Geology
EARTHQUAKE LECTURE
Name_______________
Standards
• Describe the geological manifestations of plate tectonics, such as earthquakes
• Describe the impact of plate motions on societies and the environment
• Describe how waves are used for practical purposes (e.g., seismic data)
• Examine investigations of current interest in science
I. Major Earthquakes in History
The following are just a few of many notable earthquakes through history
A. 1811: New _______________ Missouri
1. Magnitude 7.5
2. Large areas sank into the earth
3. New lakes were formed
4. The Mississippi River changed its course and even flowed backward
5. Sand blows (geysers) occurred, can still see remnants today
B. _________: San Francisco
1. Felt from southern Oregon to south of Los Angeles and inland to central Nevada
2. Estimated magnitude of 7.8
3. >3000 killed
4. Massive fires
C. 1985: _________________ City
1. Magnitude 8.1
2. Epicenter 350 km away off Pacific coast
3. Shaking lasted 3 – 4 minutes
4. Collapse of poorly constructed buildings
5. Liquefaction of soils under city
6. ~10,000 killed
D. 1960: Valdivia, ______________
1. Largest earthquake ever recorded
2. Magnitude 9.5
3. Caused tsunamis in many parts of the Pacific, including Hilo, Hawaii
4. 1,655 people killed
E. 2004: Sumatra EQ and Indian Ocean ________________
1. Magnitude 9.2
2. Rupture continued for 9 minutes & moved 1300 km along a thrust fault – the
longest single fault break ever recorded
3. Resulted in tsunamis that killed 300,000 on Sumatra, Sri Lanka, Thailand, the
Maldive Islands and Somalia
F. 2008: Eastern Sichuan, _____________
1. Magnitude 7.9
2. Schools and hospitals collapsed
3. ~70,000 killed
4. ~18,000 missing
1
5. Strong aftershocks and landslides
6. May have been triggered by dam holding 315 million tons of water
G. 2010: _________ Earthquake
1. Magnitude 7.0
2. ~212,000 killed
3. ~ 1 million homeless
4. Major damage to city of Port-au-Prince
H. 2011: ______________ Earthquake and Tsunami
1. Magnitude 9.0 earthquake
2. Rupture along thrust fault at the _________________ zone between the Pacific &
N. American plates
3. Fault moved upwards by 30-40 m and slip occurred over an area 300 km long
4. Foreshocks occurred over 2 days preceding the quake (a M 7.2 and 3 greater than M
6 on same day)
5. Several aftershocks have occurred, many over a M 6
6. Resulting 30 ft ______________ swept through many coastal regions of Japan,
reaching as far as 6 mi inland
7. 13,116 people killed, 14,377 missing
II. What is an Earthquake?
A. We inhabit a fragile __________ environment of houses, ________________ &
transportation systems that is anchored in Earth’s ____________
B. This environment is ___________________ to seismic vibration, ground rupture,
landslides and tsunamis
C. Plate movements generate ______________ at the boundaries that can be described in
terms of stress, strain and strength
1. ____________ – local forces per unit area that cause rocks to deform
2. ____________ – relative amount of deformation caused by the stress
3. Rocks fail – break – when they are stressed beyond a critical value called their
______________
D. Earthquakes are the result of stress that builds up over time, as tectonic forces deform
rocks on either side of a fault
1. They occur when the stress exceeds the strength of the rocks, which
suddenly ___________ along a new or preexisting _____________
2. The two blocks of rock on each side of the fault ________, releasing the stress
suddenly, causing an ____________________, which generates seismic waves
E. Elastic Rebound Theory
1. Faults remain ___________ while strain ____________ accumulates in the
rocks on either side, causing them to deform until a sudden slip along the fault
releases the energy
2. _______________ means the rocks spring back to their undeformed shape when
the fault unlocks
3. The distance of displacement is called the ___________ _________
2
III. Focus and Epicenter
A. ____________ – point at which the slip begins – somewhere _________ the surface
1. Most earthquakes in continental crust have focal depths from 2 – 20 km (rocks
behave in a ductile manner below 20 km)
2. Subduction zone earthquakes can have foci as great as 690 km deep
B. ____________ – the geographic point on Earth’s __________ directly above the focus
IV. Fault Rupture
A. Does not happen all at once
1. Starts at _____________ and expands outward on fault plane at ~2 – 3 km/s
2. Rupture stops when stress can no longer break the rocks
B. __________ of earthquake is related to total _________ of fault rupture
1. Most earthquakes are very small and the rupture never breaks the surface
2. However, in large, destructive earthquakes, surface breaks are common
a. Ex: 1906 San Francisco EQ caused surface displacements averaging
___ m (~12’) along a ______ km section of the San Andreas
3. Faulting in largest Earthquakes can extend more than 1000 km and the slip can
be as large as 20 m (~60’)
C. Stored strain energy is released in the form of frictional ___________ and seismic
