EARTHQUAKES
Dr. R. B. Schultz
Global Earthquake Locations
What are Earthquakes?
• The shaking or trembling caused by
the sudden release of energy
• Usually associated with faulting or
breaking of rocks
• Continuing adjustment of position
results in aftershocks
• Shaking of earth due to movement
of rocks along a fault.
• Rocks under stress accumulate
strain energy over time.
• When stress exceeds strength
of rocks, rock breaks.
• Strain energy is released as
seismic waves. The longer that
energy is stored up and is
maintained without release, the
more likely that a strong
earthquake will occur.
The animation below of a subduction zone, where an
oceanic plate is converging with a continental plate,
attempts in cartoon fashion to show the deformation of
the continental plate that precedes each earthquake.
What is FAULT?
•A break or crack in rock
along which movement of
the earth’s crust occurs.
•Can be visible on the crust
or can be far underground.
•Can be only a meter long or
they can extend for
hundreds of kilometers.
FAULT
Earthquakes can happen along any type of plate
boundary. They also occur along faults which are
large cracks in the earth’s crust. Most faults
are associated with large plate boundaries
where violent earthquakes usually occur.
Movement of rock along a fault can release
a tremendous amount of energy: This
movement can release three main forces on
adjoining areas of the crust:
1.Tension – results when a system of forces
pulls or stretches an object from opposite
sides.
2.Compression – results when a system of
forces pushes an object from opposite
sides.
3.Shearing - results when a system of
forces is applied to an object in different
directions.
What is the Elastic Rebound
Theory?
All rocks have an elastic limit.
If the forces acting on these
rocks exceed this limit, or
overcome the friction between
blocks, the crust snaps and an
earthquake occurs.
What is the Elastic Rebound Theory?
• Explains how
energy is stored in
rocks
– Rocks bend until
the strength of
the rock is
exceeded
– Rupture occurs
and the rocks
quickly rebound to
an undeformed
shape
– Energy is released
in waves that
radiate outward
from the fault
KINDS OF FAULT
1. Normal Fault
 one block can move down and the
other can move up, or vice versa.
 occurs where forces tend to
pull section of the earth’s crust
apart.
 tends to follow the gravitational
pull on the fault blocks involved.
 The fault plane on the normal fault
is generally very steep. In a normal
fault the two involved blocks are (by
gravity) pulling away from one
another causing one of the fault
blocks to slip upward and the other
downward with respect to the fault
plane (it is hard to determine
whether both or just one block has
moved.).
 The exposed upward block forms a
cliff-like feature known as a
FAULT SCARP. A scarp may range
from a few to hundreds of meters
in height and their length may
continue for 300 or more
kilometers (around 200 miles).
 FAULT SCARP – landforms that
form cliffs and ridges along the
fault. (book definition)
Normal Fault
KINDS OF FAULT
2. Thrust Fault
 one block moves over another block
 occurs where forces tend to push
sections of the crust together.
 When thrust faults are exposed on the
surface overburnden material lies over
the main block. They are normally
associated with areas of folded surfaces
and or mountaineous regions. The dip
angles of thrust faults are normally not
as steep as a normal fault.
Thrust Fault
KINDS OF FAULT
3. Strike-slip Fault (Transcurrent Fault)
 the movement of the blocks is
horizontal
 known as the San Andreas fault of
California. This fault marks the
margin line between the Pacific and
North American Plates.
 On the surface, scarps form as hills
crossing the fault zone are torn apart
by movement over time.
• Actually anything crossing this
fault zone is either slowly torn
apart, or offset.
• Rivers crossing the fault line are
called offset streams and are
classic signatures of fault
activity along the San Andreas.
• These faults can be very long,
the San Andreas is nearly 600
miles long.
Strike-slip Fault
Three Types of Faults
Strike-Slip
Thrust
Normal
EARTHQUAKE
AND PLATE
The lines TECTONICS
on the map indicate plate
boundaries.
