Lab 2
Seismogram Interpretation
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• Refraction Seismology – Refraction experiments are based on the
times of arrival of the initial ground movement generated by a source
recorded at a variety of distances. Thus the data set consists of a series of
times versus distances.
• Reflection Seismology – In reflection experiments, analysis is
concentrated on energy arriving after the initial ground motion. The
analysis focuses on the waves reflected from the subsurface layers.
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Elastic waves
• Amplitude is the peak to trough height divided by two.
• Wavelength is the distance over which the wave goes through one complete cycle.
• Period is the time over which the wave is observed to complete a single cycle.
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Snell’s Law
• Snell’s Law governs the path by which a wave
would take the least amount of time to propagate
between two fixed points.
• In this case, the velocity of the overlying layer is
less than that of the underlying layer.
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Seismic Velocity
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Seismic velocity is a material property (like density).
There are two kinds of waves – Body and Surface waves.
There are two kinds of body wave velocity – P and S wave velocities.
P waves always travel faster than S waves.
Seismic velocities depend on quantities like chemical composition,
pressure, temperature, etc.
Faster Velocities
Slower Velocities
• Lower temperatures
• Higher temperatures
• Higher pressures
• Lower pressures
• Solid phases
• Liquid phases
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Seismic Velocity and Travel Times
• A common method to determine the structure of the earth’s interior is to
analyze the variations in the travel times of seismic waves:
– Earthquake occurs and generates seismic waves
– Earthquake is identified and located
– Travel times of seismic waves are compared to a reference model
– Anomalous travel times are converted to heterogeneities inside the earth (inverse
modeling)
• Travel time of a seismic wave is the time taken to travel from the hypocenter
of the earthquake to the seismometer
• Seismologists use the travel time curve to identify seismic phases (P, S, etc.)
by determining when they will arrive on a seismogram given how far away
the earthquake epicenter is from the station. It can also be used in reverse.
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Seismic Velocity-Depth Model for Whole Earth
• Pressure and temperature increase
as we go deeper into the earth
• These have opposite effects on
seismic velocity
• Seismic velocity tends to increase
with depth (increasing pressure)
• Exceptions include regions of
partial melt (LVZ-asthenosphere),
and total melt (the outer core)
•What is the relationship of seismic
velocities with density?
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Possible Ray Paths for Seismic Waves Penetrating the Earth
• In the mantle and inner core, the
velocities increase with depth, so
the ray bend away from the normal
• At the mantle-outer core (fluid)
boundary the decrease in velocity
causes those rays refracted into the
core to bend towards the normal
• What is mode conversion?
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Seismogram Example
P wave
S wave
Pick two seismic phases and measure the time interval
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Jeffreys-Bullen Travel Time Curve for Earthquake Focus at the Surface
Slide right or left on the graph below until that amount
of time (vertical axis) fits on the curves representing
the two seismic phases identified on the seismogram
Epicentral distance (1 degree = 111 km) is the angle,
subtended by the earthquake epicenter and
seismometer, at the center of the earth (Bullen and
Bolt, 1985)
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Surface Waves
• Surface waves propagate along the earth’s surface.
• Surface waves are larger in amplitude and longer in duration than body
waves.
• Surface waves propagate at a speed lower than body waves and are
recorded after the P and S waves.
• There are two types of surface waves: Rayleigh and Love waves.
• Rayleigh waves are denoted by LR or R, and Love waves are denoted by
LQ or Q (L for long; R for Rayleigh; Q for Querwellen, German,
‘transverse waves’).
• Surface wave amplitudes decays exponentially with depth.
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Surface Wave Dispersion
• Surface waves are dispersive, which means that their velocities depend on
frequency.
• Longer wavelength (longer-period, lower-frequency) surface waves
contain more information about the deep velocity structure.
• Shorter wavelength (shorter-period, higher-frequency) surface waves
provide information about the shallow structure.
• The first surface wave energy to arrive at any seismometer is of those
frequencies that have the greatest velocities.
• The other frequencies will arrive later according to their frequencies.
• So, seismometers at increasingly greater distances from an earthquake
record surface waves that are increasingly spread out.
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Seismogram Example
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Group Velocity of Surface Waves
• The velocity with which a surface wave energy associated with a particular
frequency travels is called the group velocity (Delta/Time).
• The group velocity of surface waves is constant for a given frequency.
• Generally, Love wave group velocities are greater than Rayleigh wave
group velocities, which means that on seismograms Love waves usually
arrive before Rayleigh waves.
• A plot of velocity against period is called the dispersion curve.
• Dispersion curves contain much information about the velocity structure of
the crust and upper mantle.
• A linearized inversion technique, is often used to obtain a velocity-depth
structure appropriate for particular dispersion curves.
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Comparison of Refraction and Reflection Methods
Refraction Methods
Reflection Methods
Advantage
Disadvantage
Advantage
Disadvantage
Refraction observations generally
employ fewer source and
receiver locations and are thus
relatively cheap to acquire.
Refraction seismic observations
require relatively large sourcereceiver offsets (distances
between the source and where
the ground motion is recorded,
the receiver).
Reflection seismic observations are
collected at small sourcereceiver offsets.
Because many source and receiver
locations must be used to
produce meaningful images of
the Earth's subsurface,
reflection seismic observations
can be expensive to acquire.
Little processing is done on refraction
observations with the
exception of trace scaling or
filtering to help in the process
of picking the arrival times of
the initial ground motion.
Refraction seismic only works if the
speed at which motions
propagate through the Earth
increases with depth.
Reflection seismic methods can work
no matter how the speed at
which motions propagate
through the Earth varies with
depth.
Refraction seismic observations are
generally interpreted in terms
of layers. These layers can
have dip and topography.
Reflection seismic observations can
be more readily interpreted in
terms of complex geology.
Reflection seismic processing can be
very computer intensive,
requiring sophisticated
computer hardware and a
relatively high-level of
expertise. Thus, the
processing of reflection
seismic observations is
relatively expensive.
Refraction seismic observations only
use the arrival time of the
initial ground motion at
different distances from the
source (i.e., offsets).
Reflection seismic observations use
the entire reflected wavefield
(i.e., the time-history of ground
motion at different distances
between the source and the
receiver).
A model for the subsurface is
constructed by attempting to
reproduce the observed arrival
times.
The subsurface is directly imaged
from the acquired observations
Because such a small portion of the
recorded ground motion is
used, developing models and
interpretations is no more
difficult than our previous
efforts with other geophysical
surveys.
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Because of the overwhelming amount
of data collected, the possible
complications imposed by the
propagation of ground motion
through a complex earth, and
the complications imposed by
some of the necessary
simplifications required by the
data processing schemes,
interpretations of the reflection
seismic observations require
more sophistication and
knowledge of the process.
Earthquakes and Aftershocks
• An earthquake occurs as the result of a slow build up of strain
(deformation) in rock, usually caused by the relative motion of adjacent
plates.
• When a fault or volume of rock can no longer resist movement, the stored
strain energy is released, causing an earthquake.
• A strong earthquake is generally followed by a sequence of aftershocks,
which can continue for months.
• The aftershocks occur during a period of readjustment of the earth
following the main shock, in which small localized strains on the fault are
released.
• Deep focus earthquake usually do not have aftershocks.
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