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Earthquake Introduction
What is an Earthquake?
A. Earthquake: Sudden slip on a fault produces radiated seismic energy
Earthquakes
What is an earthquake?
Sudden slip on a fault, and
the resulting ground
shaking and radiated
seismic energy caused by
the slip.
When seismologists think of an earthquake,
this image of a seismogram is what comes to
their minds.
Or by volcanic or magmatic
activity, or other sudden
stress changes in the
earth.
Faults and earthquakes
A fault is a fracture or a
zone of fractures in a rock
where the two sides have
been displaced relative to
each other.
Movement may occur
rapidly, in the form of an
earthquake - or may occur
slowly, in the form of creep.
The total offset may be
centimeters to kilometers.
1999 Izmit earthquake, Turkey
Faults and earthquakes
Offset road, 1999 Izmit earthquake, Turkey (and collapsed
condos)
Faults and earthquakes
Offset road, 1992 Landers earthquake, California
Faults and earthquakes
Offset parking lot, 1992 Landers Earthquake, California
Earthquakes
Why do earthquakes occur?
B. Elastic Rebound: the mechanism of
earthquake generation
1) Stress in the Earth mostly due to
movements of tectonic plates
2) Strain energy accumulates
a) stored potential offset of fault
b) the fault is locked
3) Yield Point is reached strength of the
rock is exceeded
4) Sudden brittle failure, faulting produces
earthquake
Thrust fault scarp at El Asnam, Algeria.
The fault scarp from the earthquake of
October 10, 1980, at El Asnam, Algeria,
shows a 3-m vertical offset
Earthquakes
Why do earthquakes
occur?
B. Elastic Rebound: the
mechanism of earthquake
generation
4) Sudden brittle failure
5) Deformed rock springs back
into place releasing energy
6) Energy is transmitted in all
directions as seismic waves.
This produces ground
shaking
Earthquakes
B. Elastic Rebound Theory
Earthquakes
B. Elastic Rebound Theory
Earthquakes
B. Elastic Rebound Theory
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10_06b.jpg
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Earthquake hypocenter and epicenter
Fault: a zone of weakness in the
Earth between two crustal blocks
C. Hypocenter or Focus: where
slippage along the fault zone
results in an earthquakes
D. Epicenter: a point on the
Earth’s surface directly above the
hypocenter
Earthquake hypocenter and epicenter
C. Focus (Hypocenter)
The precise underground spot where
the rocks begin to rupture or shift—
the weakest point
The energy released radiates out in
all directions like a stone dropped
into a calm pool
D. Epicenter
The location on Earth’s surface
directly above the focus
The energy is released from the
focus and travels out in all directions
like waves.
Waves are strongest closest to the
epicenter and the damage is greatest
there, and dissipates further away
II. Seismic Waves—the products of Earthquakes
Examples:
When you toss a large stone into a
quiet lake the energy from the
falling stone is transferred to the
water on impact.
Ripples radiate in all directions, but
eventually die out at some distance
from the point of impact.
Sound energy produced by your
vocal cords is transmitted through
the air in the form of sound waves.
These also die out with great
distance from their source
10_04.jpg
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II. Seismic Waves—the products of Earthquakes
Seismic waves
With brittle failure (due to stress
that exceeds their strength) the
elastic energy that is released is
similarly transmitted at great
speeds through the surrounding
rocks in all directions.
The energy is transmitted in the
form of waves: seismic waves
Earthquakes
Seismic Waves
A. Body Waves: transmit energy
through the Earth’s interior in all
directions from the focus
1. P waves (primary waves)
a. Speed: Fastest seismic waves—4
miles/second = 14,400 miles/hour
(in cont crust)
So they’re the first signal to arrive at
an earthquake-recording station
b. Movement: compression and
contraction in the direction of wave
propagation.
Earthquakes
Seismic Waves
A. Body Waves
When the rock first slips, it pushes into
neighboring rock compressing it.
Then the rocks expand elastically
(dilate) past their original volume
compressing its neighbor.