The Focus and Epicenter of an Earthquake
• The point within
Earth where
faulting begins
is the FOCUS,
or hypocenter
• The point
directly above
the focus on the
surface is the
EPICENTER
Three Types of Focus
1. Shallow-focus
 earthquake originates between the
earth’s surface and a depth of 70
kilometers below the surface.
 earthquakes in this class are
detected at all three types of plate
boundaries-trenches, ridges and
fault boundaries.
 most earthquakes have shallow focus.
Three Types of Focus
2. Intermediate-focus
 earthquake originates between 70
and 300 kilometers beneath the
surface.
3. Deep-focus
 earthquake originates 300 to 700
kilometers below the surface.
 Both occurs at trench boundaries,
where one plate moves under another.
Detecting Earthquakes
• An earthquake’s energy moves from the
focus as Seismic Waves. (Siz’mik)
• These waves travel to all directions.
• An earthquake generates 3 kinds of seismic
waves.
• These waves are:
• P-waves (Primary)
• S-waves (Secondary)
Waves
• In the illustration below, P-waves have the
movement like that of Slinky. While Swaves have the movement of a snake or a
rope. L-waves has movements of an ocean
wave.
P or Primary Wave
• The fastest seismic wave
• Can travel through any material on earthsolid or liquid
• P-waves compress and expand particles that
lie on their path
S or Secondary Wave
• It moves at half the speed of P-waves
• Can travel only through solids
• Movement within the earth resembles to
wavelike motion that results when one
shakes a stretched rope from side to side.
L or Surface Wave
•
•
•
•
The slowest wave
Moves along the earth’s surface
Causes most damage among all waves
Resembles thw motion of ocean waves.
Using Seismographs to Find the
Epicenter
• Seismograph- A sensitive instrument that
measures and records seismic waves.
How does it work?
• A seismograph has a heavy weight
suspended from a support that is attached to
bedrock. During an earthquake, inertia
keeps the weight and the pen attached to it
still. When a seismic wave passes, the
support, base and revolving paper drum
shake. Thus, the pen marks zigzag lines on
a paper drum, recording the movements of
the earth. The recorded lines are the...
Seismogram
• A recording or the recorded lines of a
seismograph.
• In the figure below, there are 3 distinct
groups of zigzag lines that correspond to the
different kinds of seismic waves.
The Epicenter and the Seismogram
• Scientists analyze the time difference
between the arrival of the P and S waves.
• By doing so, they can calculate the distance
to the earthquake’s epicenter.
How do they do it?
• Note: Please refer to the pictue at the past slide.
• Readings from 3 seismograph stations are needed to
locate the epicenter of the earthquake. Assume that a
scientist calculates the distance from Station A to the
epicenter to be 1000 km. The distance is determined on
the difference of arrival time of the 2 waves. The
scientist knows the distance of A to the epicenter. The
scientist draws a circle on a map which has a radius the
equivalent of 100 km. Meanwhile,Stations B and C
determines the distance to be at 500 and 400 km,
respectively. Again, they draw circles. The meeting
point of the circles is the epicenter.
Seismic Waves and The Earth’s
Interior
• The seismic waves that result from an
earthquake spread throughout the earth.
• Geologists study the speed and the paths of
seismic waves to learn about the earth’s
interior.
• Our knowledge of the earth’s layers relied
on the changes in speed and direction of
seismic waves.
Seismic Waves and The Earth’s
Interior
• Early in the century, geologists discovered
that large areas on the side of the earth away
from the earthquake focus do not receive
waves. Look at the image below.
P-Waves and the Earth’s Interior
• In the case of P-waves, they can pass
through the mantle, the liquid outer core,
and the solid inner core. Notice the small
shadow zone at the image. It has been stated
that P-waves can pass through all particles.
S-Waves and the Earth’s Interior
• While at the S-waves, a large shadow zone is
formed. Again, it has been staterd that S-waves
can only pass through solids.
