Download Notes Earthquakes

Earthquake
Types &
Analysis
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Earthquakes are events of
sudden ground-shaking,
which in many cases can be
very devastating.
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What type of boundaries does
California have?
Northern California
is along a
subduction zone
due to converging
plates. Which
plates are
converging?
Southern California
is along a transform
fault boundary.
What direction are
the 2 plates
moving?
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Background
• Most earthquakes are caused by
sudden movements on faults or by
volcanic eruptions. Of course plate
boundaries are characterized by faults
of different types.
• These sudden movements are
described by the elastic rebound
theory.
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Elastic Rebound Theory
• Energy builds - The
crust will stretch
and deform.
• Release - Build up
is released at
rupture point
• Slippage - The
breaking crust is
felt as vibrations
• Back to normal Energy is released
and the rock
returns to its
normal shape
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3 Types of Faults:
• Reverse faults occur at convergent plate boundaries.
• Normal faults occur at divergent plate boundaries.
• Strike-slip faults occur at transform plate boundaries.
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Reverse Faults
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Normal Fault
Dixie Valley-Fairview Peaks, Nevada
earthquake
December 16, 1954
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Strike Slip Faults
1906 San Francisco Earthquake
Before earthquake manure pile was
under window where farmer shoveled it
out from inside
Fault runs right under corner of barn
After earthquake manure pile moved
over about 10 feet
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SEISMIC WAVES
• Most earthquakes are caused by
sudden movements on faults or by
volcanic eruptions.
• The place where the disturbance
starts below the surface is called
the FOCUS.
• Most foci are restricted to depths
less than about 100 km, except
along subduction zones where
foci can be as deep as 700 km.
• The kinetic energy (energy of
motion) of the disturbance
radiates in all directions away
from the focus in the form of
seismic waves.
• The wavefront first reaches the
surface at the epicenter, which is
directly above the focus, or very
nearly so.
Epicenter
Focus
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How Earthquake vibrations are
measured: The Seismograph
• Seismogram: the
recorded image
from a seismograph
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WAVE TYPES: Body Waves
• BODY WAVES: These can pass into the deeper
layers of the earth, beneath the lithosphere. There
are two types of body waves, P-waves and Swaves. Body waves are NOT responsible for the
ground-shaking during an earthquake.
P-waves: "P" stands for primary. These waves
are the fastest of the wave types. After passing
through the deep interior of the earth and reemerging at the surface, P-waves are the first to
arrive. P-waves are longitudinal waves.
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WAVE TYPES: Body Waves
S-waves: "S" stand for secondary. These are the
second fastest kind of seismic wave. S-waves are
transverse waves. This kind of motion can only be
support by materials that are rigid. For this reason,
S-waves cannot travel through liquids.
After passing through the deep interior of the earth, Swaves re-emerge at the surface only along paths that
do not intersect the core. S-waves that intercept the
core are stopped, the energy is absorbed completely,
and none of the energy reaches the opposite side of
the earth. Thus, a "shadow zone" is formed at the
surface of the earth, defined by the area where Swaves are not detected. These waves helped
scientists determine the structure of the Earth’s core.
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WAVE TYPES: Surface Waves
SURFACE WAVES: These are restricted
to the lithosphere, or surface of the Earth,
and are responsible for most of the actual
ground-shaking. Most of the kinetic
energy of an earthquake is released in the
form of surface waves. In fact practically
all of the damage done by an earthquake
is caused by surface waves. There are 2
types of surface waves.
Two types: L-Waves and R-waves
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WAVE TYPES: Surface Waves
L-waves: "L" comes form the name of the
mathematician, Love, who discovered these
types of waves. However, it's helpful to think
of "L" as standing for "lateral," which relates
to the nature of the resulting groundshaking. The ground moves laterally, or
shimmies side-to-side with no vertical
component. These are the most damaging
type of waves. They can literally move the
ground beneath a building faster than the
building itself can respond, effectively
shearing the base off of the rest of the
building.
