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Earthquakes
How and Where Earthquakes Occur
PPT Modified from Troy HS
Is there such thing as
“earthquake weather?” Absolutely NOT!!!
Geologists believe that there is no connection between
weather and earthquakes. They are the result of geologic
processes within the earth and can happen in any weather and
at any time during the year. Earthquakes originate miles
underground. Wind, precipitation, temperature, and barometric
pressure changes affect only the surface and shallow
subsurface of the Earth. Earthquakes are focused at depths
well out of the reach of weather, and the forces that cause
earthquakes are much larger than the weather forces.
Earthquakes occur in all types of weather, in all climate zones,
in all seasons of the year, and at any time of day.
When do earthquakes occur?
• Earthquakes happen
once every 10 seconds
on Earth.
• About 3 million
earthquakes happen per
year.
• On average there is one
8 magnitude, 10 7
magnitudes, 100 6
magnitudes and so on
per year.
– Most earthquakes are
of very low
magnitudes
What is an earthquake?
• Movement or trembling of the ground that
is caused by a sudden release of energy
when rocks along faults move
• Reasons Earthquakes happen:
– Plates Moving
– Faults (cracks in Earth’s surface)
– Caverns collapsing
– Meteor Impacts
– Volcanic Eruptions
Where do most earthquakes
occur? • Plate boundaries or
• Faults
What is a Fault?
• a fracture along which the blocks of crust
on either side have moved.
• Plate boundaries are always faults, but not
all faults are plate boundaries.
• Fault zones or systems – region of
numerous, closely spaced faults
In California earthquakes are
caused by
• The San Andreas
Fault System
– San Andreas + Faults
= EARTHQUAKES
Three Types of Faults
• Strike-slip
• Dip-slip
– Normal
– Reverse
• (Thrust is a reverse fault with a small
dip angle)
Click here to see the faults in action
Strike-Slip Fault
• Also called transform faults or boundaries
• are vertical (or nearly vertical) fractures
where the blocks have mostly moved
horizontally.
• If the block opposite an observer looking
across the fault moves to the right, the slip
style is termed right lateral; if the block
moves to the left, the motion is termed left
lateral.
Dip-Slip Faults
• Dip-slip faults are inclined fractures where
the blocks have mostly shifted vertically.
• If the rock mass above an inclined fault moves
down, the fault is termed normal
• if the rock above the fault moves up, the fault
is termed reverse.
• A thrust fault is a reverse fault with a dip of 45
degrees or less.
Dip-slip Faults Animations
San Andreas Fault In Action
Draw California and the San
Andreas
Terms used to describe an
earthquake
Epicenter
Focus
Fault
Seismic Waves
Focus Vs. Epicenter
• Focus – the location
within Earth along a
fault at which the first
motion of the
earthquake occurs
• Epicenter- point on
Earth’s surface directly
above an earthquake’s
starting point, or focus
How does energy travel in an
earthquake?
• Energy travels in waves from the focus
• The focus is the part of the fault that
moved where energy was released
• The epicenter is located directly above
the focus on the surface
Elastic Rebound Theory
• The elastic rebound theory is an explanation for how energy is spread during earthquakes.
• As plates on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded.
• At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.
How does energy travel in an earthquake?
Waves
When P and S waves reach the
surface they make surface waves
Body Waves Surface Waves
Travel Through
the Earth
P Waves
S Waves
Raleigh Waves
Love Waves
Travel Through
the Earth
P Waves
Primary Waves
Reach an area first Can travel through
solid, liquid and air
Compress
and Expand
Body Waves
Travel Through
the Earth
S Waves
Shear Waves
Reach an area
second
Can only travel
through
solid
Move at right
angles to the
direction they
are traveling
Body Waves
P-Waves Vs. S-Waves
P-Waves (Compression)
S-Waves (Right angles)
Interior Of Earth
Interior Of Earth
Surface Waves
Raleigh
Love
Rolling Waves Jolting Waves
Like a ripple in a pond Move at right angles
Travel Along the
Surface
Love Waves Vs. Rayleigh Waves
Surface
Surface
MythBusters
• https://www.youtube.com/watch?v=SsABn
1zH_Js
Locating and Measuring Earthquakes
Loma Prieta, Ca 1989
Kobe, Japan 1995
How are earthquakes detected? • Earthquakes are detected
by seismographs (detects
and records EQ)
• Seismographs record the
information on a
seismogram (the EQ
recording)
• Seismology is the study of
earthquakes and their
seismic waves
Locating the Epicenter
In order to determine the location of an earthquake, the earthquake needs to be recorded on three different seismographs that are at significantly different locations.
The other piece of information needed is the time it takes for P-waves and S-waves to travel through the Earth and arrive at a seismographic station.
The Triangulation Method
Triangulation
A mathematical method for locating the
epicenter of an earthquake using three
or more data sets from seismic stations.
Data is collected using earthquake
monitoring instruments called
seismographs which record the seismic
waves of the earthquake.
Triangulation
If three arrival times are available at three
different seismic stations then triangulation
can be used to find the location of the focus or
epicenter and the time of occurrence of the
earthquake.
The distance between the beginning of the
first P wave and the first S wave tells you how
many seconds the waves are apart.
Triangulation
• Since the P waves travel faster than the S
waves, P waves will arrive at a given
seismograph station sooner than S waves.
• In other words, the S waves lag behind the P
waves.
• In fact, the time difference between when the
P waves arrive at a seismograph station and
when the S waves arrive at the same station
is called Time Lag.
The difference in
arrival times can tell us
the distance from the
earthquake
The amplitude of the
s-wave can be
compared with the
time to determine the
magnitude
How do you find the epicenter of an
earthquake?
• You must have at least 3 seismograms from different stations
• You determine the distance each station is by determining the lag time between the p wave and the s wave – The farther away the station the greater lag
• You draw circles out at those distances on a map
• Where the circles intersect is the location of the epicenter
** Three stations does not provide exact data the
data from many stations is compiled to determine the
exact epicenter
How is the strength of the
earthquake determined? • Intensity
– The effect of the Earthquake on the area or strength of shaking
– changes with distance and type of ground the location is on
– Measured by the Mercalli Scale
• Magnitude
– The actual energy released by the Earthquake
– This number does not change with distance
– Measured by the Richter Scale
How does the Richter Scale show
magnitude?
• Every number on the Richter scale shows an increase of 31 times in energy released
• What is the difference in energy between a 3 and a 4?
– 31 times
• What is the difference between a 7 and a 9?
– 7 8 9 - 31 x’s 31 = 961 times more powerful!
31x 31
Bigger Faults Make Bigger Earthquakes
1
10
100
1000
5.5 6 6.5 7 7.5Magnitude
Kilo
mete
rs
8