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Earthquakes
Chapter 16
Outline
Introduction to Earthquakes and Earth’s Interior
Why Should You Study Earthquakes?
What is the Elastic Rebound Theory?
What is Seismology?
Where Do Earthquakes Occur and How Often?
What Are Seismic Waves?
How Is an Earthquake’s Epicenter Located?
How are the Size and Strength of an Earthquake
Measured?
What Are Destructive Effects of Earthquakes?
Can Earthquakes Be Predicted? Controlled?
What Is Earth’s Interior Like?
Earth’s Internal Heat
Objectives
Energy is stored in rocks & released when they
fracture, producing various types of waves that
travel outward in all directions from their source.
Most earthquakes (EQs) take place in well-defined
zones at transform, divergent, and convergent
plate boundaries.
An EQ’s epicenter is found by analyzing EQ waves
at, at least 3 seismic stations.
The amount of damage and people’s reactions to
an EQ are used to determine an EQ’s intensity on
the Modified Mercalli Intensity Scale.
The Richter Magnitude Scale is used to express
the amount of energy released during an EQ.
Great hazards are associated with EQs such as
ground shaking, fire, tsunami and ground failure.
Introduction
Earthquake  the shaking or trembling of part
of Earth’s surface caused by a sudden release
of energy, usually by slippage of rocks along a
fracture (i.e., faulting).
Aftershocks  continuing adjustments along
the fracture may generate a series of generally
smaller quakes (occur after an EQ)
Earthquakes are an indication that Earth is an
internally active planet.
Some Significant Earthquakes
YEAR
1556
1755
LOCATION
China (Shanxi Province)
Portugal (Lisbon)
MAGNITUDE
8.0
8.6
DEATHS
1,000,000
70,000
1811-1812
1886
1906
USA (New Madrid, Missouri)
USA (Charleston, SC)
USA (San Francisco, CA)
7.5
7.0
8.3
20
60
700
1923
1964
1970
1976
1985
1988
1989
1990
1993
1994
1995
1997
1998
1999
Japan (Tokyo)
USA (Alaska)
China (Yunnan Province)
China (Tangshan)
Mexico (Mexico City)
Armenia
USA (Loma Prieta, CA)
Iran
India
USA (Northridge, CA)
Japan (Kobe)
Iran
Afghanistan
Izmit, Turkey
8.3
8.6
7.7
8.0
8.1
7.0
7.1
7.3
6.4
6.7
7.2
7.3
6.1
7.4
143,000
131
15,621
242,000
9,500
25,000
63
40,000
30,000
61
5,000 +
2,400 +
5,000 +
17,000
Why Should You Study Earthquakes?
Earthquakes:



are destructive (take many lives & cause injuries
each year).
affect national economies (EQs can disrupt the
socio-economic fabric of countries, which has
political ramifications of local, regional, & global
scale.)
affect individuals (basic knowledge of risk &
hazard types assoc. with EQs is valuable)
What Is the Elastic Rebound Theory?
Elastic rebound theory explains how energy is
released during earthquakes.
A.) On Earth’s surface, any straight line like a road or a fence
crossing a fault would be gradually deformed
B.) or bent as rocks on one side of the fault move relative to
those on the other side.
A
C
B
D
C
D
C.) When the strength of rock is exceeded,
movement occurs along the fault & energy is
released, causing an earthquake.
D.) After energy is released, the rocks rebound or
“snap back” to their original undeformed shape.
How do we measure earthquakes?
Epicenter  the point on Earth’s surface directly
above the focus.
Energy released by
movement along a fault
travels as seismic waves
outward in a concentric
pattern from the place of
movement.
Focus  point w/in Earth where fracturing first begins
(i.e., the point where energy is first released).
What Is Seismology?
Seismology is the study of earthquakes.
The passage of these waves through Earth
materials is detected, recorded, & measured
by seismographs (A).
B
A
The record made
is a seismogram (B).
3 Types of Earthquakes
(based on focus-depth):
1. shallow-focus  depths <70km
2. intermediate-focus  depths 70 to 300 km
3. deep-focus  depths >300 km
90% of all EQs  foci depths <100 km
Only 3% are deep-focus.