• This discovery has enabled scientists to
conclude that the outer core is liquid.
Types of Earthquake
1. Tectonic type- earthquake that result from
the upward and downward movements of the
crustal plates due to pressure.
2. Volcanic type- earthquakes produced by
the pressure generated by magma rising
through faults towards the earth’s surface.
Intensity
• Intensity- measure of an earthquake’s
effect or damage in a place.
• Modified Mercalli Scale named after
Guiseppe Mercalli is used as a basis for the
intensity of an earthquake.
Modified Mercalli Scale
Intensity Scale- Description
Intensity Scale- Description
I-Not felt
II-Felt by persons at rest
III- Felt indoors
IV- hanging objects swing
V- felt outdoors
VI- felt by all
Modified Mercalli Scale
Intensity Scale- Description
VII- difficult to stand
VIII- steering of cars affected
IX- general panic
X-most masonry and frame structures
destroyed with their foundations
XI- rails bent gently
XII- damage nearly total
Magnitude & Richter Scale
Magnitude- measures the energy released
during quake by measuring the amplitude of
the largest wave recorded on the seismogram
Richter Scale- named after Charles Richter
used as basis for the magnitude. The scale is
logarithmic, one point increases in the scale
means tenfold increases
Richter Scale
Magnitude Scale- Description
1- only detectable when an
ultrasensitive seismometer
2- hardly susceptible shocks
3- very feeble shocks
4- feeble shocks
5- earthquakes with moderate
strength
Ritcher Scale
Magnitude Scale- Description
6- strong earthquakes
7-major earthquakes
8- great earthquakes
9-never occurred
How are the Size and Strength of an Earthquake Measured?
•
•
Intensity
– subjective measure
of the kind of
damage done and
people’s reactions
to it
– isoseismal lines
identify areas of
equal intensity
Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake,
magnitude 6.7
Measuring Earthquakes
The power (magnitude) of an earthquake is measured on the Richter
scale, using an instrument called a seismometer.
The Richter scale is numbered 0-10 with 10 being the most powerful. The
Richter scale is logarithmic – an earthquake measuring 7 is 10 times more
powerful than one measuring 6 and 100 times more powerful than one
measuring 5. Up until 2 on the Richter Scale only instruments will detect
the earthquake. Earthquakes above 6 cause serious damage and sometimes
many deaths
The Mercalli scale measures the damage caused by an earthquake. The
Mercalli scale goes from I to XII e.g. “VI. Everyone feels movement.
People have trouble walking. Objects fall from shelves. Pictures fall off
walls. Furniture moves. Plaster in walls might crack. Trees and bushes
shake. Damage is slight in poorly built buildings. No structural damage.”
How are the Size and Strength of an Earthquake Measured?
 Magnitude
› Richter scale measures
total amount of energy
released by an
earthquake;
independent of
intensity
› Amplitude of the
largest wave produced
by an event is
corrected for distance
and assigned a value
on an open-ended
logarithmic scale
Effects Of Earthquakes: The Richter
Scale
• After an earthquake,
its description usually
includes a number
from 1 to 9 in the
Richter Scale.
• This number indicates
the magnitude or the
total amount of
energy released.
Charles F. Richter: The Inventor
• He was a geologist stationed at the
California Institute Of Technology.
• Before he invented it, earthquakes were
measured at a much less scientific scale.
Personal observations were made, and
scientists had to use this to base their
conclusions.
• This method greatly reduced the guesswork
in determining the magnitude of an
What do earthquakes do to the
Earth?
Earthquakes do a lot of effects on the earth.
These include:
• Structural damage
Benito Juarez Hospital, Mexico City. (taken
in 1985)
What do earthquakes do to the
Earth?
• Tsunami, a Japanese word meaning harbor
wave. These are really different from the
actual harbor waves, but can do a
significant amount of damage.
What Controls the Level of Shaking?
