EARTHQUAKES
What is an Earthquake?
An earthquake is the vibration of Earth
produced by the rapid release of energy.
Can cause catastrophic destruction to
humans and our civilizations.
San Francisco in flames after the
1906 Earthquake
Damage in San Francisco’s
Marina District following the
1989 Loma Prieta Earthquake
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What Causes an Earthquake?
The Plate Tectonic Theory shows
that the lithospheric plates of the
Earth are in continual motion.
Lithospheric movements produce
faults that cause earthquakes.
Elastic
Rebound
As rocks deform
along the fault
They bend,
Storing elastic energy
Once strained beyond
the breaking point,
Frictional resistance
holding the rocks
together is overcome.
The rock cracks,
releasing its storedup energy in the form
of earthquake waves.
Vibrations
(earthquakes) occur
as the deformed rock
“springs back” to its
original shape (elastic
rebound).
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What Causes an Earthquake?
Elastic Rebound
Animation #25
Where Do Earthquakes Occur?
Earthquakes are evidence of plate tectonics: A good fit exists between the plate
tectonics model and the global distribution of earthquakes.
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Where Do Earthquakes Occur?
Earthquakes originate at depths
ranging from 5 to nearly 700 km.
Shallow (surface to 70 kilometers)
Intermediate (between 70 and 300 kilometers)
Deep (over 300 kilometers)
Where Do Earthquakes Occur?
Definite Patterns Exist
Most deep-focus earthquakes only occur along
convergent boundaries with subduction.
Foci depths increase with increasing distances from
deep-ocean trenches.
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Where Do Earthquakes Occur?
Definite Patterns Exist
Earthquakes
generated along
oceanic ridge
systems always
have a shallow
focus, and none
are very strong.
Where Do Earthquakes Occur?
Definite Patterns Exist
Earthquakes
generated along
transform plate
boundary systems
are predominantly
shallow focus,
and can be very
strong.
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Where Do Earthquakes Occur?
Approximately 90% of
earthquakes occur at
depths less than 100 km.
Nearly all very damaging
earthquakes originate at
shallow depths.
When Do Earthquakes Occur?
The San Andreas Fault is an active fault zone.

Displacement occurs along discrete segments
100 to 200 kilometers long.
Some portions exhibit slow,
gradual displacement known as
fault creep.
Other segments regularly slip
producing small earthquakes.
Still other fault segments store
elastic energy for hundreds of
years before rupturing in great
earthquakes (Seismic Gaps).

Great earthquakes may occur
about every 50 to 200 years
along these sections.
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What Happens During an Earthquake?
Energy is released in the form of waves
and radiates in all directions from its
source, the focus.
Types of Seismic Waves
Body Waves

Travel through Earth’s interior.

Two types based on mode of travel.
Primary (P) Waves



Push-pull (compress and expand – compressional waves) motion,
changing the volume of the intervening material.
Therefore, can travel through solids, liquids, and gases.
Generally, in any solid material, P waves travel about 1.7 times
faster than S waves.
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Types of Seismic Waves
Seismic Wave
Motion Animation
#77
Types of Seismic Waves
Body Waves

Secondary (S) Waves
“Shake” motion at right angles to their direction of travel that
changes the shape of the material transmitting them (shear
waves).
Therefore, can travel only through solids.
Slower velocity than P waves.
Slightly greater amplitude than P waves.
Lesser amplitude than L Wave.
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Types of Seismic Waves
Seismic Wave
Motion Animation
#77
Types of Seismic Waves
Surface Waves
Travel along outer part (surface) of the Earth.
Complex motion (up-and-down motion as well as side-to-side
motion).
Cause greatest destruction.
Exhibit greatest amplitude and slowest velocity.
Waves have the greatest periods (time interval between crests).
Often referred to as long waves, or L waves.
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Types of Seismic Waves
Seismic Wave
Motion Animation
#77
Effects of Seismic Waves
Seismic Wave
Motion and
Surface Effects
Animation #78
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How are Earthquakes Located?
Sensitive instruments, called
seismographs, around the world
record the earthquake event.
Seismographs record seismic
waves.
How are Earthquakes Located?
Seismographs record the movement of Earth
in relation to a stationary mass on a rotating
drum or magnetic tape.
More than one type of seismograph is
needed to record both vertical and horizontal
ground motion.
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How are Earthquakes Located?
Seismographs Animation #79
Records obtained are called seismograms.
Seismogram Showing P, S, and L Waves
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Locating the Source of
Earthquakes
1. Three station
recordings are needed
to locate an epicenter.
2. Each station
determines the time
interval between the
arrival of the first P
wave and the first S
wave at their location.
3. A travel-time graph is used to determine
each station’s distance to the epicenter.
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Locating the Source of
Earthquakes
4. A circle with a
radius equal to the
distance to the
epicenter is drawn
around each station.
5. The point where all
three circles
intersect is the
earthquake
epicenter.
6. This method is
called triangulation.
How Are Earthquakes Measured?
Intensity – a measure of the degree of
earthquake shaking at a given locale
based on the amount of damage.
The drawback
of intensity
scales is that
destruction may
not be a true
measure of the
earthquake’s
actual severity.
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How Are Earthquakes Measured?
Magnitude – estimates the amount of
energy released at the source of the
earthquake.
How Are Earthquakes Measured?
Richter Scale