Nearly (95%) all EQs occur in seismic belts along
plate boundaries, but no place on Earth is immune.
Where Do Earthquakes Occur?
Majority (80%) occur in the circum-Pacific belt.
(Along this belt, most EQs are assoc. with convergent plate boundaries
but also occur along divergent & transform boundaries.)
15% of EQs occur along the Mediterranean-Asiatic belt.
(along a predominantly convergent zone)
5% occur in plate interiors & along divergent margins.
(Intraplate EQs take place along ancient faults commonly assoc. with
buried plate margins.)
How Often Do Earthquakes Occur?
~150,000 EQs strong enough to be felt are recorded
each year.
Another 900,000 are recorded, but are too small to be
cataloged individually.
What Are Seismic Waves?
Oakland, Calf.,1989
Seismic waves  produced as energy is released by
movement along a fault.
Seismic waves produce vibrations that cause EQs.
Categories of Seismic Waves
Body waves  seismic waves that travel through
the solid body of Earth (much like sound waves).
Two types of Body Waves:
P-waves (primary waves)
S-waves (secondary waves)
Surface waves  travel along the ground surface
(are similar to waves on water).
Two types of Surface Waves:
Rayleigh waves (vertical motion)
Love waves (horizontal motion)
Body Waves
P-waves:
-- are the fastest seismic waves.
-- travel in solids, liquids, and gases
-- are compressional (b)
-- cause material to vibrate back-&-forth in the
direction of travel.
S-waves:
-- are slower than P-waves
-- travel only in solids
-- move material perpendicular to the direction of travel
Travel velocity of body waves is proportional
to the elasticity of the Earth material.
-
Surface Waves (esp. damaging to building foundations)
1. Rayleigh waves: (like water waves)
-- move material in an elliptical path (a)
-- are slower than Love waves
2. Love waves:
-- motion is similar to S-waves (back & forth motion)
except movement is horizontal (b)
How does one locate an EQ’s epicenter?
based on the difference in the arrival time at
seismograph stations of the first P- & S-waves
(P-S time interval)
P-waves arrive first, followed in order by S-waves,
Love waves, & Rayleigh waves.
How Is an EQ’s Epicenter Located?
Seismologists:
-- estimate ave. speeds of
P- & S-waves from data
-- determine P- & S-wave
travel times for distances
b/w focus & seismograph
-- produce Time-distance
graphs (P-S time interval
vs. travel distance) that
are used to locate
epicenters
How do you locate the epicenter of an EQ?
Must know: “P-S time interval” for 3 seismograph stations
Use each P-S time interval & determine travel distance
from the time-distance graph.
A circle with a radius
equal to the travel
distance is drawn for
each of the three
seismograph stations.
The intersection of the
3 circles is the location
of the epicenter.
How Are the Size & Strength
of an EQ Measured?
Magnitude  the amount of energy released by an
EQ at its source.
It’s determined based on the amplitude of the largest
seismic wave recorded for a given EQ.
*Magnitude is unchanging!! (It doesn’t vary at each recording station.)
Intensity  subjective measure of the damage done
by an EQ, as well as what people felt
*Intensity can VARY with each recording station.
How Are the Size & Strength
of an EQ Measured?
The Richter Magnitude Scale (ranges from 1 to 9) 
quantitative measure of EQ magnitude
An increase of one unit on the Richter Scale (5 to 6) is
equivalent to a 10-fold increase in amplitude of the
largest seismic wave produced.
In terms of energy released, a one unit increase on the
Richter Scale equals a 30-fold increase in energy
released at the focus.
(It would take ~30 EQs of 5.5 magnitude to release as much energy as
one 6.5 EQ.)
Average Number of Earthquakes of Various
Richter Magnitudes per Year Worldwide
MAGNITUDE
EFFECTS
<2.5
Typically not felt but
recorded
2.5 -6.0
Usually felt; minor to
moderate damage to
structures
6.1-6.9
Potentially destructive,
especially in populated
areas
7.0-7.9
Major earthquakes;
serious damage results
>8.0
Great earthquakes;
usually result in
total destruction
AVERAGE NUMBER
PER YEAR
900,000
31,000
100
20
1 every 5 years
How Are the Size & Strength
of an EQ Measured?