___________
V. Foreshocks and Aftershocks
A. __________________ occur as a consequence of a previous EQ of larger magnitude
1. Their foci are distributed in and around the rupture plane of the main shock
2. They can last from weeks to years
3. They can compound damage from the main shock
B. __________________ are small earthquakes that occur near, but before, a main shock
1. Many large earthquakes have been preceded by foreshocks
2. Scientists have tried to use them to predict large earthquakes
3. Hard to distinguish foreshocks from other small earthquakes
VI. _________________ Waves
A. Ground ________________ produced by an earthquake
B. Enable us to _____________ earthquakes and determine type of ________________
that produced them
C. 4 types:
1. Body Waves
a. P waves
b. S waves
2. Surface Waves
a. Rayleigh waves
b. Love waves
3
D. _____________ or ____ Waves
1. Travel through Earth and are ___________ to arrive at seismic station
2. _____________________ waves
3. Can be thought of as push-pull waves: they push or pull particles of matter in
the direction of their travel
E. __________________ or ___ Waves
1. Follow the P waves through Earth, and arrive ______________ at the seismic
station
2. ___________ waves
3. Displace material at right angles to their path of travel
F. ______________ Waves
1. Arrive __________ after traveling around Earth’s surface
2. Speed slightly less than S waves
3. _______________ waves – travel in ____________ motion over surface
4. ________ waves – shake the ground in _______________ motion
H. Locating the Epicenter
1. Time _______________ between P and S wave arrival depends on
________________ waves have __________________ from focus
2. If __________ or more seismic stations know the distance, then the epicenter
can be located using _______________________
VII. Measuring the Size of an Earthquake
A. ________________ of an earthquake is the main factor that determines the
____________ and potential _________________ of an earthquake
B. Two scales:
1. Richter magnitude
2. Moment magnitude
C. _______________ Magnitude
1. Developed by Charles Richter in 1935
2. Each earthquake is assigned a number on a ____________________ scale
3. Two earthquakes ________ by _______ magnitude if the size of their ground
motions differs by a factor of ____
4. This means the ground motion of a magnitude 6 earthquake is 10 times greater
than a magnitude 5 and 100 times greater than a magnitude 4
5. The energy released as seismic waves increases by a factor of 33 for each
Richter unit
D. __________________ Magnitude
1. Seismologists now prefer a measure of EQ size more directly related to the
physical ____________ of _______________ that causes the EQ
2. Moment magnitude is the ________________ of the __________ and the
average ___________ across the fault break
3. It increases by about 1 unit for every 10-fold increase in the area of faulting
4
4. It produces roughly the same numerical values as Richter’s method, but can be
measured from seismograms and determined by field measurements of the fault
E. Earthquake Size and Frequency
1. _______________ earthquakes occur much ________ often than ________ ones
2. Worldwide figures of earthquake size per year:
a. 1,000,000 with magnitudes greater than 2.0
b. 100,000 greater than 3.0
c. 1000 greater than 5.0
d. 10 greater than 7.0
e. Earthquakes with magnitude above 8.0 occur about once every 3 years
f. Very large ones like the 2004 Sumatra quake (magnitude 9.2), 1964
Alaska (9.2) and 1960 Chile (9.5) are rare
F. ______________ Intensity
1. ______________ of shaking depends on ______________ from fault rupture
2. Damage from shaking depends on distance from populated areas
3. Estimated shaking determined with modified _______________ intensity scale
– values from I (not felt) to XII (damage total) (for full scale see page 307 of
textbook)
VIII. Earthquakes and Faulting
A. Most earthquakes occur at plate _______________
1. Largest earthquakes occur at ___________________ boundaries on
_______________ faults that form where one plate __________ beneath another
a. Exs: Sumatra (2004), Alaska (1964) & Chile (1960): largest EQ ever
recorded, magnitude 9.5
B. ______________ Earthquakes
1. A small percentage of earthquakes occur in plate interiors
2. Foci are shallow and occur mostly on continents
3. Many occur on ________ faults that use to be part of plate boundaries and are
now areas of crustal _____________
4. Examples include some of most famous in American history: New Madrid,
Missouri (1811-1812), Charleston, South Carolina (1886), and Cape Ann,
near Boston Massachusetts (1755)
C. Regional Fault Systems
1. ____________ of deformation between plate boundaries usually have a network
of _______________ faults – a fault ___________ – rather than a single fault
a. Ex: in California, the “master fault” is the San Andreas, however, there
are many subsidiary faults on either side that generate large earthquakes.