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WAVE TYPES: Surface Waves
R-waves: "R" stands for Rayleigh, the
English physicist who first predicted this
kind of seismic movement. Here, it's helpful
to think of "R" as standing for "rolling," which
relates to the nature of the ground-motion,
like the motion of an anchored boat as it
bobs up and down on sea waves.
R-waves behave like sea-waves. As an
R-wave passes beneath a building, for
instance, the building first moves up and is
then set back down.
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VELOCITY
The velocity (speed and direction) of the p-wave and
s-wave are 2 key pieces of information when
determining the event of an earthquake. These 2
important pieces of information used to determine
the place, magnitude, and time of an earthquake.
The velocity of the P-wave is nearly twice the
velocity of the S-wave. At any place on the surface
where both the P-wave and the S-wave can be
detected, there is a delay in time between the arrival
of the P-wave and the arrival of the S-wave.
From the perspective of an observer watching a
seismometer, the first indication that an earthquake
has happened somewhere is the arrival of the Pwave. A short period of time passes before the
arrival of the S-wave.
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Richter Scale
The Richter scale is a scale that
expresses the magnitude of
an earthquake. Each step on
the scale represents a 30 fold
increase in the amount of
energy released.
An earthquake with a magnitude
of 6.0 would be 306 times
more powerful than an
earthquake with a magnitude
of 1.0.
Using the distance and the
amplitude of an earthquake
from 3 earthquake stations,
we can calculate the
magnitude of an earthquake
on the Richter scale.
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#36-39??
•
When looking at a seismogram, we first see the arrival of
the p-wave. The next wave to arrive is the s-wave. The
time in between the arrival of the 2 waves is known as
the s-p interval. This s-p interval is used to calculate the
distance to the epicenter of the earthquake. After the p & s
waves arrive, then the surface waves arrive. These are the
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waves that make the large squiggles on the right.
AMPLITUDE
• Amplitude is the maximum amount of vertical distance
of the seismogram, and reflects the amount of groundshaking. Notice that most of the "ground-shaking" is
associated with the surface waves.
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Determining the Distance to the Epicenter
The red line shows how the S-P interval relates to the distance to
the epicenter.
For each seismic station determine the distance to the epicenter
using the graph.
For each seismic station, find the S-P interval time on the vertical
scale, move across horizontally to the red line, then vertically
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down to the distance scale.
Record your distance for each station in your data table.
The exercise refers to
earthquakes in and
around the Caribbean
basin, an area that
experiences many
earthquakes every year.
The map shows the
locations of
earthquakes
(epicenters) in the
Caribbean region.
Note how many
noteworthy earthquakes
(Magnitude > 3.5)
occurred in just the
twenty-two years from
1967 to 1999.
Note also the relationship
between the epicenters
and plate boundaries.
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BACKGROUND
All three types of plate
boundaries can be found
around the edges of the
Caribbean plate, although
the divergent type of
boundary is restricted to
a very small region west
of Jamaica, just south of
the Cayman Islands. Near
the eastern margin of the
Caribbean plate there are
recently active volcanoes
in the lesser
Antilles. These are
associated with a
convergent plate
boundary and caused by
subduction of the North
American plate beneath
the eastern edge of the
Caribbean plate.
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PRACTICE
Here are 2 seismographs. Find the arrival time, s-p interval, and amplitude for each.
Record these in the data table and answer the questions below:
1. Which seismometer station was closer to the epicenter, A or B? Explain briefly.
2. Which seismogram records the most ground-shaking at the seismic station?
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3. Give two reasons why these seismograms cannot be records of the same earthquake.
Determining the Distance to the Epicenter
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Location of an Earthquake
Once you have
calculated the distance
which the epicenter
occurred from the
location of the
seismograph, then we
can locate the actual
epicenter. We must
have 3 distances to find
the actual location.
This process of using 3
locations is called
triangulation.
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Richter Scale
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