Push-pull waves (slinky analog)
Temporarily change volume of rock as
they travel through
c. What we feel: a series of sharp jolts,
like a sonic boom—rattles windows
Vibration is parallel to the direction of
wave travel
Earthquakes
Seismic Waves
2. S waves (secondary waves)
a. Speed: slower than P waves—2
miles/second = 7,800 miles/hour (in
cont crust
b. Movement: it shears the rock
sideways at right angles to the
direction of travel
Imagine a rope tied at one end that
was then sharply snapped up and
down
Temporarily change the shape of the
material they are being transported
through
c. What we feel: up-and-down and sideto-side motion, shaking the earth vertically
and horizontally, continuous wriggling
motion
Vibrations are perpendicular to the
direction of wave travel
Earthquakes
Seismic Waves
B. Surface Waves: waves that
travel along the earth’s surface—
like lake ripples
Created by the vibration of Body
Waves
Travel along the surface so longer
time travel to reach a point
1. Love Waves: move the ground
side to side in a horizontal plane
(at right angles to the direction of
propagation.
Whip objects from side to side.
Earthquakes
Earthquakes
Seismic Waves
B. Surface Waves: waves that
travel along the earth’s
surface—like lake ripples
2. Rayleigh Waves: like rolling
ocean waves, move both
vertically and horizontally in the
direction of wave propagation
Similar to a brisk walk across a
waterbed, some people get
“seasick”
Animation of seismic waves
III. Measuring Seismic Waves and Locating Earthquakes
A. Locating and Measuring size of
EQ’s
We know that P and S waves travel
at different speeds, so we can use
them to determine the distance to
an earthquake
1) We need a measuring device:
Seismograph: earthquake
recorders
Pendulum, allowed to swing
A frame, attached to the ground,
supports a recording device
When the ground moves, so does
the recording device and the
pendulum swings
III. Measuring Seismic Waves and Locating Earthquakes
A. Measuring size of EQ’s
seismographs records
EW motion
NS motion
Vertical motion.
Used to require three seismometers.
Recording device traditionally used
a rotating drum of paper.
When the earth moves, the pen at
the end of the pendulum leaves a
trace on the paper around the
drum:
This record is called a seismogram
III. Measuring Seismic Waves and Locating Earthquakes
B. Interpreting a Seismogram
Time-series: Plot time versus
amplitude of wave
1. Record is fairly flat
2. First hint of motion is where
the P waves arrive
3. Next large jump in amplitude
is the S waves
How do seismometers work?
A seismograph records the motion of the ground.
Originally, a drum covered with paper rotates under a pen. The pen moves from one
end of the cylinder to the other creating a helical spiral line around the cylinder. A
sensor converts the motion of the ground into an electrical signal which is amplified
and converted once again into motion of the pen.
Seismic shaking is shown in the top center of these recordings (the pen makes a
regular mark every minute). Seismographs are being replaced with digital recorders.
Animation of seismometer
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III. Measuring Seismic Waves and Locating Earthquakes
C. Locating the epicenter of
earthquakes
1. P and S waves start out
from the focus at the same
time, but they travel different
speeds
So the P waves arrive before S
waves, and the further the
seismograph is from the
focus, the further apart the P
and S wave arrival times
are.
2. The media uses the different
velocities and arrival times of
P and S waves to determine
the location of a quake’s
epicenter.
III. Measuring Seismic Waves and Locating Earthquakes
C. Locating the epicenter of
earthquakes
1. P and S waves start out from
the focus at the same time,
but they travel different
speeds
So the P waves arrive before S
waves, and the further the
seismograph is from the
focus, the further apart the P
and S wave arrival times are.
2. The media uses the different
velocities and arrival times of
P and S waves to determine
the location of a quake’s
epicenter.
III. Measuring Seismic Waves and Locating Earthquakes
C. Locating the epicenter of
earthquakes
a. Measure distance
between P and S wave
arrival time
b. Use travel time chart to
determine distance to
city
c. Use compass to draw
distance radius around
city
Need 3 seismograph
stations to determine
epicenter location
III. Measuring Seismic Waves and Locating Earthquakes
C. Locating the epicenter of
earthquakes
a.
Measure distance
between P and S wave
arrival time
b. Use travel time chart to
determine distance to
city
c. Use compass to draw
distance radius around
city
Need 3 seismograph
stations to determine
epicenter location
Question of the week
What is the difference between the earthquake
focus (hypocenter) and epicenter?