EARTHQUAKES
ESSC 100 – Intro to Earth Science
Ancient cultures offered a variety of
explanations for earthquakes activity
(seismicity), most of which involved
the action or mood of a giant animal
or god.
EARTHQUAKES AND Today we know that an earthquake is
the motion or trembling of the ground
EARTH’S INTERIOR produced by several factors,
including:
• sudden displacement of rock in the
Earth's crust;
• volcanic eruptions;
• giant landslides;
• a meteorite impact, or
• underground nuclear-bomb tests.
STRESS AND STRAIN, PLASTIC AND ELASTIC REBOUND
BRITTLE DEFORMATION • Along plate boundaries,
rocks are under stress.
•Stress is the push, pull or shear • Rocks initially deform
that a material feels when it is plastically, but when the
subjected to a force. stress exceeds the
•Strain is the change in shape of strength of rocks, they
a material in response to the break along a fault.
application of a stress. • The accumulated strain is
•Brittle deformation: permanent suddenly released as
deformation in which the rock seismic waves.
fractures or crack, instead of • Rocks are somewhat
flowing or bending. elastic, so they try to snap
•Plastic (ductile) deformation: back after they break.
permanent deformation in which • The fault remains as a
a rock may change its shape by weakness in the rock.
flowing or bending.
See textbook figure 7.4
SAN FRANCISCO, 1906
EARTHQUAKES
General features
• Vibration of Earth produced by a rapid
release of energy
• Associated with movements along faults
• Explained by the plate tectonics theory
• Mechanism for earthquakes was first explained
by H. Reid
• Rocks "spring back" – a phenomena called
elastic rebound
• Vibrations (earthquakes) occur as rock
elastically returns to its original shape
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TERMINOLOGY EARTHQUAKE WAVES
• When an earthquake occurs, the sudden movement of the rock causes
seismic waves to radiate out from the area where the movement occurred Earthquake waves
(the earthquake hypocenter or focus) at a speed of several kilometers
per second. • Types of earthquake waves
• Body waves
• EPICENTER: directly above the
• generated at the focus when an earthquake occurs.
hypocenter, is the location of earthquakes
projected at the surface (in latitude and • travel in the interior of the Earth (3-7 km/sec)
longitude. • P waves - Primary
• S waves - Secondary
• Main Shock: largest and generally First
• Surface waves
earthquake in a sequence.
• Produced when body waves “hit” the surface
• Aftershock: smaller earthquakes after • Complex motion
first main shock. Can last as much as a • Slowest velocity of all waves
month afterward. Can be almost as large
as main shock, generally smaller.
• Study of earthquake waves is called seismology
Decrease in magnitude with time. • Earthquake recording instrument (seismograph)
• Records movement of Earth
• Foreshock: an earthquake that occurs
• Record is called a seismogram
prior to a large one.
PRIMARY (P) WAVES SECONDARY (S) WAVES
• Push-pull (compressional) • "Shake" motion: they propagate
motion: particles are displaced by laterally displacing the
parallel to the direction of wave medium through which they
propagation. move
• Travel through solids, liquids, • Travel only through solids
and gases • Slower velocity than P waves
• Greatest velocity of all (about 3.5 km per second.)
earthquake waves (up to 7 • However, S waves, because of
km/sec) their shearing motion, are far
• Because P waves are like sound more damaging to structures
waves, when they reach the than P waves.
surface they can create sound
waves in the air that are audible
to humans and animals.
RECORDING EARTHQUAKES:
SURFACE WAVES SEISMOGRAPHS
Surface waves are slower than body waves. Because their motion is What we want to do to record an earthquake is to
restricted to the surface of the earth, they generally have a longer distance to measure the shaking of the earth.
travel to reach a particular point than do body waves.
•However, everything attached to the earth,
Rayleigh (R) waves - make the surface of the ground to go up and down, including our measuring instruments, will move
like ripples on the surface of a pond. with the earth.
Love (L)waves - are a horizontal displacement at the surface, that is, they •We need a stationary frame of reference from
cause the surface of the ground to shear sideways. The horizontal shaking of which we can measure the shaking without being
Love waves is particularly damaging to building foundations. a part of it.
•Although we cannot easily detach our
instruments from the earth, we can take
advantage of inertia to isolate them from earth
movements.
•Inertia is the tendency of an object at rest to
remain at rest.
Rayleigh • The key to a seismograph is the presence of
Love (L)
(R) waves a weight that stays fixed in space while
waves
everything else moves around it.