b. Most of the damaging earthquakes in California during the last century
have occurred on these subsidiary faults
5
IX. Earthquake ____________________
A. Over the last century, earthquakes worldwide have caused an average of 13,000 deaths
per year and hundreds of billions of dollars of damage
B. Two California earthquakes – 1989 _______ __________ (mag 7.1 & $10 billion in
damage) and 1994 ________________ (mag 6.8 & $40 billion in damage) – were
among the _______________ disasters in U.S. history because of nearby urban areas
C. Destructive earthquakes are even more common in ___________ than in California
1. Japan is the best prepared nation to deal with earthquakes, with strong public
education campaigns, building codes and warning systems
2. Despite this, more than 5600 people were killed in a mag 6.9 EQ in Kobe in
1995
3. Casualties and structure failure (50,000 buildings destroyed) occurred because
of less stringent building codes that were in effect when most of the city was
built and the proximity of the rupture to the city
D. How Earthquakes Cause Damage
1. _____________ effects:
a. Faulting (breaks in ground surface)
b. Ground shaking (from seismic waves)
2. _____________ effects:
a. Landslides
b. Tsunamis
c. Fires
E. _____________ and ______________
1. Ground ________________ can subside or uplift during faulting
2. Ground __________________ near the epicenter can ________________ the
acceleration of ______________, so objects lying on the surface can be
____________ into the air
3. Seismic _________ can ___________ structures so hard that they
____________, which is the leading cause of casualties and economic damage
4. Examples:
a. Tangshan, China 1976: >240,000 killed
b. Spitak, Armenia 1988: 25,000 killed
c. Izmit, Turkey 1999: 15,600 killed
d. Etc…
F. Landslides and Other Ground Failures
1. _________________ can bury towns
a. Ex: debris flow in China’s Kansu Province, 1920, covered >100 km2,
200,000 killed
2. ___________ saturated soils can behave like a liquid – called _____________ –
and flow away, taking buildings, bridges, etc along with it
a. Ex: cause of massive building collapse in Mexico City EQ: Mexico City
built on unstable soils of ancient lakebed
6
G. ______________
1. Destructive _________ wave triggered by earthquake beneath the ocean
2. NOT called tidal wave – this term is incorrect, has nothing to do with tides
3. Deadliest and most destructive hazard associated with largest earthquakes –
_______________ quakes that occur in _______________ zones
a. Megathrust ruptures can push the seafloor upward by as much as 10 m,
displacing the overlying ocean water
b. Resulting wave travels at speeds of up to _____ km/hr, as fast as a jetliner
c. They are hardly noticeable in deep ocean, but waves ________ down and
________ up when they reach ___________ coastal waters
d. Resulting wave can be ________ of meters tall
4. Most common in __________ Ocean, why?
a. ________ of _________ – subduction zones ring the Pacific
b. Examples:
i. 1964 Alaska EQ caused tsunamis that hit thousands of kilometers
from epicenter. At one location, near Valdez, AK, the tsunami ran
up a mountainside to a height of 67 m (that’s ______ feet)!
ii. 2004 _____________ Ocean EQ caused tsunamis that killed
__________ people in several countries
H. _____________
1. Are ignited by ruptured ________ lines or downed ____________ power lines
2. Damage to ____________ mains can making fighting them impossible, as
happened in the 1906 San Francisco EQ
X. Reducing Earthquake Risk
A. Seismic _____________ – describes the ______________ of seismic shaking and
ground disruption that can be expected
B. Seismic _______ – describes the ______________ that can be expected for a specific
region
1. Risk depends on the seismic hazard, ________________, and number of
____________ structures
2. ____________________ leads the nation in seismic risk at ____% of the
national total, with Los Angeles county accounting for ____%
3. But 46 million people are at risk outside of California, including: Hilo,
Honolulu, Anchorage, Seattle, Tacoma, Portland, Salt Lake City, Reno, Las
Vegas, _____________________, Charleston, Memphis, Atlanta, ST. Louis,
New York, Boston & Philadelphia
C. Land Use Policies
1. Exposure of built structures to earthquake risk can be reduced by policies that
________________ land use
2. It is unwise to erect buildings on known active faults, as was done in residential
areas of San Francisco.
3. California law now restricts construction across active faults.
7
4. Real estate agents are required to disclose information about houses built on a
fault
D. Earthquake Engineering
1. Risk from seismic shaking can be reduced by good ________________ and
_________________
2. Building codes specify the _____________ a structure must be able to withstand
from a seismic hazard
3. U.S. building codes have been largely successful in preventing loss of life
during earthquakes
a. Ex: from 1983 to 2004, 131 people died in nine severe earthquakes in the
western U.S., whereas >460,000 people were killed by earthquakes
worldwide
E. ________________ Systems
1. When an earthquake occurs, automated ______________ systems can send
warnings tens of seconds before the arrival of destructive seismic waves
2. tsunamis travel 10 times slower than seismic waves, so distant shorelines can be
given up to _________ of warning time
a. Unfortunately, no system had been installed in the Indian Ocean during
the 2004 quake
XI. Can Earthquakes be Predicted?
A. Prediction means specifying ___________, __________________ and ________
B. Information from plate _______________ and geologic _______________ of fault
systems can allow geologists to ________________ which faults are likely to produce
earthquakes over the ___________ term
C. To specify precisely when a particular fault will rupture is very difficult
D. ___________-Term ___________________
1. The longer the time since the last big EQ, the sooner the next one will be
2. _________________ interval – the average time between large earthquakes.
a. Determined by _________ rate – how long it takes for a fault to build up
enough strain that rock strength is exceeded
E. ___________-Term ________________
1. There have been a few successful short-term predictions
a. Ex: in 1975, an EQ was predicted only hours before occurring near
Haicheng, China
b. Seismologists used precursors of swarms of tiny earthquakes to make
prediction
c. The next year, however, an unpredicted quake struck the Chinese city of
Tangshan, killing more than 240,000
2. No reliable method of short-term prediction has been found

L. Brown
Updated April 2013
8