Based on the amplitude of the largest seismic wave recorded.
Accounts for the decrease in wave amplitude with increased distance.
Each unit of Richter magnitude increase corresponds to a tenfold increase
(logarithmic scale) in wave amplitude and a 32-fold energy increase.
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Why Should We Care?
Expeditions in Geology Video Clip #1: The
Wasatch Fault – Active Fault in the Rockies
Offset
Produced by
the 1906 San
Francisco
Earthquake
Earthquake Destruction
Destruction from Seismic Vibrations
1.
2.
3.
4.
5.
6.
Ground Shaking
Liquefaction of the Ground
Seiches
Tsunamis, or Seismic Sea Waves
Landslides and Ground Subsidence
Fire
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Earthquake Destruction
Amount of structural damage
attributable to earthquake vibrations
depends on:
Proximity to populated areas
Magnitude
Intensity and duration of the vibrations
Nature of the material upon which the
structure rests
Design of the structure
Ground Shaking
Regions within 20 to 50 kilometers of the
epicenter will experience about the same
intensity of ground shaking.
Destruction varies considerably mainly due to the
nature of the ground on which the structures are
built.
Damage to I-5 during the
Northridge, CA Earthquake in 1994
Damage Caused by the 1964
Anchorage, Alaska Quake
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Liquefaction of the Ground
Unconsolidated materials saturated
with water turn into a mobile fluid.
Can cause underground structures to
migrate to the surface, and buildings
and other aboveground structures to
settle and collapse.
Earthquake Destruction
Liquefaction of the Ground
Dry Compaction and Liquefaction
Animation #21
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Tsunamis or Seismic Sea Waves
Result from vertical displacement along
a fault located on the ocean floor.
Result from a large undersea landslide
triggered by an earthquake.
Tsunamis or Seismic Sea Waves
Advance across oceans at great speeds ranging from ~500 to
950 km/hour (~310 to 590 miles/hour).
In the open ocean, height is usually < 1 meter.
Distances between wave crests range from 100 to 700 km.
In shallower coastal waters, the water piles up to heights that
occasionally exceed 30 meters (~100 feet).
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What Happens to a Tsunami as it Approaches Land?
As a tsunami leaves the deep water of the open ocean and travels into the shallower water
near the coast, it transforms.
A tsunami travels at a speed that is related to the water depth – hence, as the water depth
decreases, the tsunami slows.
The tsunami's energy flux, which is dependent on both its wave speed and wave height,
remains nearly constant.
Consequently, as the tsunami's speed diminishes as it travels into shallower water, its
height grows.
Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several
meters or more in height near the coast.
When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a
series of breaking waves, or even a bore.
http://www.geophys.washington.edu/tsunami/general/physics/physics.html
What Happens When a Tsunami Encounters Land?
As a tsunami approaches shore, it begins to slow and grow in height.
Just like other water waves, tsunamis begin to lose energy as they rush onshore – part of
the wave energy is reflected offshore, while the shoreward-propagating wave energy is
dissipated through bottom friction and turbulence.
Despite these losses, tsunamis still reach the coast with tremendous amounts of energy.
Tsunamis have great erosional potential, stripping beaches of sand that may have taken
years to accumulate and undermining trees and other coastal vegetation.
Capable of inundating, or flooding, hundreds of meters inland past the typical high-water
level, the fast-moving water associated with the inundating tsunami can crush homes and
other coastal structures.
Tsunamis may reach a maximum vertical height onshore above sea level, often called a
runup height, of 10, 20, and even 30 meters.
http://www.geophys.washington.edu/tsunami/general/physics/physics.html
Tsunami at Hilo, Hawaii (April 1, 1946) that originated in the Aleutian Islands near
Alaska, was still powerful enough to rise 30 to 55 feet when it hit Hawaii.
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Earthquake Destruction
Tsunamis or Seismic Sea Waves
http://serc.carleton.edu/NAGTWorkshops/visua
lization/collections/tsunami.html
Earthquake Destruction
Tsunami Animation #91
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Seiches
The rhythmic sloshing of water in lakes,
reservoirs, and enclosed basins.
Waves can weaken reservoir walls and
cause destruction.
Landslides and Ground
Subsidence
Landslide caused by the 1964
Alaskan Earthquake
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Fire
San Francisco in flames after the 1906 Earthquake
Can Earthquakes be Predicted?
Short-Range Predictions
Goal is to provide a warning of the
location and magnitude of a large
earthquake within a narrow time frame.
Research has concentrated on
monitoring possible precursors –
such as measuring:



uplift
subsidence
strain in the rocks
Currently, no reliable method exists for
making short-range earthquake
predictions.
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Can Earthquakes be Predicted?
Long-Range Forecasts
Give the probability of a certain
magnitude earthquake occurring on a
time scale of 30 to 100 years, or more
(statistical estimates).
Based on the premise that earthquakes
are repetitive or cyclical.

Using historical records or paleoseismology
Are important because they provide
information used to


Develop the Uniform Building Code
Assist in land-use planning
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