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Earthquakes
Chapter 14 or Chapter 11
I. Earthquake
A. Earthquake - the vibration of Earth produced
by the rapid release of energy
• fault
• focus
• Epicenter
• Surface rupture zone
I. Earthquake
B. Where earthquakes occur?
• Fault – fracture in a rock block along which
both sides of the rock move in opposite
directions.
• Sometimes fault creep slowly,
• Sometimes movement happens fast during an
EQ.
Ex. San Andreas Fault
I. Earthquake
Figure 14.5
C. How?
Elastic rebound –
1. Rocks on both sides of fault are deformed by tectonic
forces
2. Rocks bend and store elastic energy
3. Frictional resistance holding the rocks together is
overcome AND…..
4. KAPOW! EARTHQUAKE
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II. Seismology
A. Seismology - The study of earthquake waves
1. Seismographs - instruments that record seismic
waves
• Records the movement of Earth on a rotating
drum of paper
• Paper records are seismograms
Figure 14.8
Figure 14.7
II. Seismology
B. seismic waves – energy waves emanating
from the focus.
▪ Types:
1. Surface waves –
▪
▪
▪
Travel on surface,
cause most destruction,
slowest waves.
II. Seismology
2. Body waves
▪ Travel inside Earth
▪ Two types:
a. Primary (P) waves
• Compressional waves
• Fastest wave so it arrives at seismograph
first.
b. Secondary (S) waves
 Back and forth motion
 Slower than P-waves but faster than surface waves.
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Types of seismic waves
Summary – after an earthquake P-waves to
seismograph first, S-waves arrive second, and surface
waves arrive last.
Figure 14.9
Figure 14.10
II. Seismology
C. How locate the epicenter?
– use the difference in arrival times of P and S
waves. More time between P and S waves if
seismograph is farther away.
1.
2.
3.
4.
The epicenter is located using three or
more seismograph
Need 3 seismograph stations
Each station determines the arrival time difference between
the two waves.
A travel-time graph is used to determine each station’s
distance to the epicenter
Draw circles around each seismograph – where all three
intersect = epicenter.
III. Location of earthquakes
A. Where are earthquakes found?
1. On active faults.
2. Most active faults and EQ’s are on active place
boundaries
3. This occurs along earthquake belts =
a. Circum-Pacific belt,
b. Mediterranean Sea region to the Himalayan complex,
c. the mid-oceanic ridge system
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Earthquakes of magnitude 5 or greater over a 10 year period
III. Location of earthquakes
B. Earthquake depths
• Shallow (0-70 km)
• Intermediate (70-300 km)
• Deep (> 300 km)
• Only shallow EQ’s at divergent boundaries and
transform boundaries.
• Shallow, interm., and deep EQ’s at convergent
boundaries.
IV. Earthquake Size and Destruction
A. Magnitude – the amount of energy released at the
earthquake focus.
▪ Richter scale (developed by Charles Richter 1935)
▪ Measure of the amplitude of the largest seismic wave
recorded and is adjusted for distance from focus.
▪ Range from 0 to no maximum. However, largest
magnitude recorded = 9.5 in Chile 1960, second largest 9.2
Alaska 1964.
▪ Magnitudes less than 2.0 are not felt by humans
▪ Each unit of Richter magnitude increase equals a tenfold
increase in wave amplitude and a 32-fold energy increase
IV. Earthquake Size and Destruction
B. Largest Earthquake Magnitudes since 1900 –
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Chile, 1960, 9.5
Alaska, 1964 9.2
Sumatra Indonesia, 2004, 9.1
Tohoku, Japan, 2011, 9.0
Kamchatka, Russia, 1952, 9.0
Chile, 2010, 8.8
Ecuador, 1906, 8.8
Alaska, 1965, 8.7
Indonesia, 2005, 8.6
Tibet (China), 1950, 8.6
IV. Earthquake Size and Destruction
IV. Earthquake Size and Destruction
C. Amount of structural damage due to
surface shaking depends on:
D. Liquefaction of the ground
– Loose sediments saturated with water turn into a
mobile fluid during shaking.
1. How close to focus – closer to epicenter of
shallower EQ = more damage. Almost same
– Makes structures collapse, and sand volcanoes.
amount of shaking in about 20 mile radius.
2. Type of sediments or bedrock under structure
– bedrock less shaking; loose sediment more
shaking; saturated loose sediment/artificial
fill = most shaking.
3. Design of the structure
• Animation of Liquefaction
• YouTube – Japan Tsunami Liquefaction (3 min)
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IV. Earthquake Size and Destruction
E. Tsunamis, or seismic sea waves
▪ inappropriately called “tidal waves”
▪ Result from vertical movement along a fault on
the ocean floor during an EQ.
▪ In open ocean wave height is very small, but
▪ At shallower shore water piles up to heights
that occasionally exceed 100 feet!!!
▪ Notable tsunamis = 2004 Indonesian tsunami, how many people
killed ?
▪ 230,000 people
IV. Earthquake Size and Destruction
– 1964 Alaska EQ – tsunami in Crescent City, CA
F. Landslides – can be triggered by EQ.
V. California and the San Andreas Fault
A. San Andreas Zone location
1. Separates Pacific Plate from North American
Plate and runs from Mendocino Triple Junction
in north to Mexico.
2. Made of San Andreas + many other related
faults.
3. But most have right lateral movement.
i.
Mendocino Triple Junction
Pacific Plate is movnig north and NA plate is moving
south.
Figure 14.24
Big Bend
Salton Trough
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V. California and the San Andreas Fault
B. Movement of crustal blocks on sides of fault
▪ Some parts move very slowly and gradually = fault
creep
▪ Other parts regularly slip making earthquakes
▪ other parts stay locked until big release, like elastic
rebound theory. These “great earthquakes” occur
about every 50 to 200 years.
▪ How old is it?
▪ ~ 800 miles long, movement averages about 2
inches per year. (Do the calculations!)
V. California and the San Andreas Fault
Loma Prieta Earthquake
1989, Epicenter in
Santa Cruz Mountains
C. Major CA Earthquakes
1. 1868 - Hayward Earthquake – 6.8 magnitude on
Hayward fault – epicenter in Hayward
2. 1906 – Great San Francisco Earthquake and Fire – 7.8
magnitude on San Andreas Fault - epicenter just south
of San Francisco. 700 – 2,800 deaths
3. 1989 – Loma Prieta Earthquake – 6.9, on San Andreas,
epicenter at Loma Prieta in Santa Cruz Mountains. 63
deaths.
4. 1994 – Northridge Earthquake – 6.7, epicenter NW of
Los Angeles near Northridge /San Fernando
Valley/Berbank. 60 deaths
Two hot spots of
shaking!!
1) Loma Prieta
2) Fill in Oakland
3) Similar fill in flats of
Oakland, Berkeley,
and Richmond.
V. California and the San Andreas Fault
D. “The Big One” – assessing earthquake risk in Bay
Area.
1. US Geological Survey – Percent probability of a 7.6
magnitude earthquake erupting on this particular fault
by 2032.
a. Hayward Fault – makes Oakland Hills, 27%, “most
dangerous fault in United States”. Large EQ every 140
yrs – last one 142 yrs ago in 1868. YO, ITS DUE!!!!!
b. Calaveras Fault - makes Diablo Valley, 11%
c. San Andreas Fault – 21%
d. San Gregorio Fault – near Half-Moon Bay, 10%
2. Total probability for entire Bay Area – 62%
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