• Magnitude
– More energy released
• Distance
– Shaking decays with distance
• Local soils
– amplify the shaking
Is there such a thing as
“Earthquake Weather”???
Earthquake Effects - Ground Shaking
Northridge, CA 1994
Earthquake Effects Ground Shaking
Northridge, CA 1994
Earthquake Effects - Ground Shaking
Loma Prieta, CA 1989
KGO-TV News ABC-7
Earthquake Effects - Ground Shaking
Kobe, Japan 1995
Earthquake Effects - Ground Shaking
Kobe, Japan 1995
Earthquake Effects - Surface Faulting
Landers, CA 1992
Earthquake Effects - Liquefaction
Source: National Geophysical Data Center
Niigata, Japan 1964
Earthquake Effects - Landslides
Source: National Geophysical Data Center
Turnagain Heights, Alaska,1964 (upper left inset);
Santa Cruz Mtns, California , 1989
Earthquake Effects - Fires
Loma Prieta, CA 1989
KGO-TV News ABC-7
Earthquake Effects - Tsunamis
1957 Aleutian Tsunami
Photograph Credit: Henry Helbush. Source: National Geophysical Data Center
Earthquake Preparedness
Before- develop a disaster
plan; secure your home; store
flammable; pack survival kit
lightly; and conduct
earthquake drills.
Earthquake Preparedness
During- cover your head; look for
a safe place to hide temporarily;
stay in an open area; go to a
higher ground for tsunami alert;
and move away from steep
elevations.
Earthquake Preparedness
After- exit building calmly;
see if you have injuries; and do
not enter a building in attempt
to save someone.
QUIZ
1. Which activities are earthquakes usually
associated with?
2. What is a fault?
3-5. Name the 3 movements of rock.
6. What causes these movements? (2 words.)
7. Which theory explains the cause why
earthquakes occur?
QUIZ
8-10. Name all types of faults.
11. There are 3 types of focuses, the Shallow,
_______, and the Deep.
12. What is the slowest wave?
13. Name the instrument that detects
earthquakes and its recordings.
14. At least how many circles are needed to
locate the epicenter?
QUIZ
15. Which wave can penetrate the cores?
16. Which wave cannot penetrate the outer
core?
17. Who invented a scale used to describe an
earthquake?
18-19. Which main effects were described
here?
20. Name at least an engineering technique
given.
Thank you for viewing and have
a good day!
References:
• www.google.com
• Integrated Science book
QUIZ
Fill in the blanks:
1. Earthquake is the the shaking or
trembling caused by the sudden
release of ________.
2. The point within Earth where
faulting begins is the FOCUS, or
___________.
3. Shaking of earth due to movement of
rocks along a _________.
QUIZ
4. Can be visible on the ________ or
can be far underground.
5. ________ is where an oceanic
plate is converging with a
continental plate.
6. San Andreas fault is what kind of
fault?
7. _______ results when a system of
forces pushes an object from
opposite sides.
QUIZ
9. If the forces acting on these rocks
exceed this limit, or overcome the
________ between blocks, the crust
snaps and an earthquake occurs.
10. One block can move down and the
other can move up, or vice versa is a
_______ fault.
11. The point directly above the focus
on the surface is the _________.
QUIZ
12. ______ focus earthquake originates
between the earth’s surface and a
depth of 70 kilometers below the
surface.
13. _________ results when a system of
forces is applied to an object in different
directions.
14. The exposed upward block forms a
cliff-like feature known as a
______.
QUIZ
15. In ______ focus , the
earthquake originates between
70 and 300 kilometers beneath
the surface.
answers:
Fill in the blanks:
1. Energy
2. Hypocenter
3. Fault
4. Crust
5. Subduction zone
6. Strike-slip
7. Compression
Answers:
Fill in the blanks:
9. Friction
10. Normal
11. Epicenter
12. Shallow
13. Shearing
14. Fault scarp
15. Intermediate