Faults &
Earthquakes
Faults
| Earthquakes | Seismology
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- 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
- 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 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
- 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
Faults
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| Earthquake hazards | top
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