Modified Mercalli Scale (ranges from 1 to 12) 
assesses earthquake intensity (approximates size & strength)
For an earthquake of given Richter magnitude,
Intensity WILL VARY with distance from epicenter,
local geology, construction practices, etc.
II Felt only by a few people at rest, especially on upper floors of buildings.
VI Felt by all, many frightened and run outdoors. Some heavy furniture moved,
a few instances of fallen plaster or damaged chimneys. Damage slight.
IX Damage considerable in specially designed structures. Buildings shifted off
foundations. Ground noticeably cracked. Underground pipes broken.
What are the Destructive Effects of EQs?
Destructive effects of earthquakes include:
ground shaking
fire
tsunami
landslides
panic
disruption of vital services
psychological shock
Numbers of deaths & injuries depend on several factors:
magnitude
duration of shaking
local geology
population density
distance from epicenter construction practices
disaster response planning
The time at which an earthquake occurs affects its
destructiveness. EQs during working hours in urban areas
are most destructive & cause most fatalities & injuries.
Ground Shaking
Effects of ground shaking cause more injuries than any
other earthquake hazard.
Along with magnitude & distance to epicenter, Earth
materials (bedrock vs. sediment) strongly influence the
amplitude & duration of seismic waves.
Amplitude & duration of S-waves is greater in poorly
consolidated or water-saturated material than in bedrock.
Structures built on
bedrock suffer less ground
shaking than those built
on poorly consolidated or
water-saturated material.
Liquefaction  process in which the material on
which buildings are constructed behaves as a fluid
(esp. prevalent in fill & water-saturated sediment.
Niigata,
Japan, 1964
Ground Shaking
Building material & type of construction can affect
the amount of damage done by ground shaking.
Easily destroyed structures
Adobe & mud-walled buildings
Unreinforced brick & poorly built concrete
structures
Fire
Severed electrical & gas lines pose a great hazard.
1902  90% of damage in the San Francisco EQ
1923  75% of all homes destroyed (mostly of wood)
in Tokyo & almost all homes in Yokohama, Japan.
1989  (Loma Prieta EQ) Large fire in the marina
district of San
Francisco
San Francisco, CA, 1989
Killer Waves -- Tsunamis
Tsunami (tidal wave)  an ocean wave produced by an
earthquake.
Height of tsunamis
Open ocean  < 1 m high
Shallow water of coastal areas  30 m or more
Tsunamis travel at several 100 km/hr & can devastate lowlying coastal areas 1000s of miles from their source.
Following the tsunami
Hilo, Hawaii, 1946
that struck Hilo, HI
(in 1946), the U.S.
developed an early
warning system to
predict the arrival of
tsunami to coastal
areas of the Pacific
Ocean.
Ground Failure that has caused deaths:
EQ-triggered landslides in mountainous areas
(1959 Montana EQ fault scarp  large rock slide dammed the Madison R.)
Collapse of cliffs of wind-laden silt
(1920 Ganso, China EQ  100,000 killed).
EQ-induced avalanche (1970 Peru EQ  66,000 killed).
Montana, 1959
Can Earthquakes Be Predicted?
Successful prediction includes:
location
strength
time frame for occurrence
Seismic risk maps (constructed by historic records
& distribution of known faults) are used to assess the
likelihood & potential severity of future EQs
This guides EQ preparedness planning.
Earthquake Precursors
Precursors  short- & long-term changes w/in
Earth in advance of EQs
Precursors include (short-term):
tilting of the surface (as rocks deform due to inc. pressure)
fluctuations in the water levels in wells
changes in locations & frequency of small EQs
increased number of foreshocks, etc.
Long-term prediction: delineation of seismic gaps
Seismic gaps  locked areas along fault zones
Locked areas aren’t releasing energy.
Continued accumulation of pressure could lead to
major EQs in the future.
Earthquake Prediction Programs
United States, Russia, Japan, & China
are the only nations that have govt.-funded
EQ prediction programs.