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HOW A SEISMOGRAPH WORKS Horizontal (N-E)
• Let’s consider a mechanical vertical-motion seismograph consisting of a
heavy weight (like a pendulum) suspended from a spring.
• When an earthquake wave arrives and causes the ground surface to move up
and down, it makes the seismograph frame also move up and down.
• The weight, however, remains fixed in space. As a consequence, the revolving
paper roll moves up and down under the pen, which traces out the waveforms
representing the up-and-down movement.
• On a real seismograph record, one revolution of the paper cylinder
corresponds to an hour; a single paper roll holds the record for a whole day.
Figure 7.10
Vertical (Z)
SEISMOGRAMS Seismograms: the written record
The seismic trace recorded on a seismograph contains a variety of
information useful for analyzing the intensity, distance from the
of an earthquake
epicenter, and location of the earthquake.
Seismic waves reach a recording
One can recognize the different kinds of earthquake waves on the
station at different times producing
seismic trace.
identifiable sets of waves (pattern).
The first pulse of waves are the fast moving primary waves (P) waves.
As they begin to fade, the second large pulse records the arrival of the
slower secondary (S) waves. Finally, a mishmash of surface waves and P→ emerge at steep angle producing
reflected p and s waves arrive. mainly vertical ground motion
S→ last somewhat longer than P
trains (E-N components of motion)
Earthquake coda→ the dying end of Wave Dispersion
an earthquake composed of a
mixture of surface waves (L-R) and
See textbook scattered P and S late arrivals
figure 7.12 (deeper structures)
WHAT CAN WE LEARN FROM FINDING THE EPICENTER
By accumulating a tremendous amount of data, seismologists have
SEISMOGRAMS ? determined the average times of S and P waves for any specific distance.
1. Distance: we know how fast the fasted P and S waves travel, so we These travel times are published as time-distance graphs (or travel-
can use the difference in their arrival times (time lag) at the time curves), illustrating the difference between the arrival times of P
seismograph to determine how far away the earthquake was. and S waves.
The farther away a seismograph
2. Origin Time: once we know how far away an earthquake was, we can station is from the focus of an
determine the exact time that it happened. earthquake, the longer the interval
between the arrivals of the P and S
3. Location of Epicenter: If we have the distances to the earthquake
waves, and hence the distance
epicenter calculated from three or more seismic stations, then we can
between the two curves on the graph.
use triangulation to find the exact location of the epicenter.
4. Magnitude: the strength of the earthquake is indicated by the
amplitude or size of the spikes on the seismograph. However, the
farther away the seismic station is, the more attenuated the waves
become and the smaller are the spikes produced by a given
earthquake. Fortunately, since we know how far away the earthquake
was, we can compensate for distance to determine how large the
Earthquake was where it happened.
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TRIANGULATION
The epicenter of any earthquake can be determined by using a time-
distance graph and knowing the arrival times of the P and S waves at
three seismograph locations.
EARTHQUAKES
1-Determine the distance of the epicenter from each of the
seismographs. Earthquake intensity and magnitude
2-For each seismograph, draw
on a map a circle whose radius • Intensity
equals the distance from the • A measure of the degree of earthquake shaking
epicenter.
at a given locality based on the amount of
3-The intersection of the three damage
circles is the location of the
earthquake’s epicenter. • Most often measured by the Modified Mercalli
A minimum of THREE locations Intensity Scale
is necessary because two • Magnitude
locations will provide two
possible epicenters, and one • Concept introduced by Charles Richter in 1935
location will provide an infinite
number of epicenters.
MEASURING THE MAGNITUDE THE RICHTER SCALE
•Earthquake strength is measured against
a scale developed by the seismologist
Charles Richter in 1935. •The Richter scale is logarithmic, that is an increase of 1
•The magnitude of an earthquake is a magnitude unit represents a factor of ten times in
measure of the amount of energy amplitude. The seismic waves of a magnitude 6
released. Each earthquake has a unique
magnitude assigned to it.
earthquake are 10 times greater in amplitude than those
of a magnitude 5 earthquake.
•The magnitude is calculated as the
logarithm of the amplitude of waves •However, in terms of energy release, a magnitude 6
recorded by seismographs.
earthquake is about 31 times greater than a magnitude 5.
