938	8.5
Kuril Islands	1963	8.5

Historic U.S. Earthquakes (+8 M) (http://wwwneic.cr.usgs.gov/neis/eqlists/10maps_usa.html)

Location	State	Date	Magnitude
Prince William Sound	Alaska	1964	9.2
Andreanof Islands	Alaska	1957	9.1
Rat Islands	Alaska	1965	8.7
East of Shumagin Islands	Alaska	1938	8.2
New Madrid	Missouri	1811	8.1
Yakutat Bay	Alaska	1899	8
Andreanof Islands	Alaska	1986	8
New Madrid	Missouri	1812	�8
Near Cape Yakataga	Alaska	1899	7.9
Fort Tejon	California	1857	7.9
Ka'u District	Hawaii	1868	7.9
Gulf of Alaska	Alaska	1987	7.9
Andreanof Islands	Alaska	1996	7.9
Denali Fault	Alaska	2002	7.9
New Madrid	Missouri	1812	7.8
Imperial Valley	California	1892	7.8
San Francisco	California	1906	7.8
Gulf of Alaska	Alaska	1988	7.8

Historic California Earthquakes (+7 M)

LOCATION	DATE	MAG
Great 1906 earthquake	1906	8.25
Great Fort Tejon earthquake	1857	8.25
Kern County earthquake	1952	7.70
Owens Valley	1872	7.60
Landers	1992	7.30
SW of Lompoc	1927	7.30
W. of Eureka	1922	7.30
Cape Mendocino	1992	7.20
W. of Eureka	1980	7.20
Cape Mendocino	1923	7.20
Hector Mine	1999	7.10
W. of Crescent City	1991	7.10
Loma Prieta	1989	7.10
Imperial Valley	1940	7.10
Colorado R. delta	1934	7.00
W. of Eureka	1899	7.00
Hayward fault	1868	7.00
San Francisco Peninsula	1838	7.00
Wrightwood	1812	7.00
Santa Barbara Channel	1812	7.00
Northridge	1994	6.70
San Fernando	1971	6.50
Whittier Narrows	1987	5.80

What Happens During an EQ?
- 1. Tectonic stress
- 2. Crustal rocks deform
- 3. Elastic energy stored
- 4. Energy released (seismic waves) = earthquake

Seismology
- Seismology is the study of earthquakes and the structure of the earth by both naturally and artificially generated seismic waves.
- Two main types of seismic waves
  - Surface waves
  - Body waves
- Seismology - Surface Waves
  - A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion
  - A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling
- Seismology – Body Waves
  - Primary (P) waves
    - Push-pull motion, changing the volume of the intervening material
    - Travel through solids, liquids, and gases
    - P waves travel about 1.7 x faster than S waves
  - Secondary (S) waves
    - Move at right angles to their direction of travel
    - Travel only through solids
    - Slower velocity than P waves
    - Slightly greater amplitude than P waves
- Earthquake recording instrument (seismograph)
  - Records movement of Earth
  - Record is called a seismogram
- Locating the Epicenter
- 3 seismograms stations
  - Each station determines S-P interval
  - A travel-time graph is used to determine each station’s distance to the epicenter
Earthquake hazards
- Earthquake hazards
  - Ground shaking
  - Surface Faulting
    - Displacement that reaches the earth's surface during slip along a fault.
    - Commonly occurs with shallow earthquakes, (epicenter less than 20 km.)
  - Fires
  - Tsunami

EQ Hazards – Ground Shaking
- Severity of ground shaking depends on:
  - Total energy released
  - Distance from the source
  - Acceleration
  - Nature of the ground material
  - Total time of shaking

Factors that affect the strength of ground shaking:
- the softness of the surface rocks and
- the thickness of surface sediments.
Results:
- Liquefaction
- Landslides
- Ground Subsidence
- Building Collapse
  - Factors that determine structural damage
    - Intensity of the earthquake
    - Duration of the vibrations
    - Nature of the material upon which the structure rests
    - The design of the structure

EQ Hazards – Surface Faulting
- Displacement that reaches the earth's surface during slip along a fault.
- Commonly occurs with shallow earthquakes, (epicenter less than 20 km.)
EQ Hazards – Tsunami, or seismic sea waves

Are NOT “tidal waves” !
Caused major displacement of the ocean by:
- Earthquakes
- Volcanic Eruptions
- Submarine landslides
QuickTime animation

LINKS

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