EQ prediction is progressing, but
unsuccessful short-term predictions may
lead to skepticism & disregard
(like with hurricane, tornado, & tsunami warnings).
What You Can Do to Prepare For an EQ?
1. Be familiar with geologic hazards of the area where you live & work.
2. Make sure your house is bolted securely to the foundation &
that the walls, floors, & roof are all firmly connected together.
3. Heavy furniture such as bookcases should be bolted to the
walls; semi-flexible natural gas lines should be used so that
they can give without breaking; water heaters & furnaces
should be strapped & the straps bolted to wall studs to prevent
gas-line rupture & fire. Brick chimneys should have a bracket
or brace anchored to the roof.
4. Maintain a several-day supply of freshwater & canned foods.
Keep a fresh supply of flashlight & radio batteries & a fire
extinguisher.
5. Maintain a basic first-aid kit, & have a working knowledge of
first-aid procedures.
6. Learn how to turn off the various utilities at your house.
7. Have a planned course of action for when an EQ strikes.
What is Earth’s Interior Like?
Earth is concentrically layered (crust, mantle, core).
Each layer differs in composition & density and
they’re separated by distinct boundaries.
The behavior & travel times of P- & S-waves
provide info about Earth’s internal structure.
Rock density & elasticity increase with depth in the
Earth as do travel velocities of P- & S-waves.
Layers of differing density & elasticity cause seismic
waves to be bent like light is when it passes from air to
water. (refraction)
Part of the energy of P- & S-waves is reflected at
density-elasticity boundaries (like light from a mirror).
C
Depths to boundaries are
calculated by knowing:
--wave velocities
--time interval it takes for
waves to travel from the
source to density-elasticity
boundaries & back to the
surface
Seismic wave velocity
continuously changes
with depth, but the
changes are abrupt at
strong density-elasticity
boundaries.
Three discontinuities:
crust-mantle,
mantle-outer core,
outer core-inner core
Crust
Crust  the Earth’s thin skin
Mohorovicic discontinuity (Moho)  the crustmantle boundary (20-90 km deep beneath continents
& 5-10 km deep beneath oceanic crust).
Continental crust
Oceanic crust
Average density of 2.65 gm/cm3
P-wave velocity of 6.75 km/sec
Average density of 3.0 gm/cm3
P-wave velocity of 7 km/sec
Mantle
Mantle  lies b/w the Moho & the discontinuity at
2900km (marks an abrupt velocity decrease).
Low-velocity zone (depth of 100-250 km)
Here, decreased seismic wave velocities derive
from the decreased elasticity of the asthenosphere.
Composition of the mantle: peridotite
(olivine & pyroxene)
Volume of Earth: ~80%
The Core
The 2900 km discontinuity marks the boundary b/w the
mantle & outer core.
Outer core (molten)  composed of Fe & S
Inner core (solid)  composed of Fe & Ni
P-wave velocity drops rapidly at mantle-outer core boundary.
P-waves are
refracted so that
little P-wave
energy passes
thru Earth to
reach the surface
between 103o &
143o of an EQ
focus. This is the
P-wave shadow
zone.
The Core
At the mantle-outer core boundary, S-waves are blocked
& not transmitted into the outer core. The outer core
behaves as a liquid b/c liquids do NOT transmit S-waves.
The S-wave shadow
zone is larger than
the P-wave shadow
zone. It includes all
areas on the
surface greater than
l03o from the EQ
focus.
Seismic Tomography
Seismic tomograghy  a CAT scan of Earth using
seismic waves instead of x-rays
3-D areas of unusually slow or fast wave travel
can be detected.
Travel speeds in the mantle:
faster travel correspond to cold areas
(inactive continental interiors)
slower travel marks hot areas
(divergent margins & other volcanic areas)
Distribution of hot & cold areas marks the location
of mantle convection cells.
Earth’s Internal Heat
Most of Earth’s internal heat derives from
radioactive decay of U, Th & K40.
Earth’s temperature increases with depth.
(geothermal gradient)
Geothermal gradient decreases:


2.5 oC / km near the surface
1oC / km in the mantle