•The magnitude is determined by
measuring the maximum amplitude of •The total amount of energy released in the largest
the largest seismic wave (usually a
surface wave) and the difference between
earthquakes ever recorded is 9.5 (Chile, 1960) (roughly
the arrival times of the P and S waves. equal to the energy of 10,000 Hiroshima sized atom
•Adjustments are included for the variation bombs). Probably rocks are not able to store the energy
in the distance between the various necessary to generate earthquakes of higher magnitude.
seismographs and the epicenter of the
earthquakes.
1886
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WHERE AND WHY DO EARTHQUAKES OCCUR? EARTHQUAKES AT PLATE BOUNDARIES
•The majority of earthquakes (~80%) occur in the circum-pacific belt.
•The second major seismic belt (~15%) is the Mediterranean-Asiatic belt. • TRANSFORM MARGINS => shallow focus;
•The remaining 5% of earthquakes take place mostly in the interior of plates and • DIVERGENT MARGINS AND
along oceanic spreading ridges (divergent plate boundaries). CONTINENTAL RIFTS => shallow focus;
• INTRAPLATE => shallow focus
• CONVERGENT MARGINS => shallow to
deep.
CLASSIFICATION
Seismologists recognize three categories
of earthquakes:
EARTHQUAKES
•Shallow-focus-focal depth less than 70 km.
These earthquakes usually generate along
transform or divergent plate boundaries. In Earthquake prediction
general these are the most destructive.
• Short-range – no reliable method yet
•Intermediate-focus- foci between 70 and devised for short-range prediction
300 km.
•Deep-focus-foci deeper than 300 km. • Long-range forecasts
•Approximately 90% of all earthquake foci • Premise is that earthquakes are repetitive
occur at a depth of less than 100 km. • Region is given a probability of a quake
•Intermediate and deep earthquakes occur
along convergent plate boundaries, especially
along the circum-pacific belt.
•Benioff zones => convergent margins.
Denali Fault Earthquake, M7.9, Nov. 3, 2002
EARTHQUAKES DAMAGE
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
Destruction results from
•Ground shaking
•Liquefaction of the ground (saturated material turns fluid,
underground objects may float to surface
•Tsunami, or seismic sea waves
•Landslides and ground subsidence
•Fires
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Aerial view of the Trans-Alaska Pipeline and Richardson
An aerial photo of the Trans-Alaska Pipeline System (TAPS) line near the Denali
Highway, looking north. Rupture along the fault resulted in
fault, looking west. This is where the line is supported by rails on which it can move
approximately 2.5 meters (8 feet) displacement of the
freely in the event of fault offset. Here the line has moved toward the west end of the
highway, with the north side moving east relative to the rails. Alyeska Pipeline Service Company reported no breaks to the line and therefore
south side. Photo by Patty Craw, DGGS. no loss of oil. Note the transverse crack on the Richardson Highway in lower left.
STRUCTURAL DAMAGE LIQUEFACTION
•In addition to the
• Most buildings and bridges are constructed collapse of buildings,
to withstand the downward force of gravity. the shaking from
Construction materials such as brick and earthquakes can
concrete are very strong in compression cause ground
and can support great weight. composed of loose soil
• Unfortunately, these same materials are and sand to liquefy.
brittle and incapable of resisting tensional •The most impressive
forces introduced by bending. Side to side example of liquefaction
or upward motion introduces bending was seen at Turnagain
forces and these materials fail and Heights in Anchorage,
collapse. Most people killed in earthquakes during the Good Friday
die from trauma caused by building earthquake of 1964,
collapse and object falling from walls. where 60 foot high soft
• The principle goal of earthquake clay beach cliffs
engineering is to prevent loss of life from collapsed causing the
building collapse. Even if a building is slumping of developed
damaged beyond repair, if it does not land up to 900 feet
collapse on the occupants then it will not inland along more than
cause loss of life. a mile of coastline.
TSUNAMI TRAVEL TIMES
EARTH'S LAYERED
TO HONOLULU
STRUCTURE
Most of our knowledge of Earth’s
interior comes from the study of P and S
earthquake waves
• Travel times of P and S waves through
Earth vary depending on the properties of
the materials
• S waves travel only through solids
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EARTH'S SHADOW ZONE
LAYERED
•Absence of P
STRUCTURE waves from about
105 degrees to 140
degrees around the
globe from an
earthquake
•Explained if Earth
contained a core
composed of
materials unlike the
overlying mantle
Earth's layered structure
Discovering Earth’s major layers
• Discovered using changes in seismic wave
velocity
• Mohorovicic discontinuity
• Velocity of seismic waves increases abruptly
below 50 km of depth
• Separates crust from underlying mantle
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