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Mr. Litaker
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Seismology
Charles Richter, inventor of the
Richter Scale. (Image courtesy of
Caltech)
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Seismology
is the study
of
earthquakes
and the
structure of
the earth,
by both
naturally and
artificially
generated
means.
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Developed in 1935 by Charles Richter in collaboration with Beno Gutenberg
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Ring of Fire
The "Ring of Fire",
also called the CircumPacific belt, is the
zone of earthquakes
surrounding the
Pacific Ocean--about
90% of the world's
earthquakes occur
there. The next most
seismic region (5-6%
of earthquakes) is the
Alpide belt (extends
from Mediterranean
region, eastward
through Turkey, Iran,
and northern India. 3
http://neic.usgs.gov/neis/eqlists/10maps_world.html
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An Earthquake is caused by sudden
release of elastic energy stored in
rocks.
Elastic deformation: Any change
of shape or size that disappears
when the deforming forces are
removed.
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The Earth's surface is broken. Cracks in the Earth's crust known as
faults can run for hundreds of kilometers. These faults are
frequently the sites of major earthquakes as the tectonic plates that
cover the surface of the Earth shift. Pictured above is San Andreas
Fault in California, one of the longest and most active faults
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Parkfield
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The Parkfield experiment
http://earthquake.usgs.gov/eqinthenews/2004/nc51147892/
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Shaded relief map of central California showing the
situation of the Carrizo Plain in the Coast Ranges. The
orange line shows the surface trace of the SAF, and is
thicker along the portion that is inferred to have
ruptured in the 1857 earthquake [Sieh, 1978]. The blue
line is adjacent to the Parkfield Segment (PS) of the
SAF, while the red line is adjacent to the Cholame
Segment (CS
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The Dragon's Back Pressure Ridge
This large, elongate ridge (3 km along the SAF and a maximum of 500 m
wide perpendicular to the SAF) is bounded on the far side by the San
Andreas fault. View direction is east-southeast. The pressure ridge is part
of a complex of similar kilometer-scale ridges elongate parallel to the
SAF in the southern Carrizo Plain.
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Earthquakes


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Earthquakes occur as sections of
the Earth’s crust slip past one
another. Friction is a force that
prevents these plates from sliding
smoothly.
Friction – is the force that
opposes motion between two
surfaces that are touching.
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Origination of an Earthquake


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Focus – Source of the
Earthquake that originates
in the interior of the Earth.
Site of 1st Movement !
Epicenter – The point
directly above the focus
where the energy hits the
surface.
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Hypocenter
The hypocenter is
the point within the
earth where an
earthquake rupture
starts. The
epicenter is the
point directly above
it at the surface of
the Earth. Also
commonly termed
the focus.
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When an earthquake
occurs, the elastically
stored energy is
carried outward from
the focus to other
parts of the Earth by
vibrations. These
vibrations are called
seismic waves and
spread out
spherically in all
directions, just as
sound waves do.
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Fault
A fault is a fracture along which the blocks of crust on either
side have moved relative to one another parallel to the
fracture.
Strike-slip
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.
Strike-slip faults
Dip-slip
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, whereas 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° or less.
Oblique-slip faults have significant components of different
slip
styles.
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Fault plane
The fault plane is the planar (flat) surface along which there
is slip during an earthquake
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Foreshocks
Foreshocks are relatively smaller earthquakes that precede
the largest earthquake in a series, which is termed the
mainshock. Not all mainshocks have foreshocks.
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Real Time Earthquakes
http://quake.wr.usgs.gov/recenteqs/latest.
htm
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Seismic Waves
Body Waves: Travel outward in all
directions from the focus and have the
capacity to travel through the Earth's
interior.
P (primary) waves – Longitudinal
waves move by compression/expansion
S (secondary) waves - shear
Surface Waves: Travel around but not
through the Earth; they are guided by the
Earth's surface.
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Body Waves
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Body Wave
P Waves in Motion
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Body Wave
S wave in motion
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Surface Wave
L wave in motion
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TRAVEL
TIME
OF
SEISMIC
WAVES
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REFRACTION
AND
REFLECTION
OF BODY
WAVES
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P waves are the fastest wave
S waves are slightly slower
L waves are the slowest of the
three waves
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A highly simplified simulated recording of earthquake
waves (a seismogram). This seismogram is a simulation.
The actual records of earthquake waves are far more
complicated than what is presented here
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Magnitude
The magnitude is a
number that characterizes
the relative size of an
earthquake. Magnitude is
based on measurement of
the maximum motion
recorded by a
seismograph. Several scales
have been defined, but the
most commonly used are (1)
local magnitude (ML),
commonly referred to as
"Richter magnitude," (2)
surface-wave magnitude (Ms),
(3) body-wave magnitude (Mb),
and (4) moment magnitude
(Mw). Scales 1-3 have limited
range and applicability and do
not satisfactorily measure the
size of the largest
earthquakes.
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Richter scale
The Richter magnitude
scale was developed in
1935 by Charles F.
Richter of the California
Institute of Technology
as a mathematical device
to compare the size of
earthquakes. The
magnitude of an
earthquake is
determined from the
logarithm of the
amplitude of waves
recorded by
seismographs.
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Adjustments are included for the variation in the
distance between the various seismographs and the
epicenter of the earthquakes. On the Richter Scale,
magnitude is expressed in whole numbers and
decimal fractions. For example, a magnitude 5.3
might be computed for a moderate earthquake, and
a strong earthquake might be rated as magnitude
6.3. Because of the logarithmic basis of the scale,
each whole number increase in magnitude represents
a tenfold increase in measured amplitude; as an
estimate of energy, each whole number step in the
magnitude scale corresponds to the release of about
31 times more energy than the amount associated
with the preceding whole number value.
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Intensity
The intensity is a number
(written as a Roman numeral)
describing the severity of an
earthquake in terms of its
effects on the earth's
surface and on humans and
their structures. Several
scales exist, but the ones
most commonly used in the
United States are the
Modified Mercalli scale and
the Rossi-Forel scale. There
are many intensities for an
earthquake, depending on
where you are, unlike the
magnitude, which is one
number for each earthquake.
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Moho
The Moho is the boundary
between the crust and the
mantle in the earth. This is a
depth where seismic waves
change velocity and there is also
a change in chemical
composition. Also termed the
Mohorovicic' discontinuity after
the Croatian seismologist
Andrija Mohorovicic' (18571936) who discovered it. The
boundary is between 25 and 60
km deep beneath the continents
and between 5 and 8 km deep
beneath the ocean floor.
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http://pasadena.wr.usgs.gov/shake/
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