L Braile, 12282006 revised 91408

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Interpreting Seismograms
L Braile, 12/28/2006 (revised 9/14/08)
http//web.ics.purdue.edu/braile
http//web.ics.purdue.edu/braile/edumod/as1lesson
s/InterpSeis/InterpSeis.ppt

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Interpreting Seismograms - A Tutorial for the
AS-1 Seismograph
http//web.ics.purdue.edu/braile/edumod/as1lesson
s/InterpSeis/InterpSeis.htm
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Introduction Interpreting earthquake
seismograms generally requires considerable
experience and study of seismology. However,
there are some fundamental principles that
provide a basic understanding of seismic wave
propagation and seismogram characteristics.
Furthermore, some experience can be quickly
obtained by systematic study of selected
seismograms illustrating variations in amplitude
and signal character related to source-to-station
distance, the magnitude of the earthquake, and
the earthquakes depth of focus.
Seismic Wave Propagation in the Earth Catalo
g of seismograms at various distances
Catalog of Seismograms for Different Magnitudes

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AmaSeis 24-hour Screen Image
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AmaSeis Extracted Seismogram
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What factors affect the seismogram
that you see on the screen?
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What factors affect the seismogram
that you see on the screen?

• EQ epicenter-to-station distance
• EQ magnitude
• EQ depth (surface waves small or not visible for
  deep focus events depth phases)
  
• EQ mechanism (radiation pattern, freq. range)
• Propagation path (oceanic, continental, mixed)
• Instrument response, filtering
• Noise level
• Seismograph sensitivity and gain
• Site response

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Seismograph
Earthquake Source Time Function and Mechanism
Earths Surface
Surface waves
Near- surface layers
Multiple reflection
Propagation Effects (raypaths and attenuation)
P to S conversion
P
S
Site Response
Sample Teleseismic raypaths
Instrument Response and Filtering
Seismogram
Four wave types (P, S, R, L), wave conversions
(such as P to S), different paths, and multiple
reflections produce complex seismogram!
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M7.5 Oaxaca earthquake seismogram, Earth
structure and approximate P-wave raypath
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P-wave raypaths and wavefronts through the Earth
(after Gutenberg)
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What are the distinctive characteristics
of a seismogram?
(7/26/05 Earthquake)
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What are the distinctive characteristics
of a seismogram?

• Duration of signal
• Impulsive first arrival (P-wave)
• Usually 2 or more separate arrivals
• Distinct shape
• Change in frequency of the
  signal (often seen with
  
  
• S wave and surface waves)
• Signal amplitude tapers
  
  
• off at end (the coda)
• Complexity (cant explain
  every wiggle!)
  

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What are some commonly recorded Noise Sources?
(6/17/05 Earthquake and noise)
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What are some commonly recorded Noise Sources?

• Wind
• Microseisms
• Hurricanes (large microseisms)
• Local noise (trucks, machinery, walking)
• Electronic
• Spikes
• Dropouts

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Noise Examples
Quiet day (7/1-2/04)
Noisy day (9/29-30/04 same gain setting)
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Noise Comparison Quiet Day vs. Noisy Day (Same
scale 10 minute seismograms)
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Wind Noise (4/2-3/05)
Microseismic Noise (8/12-13/05)
Closeup
6 s Period
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Hurricane Ivan (9/18-19/04)
Electronic Noise
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Spike Noise (2/15-16/00)
Foot Steps (rectangle 5/27/05)
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Foot Steps (Close-up)
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Dropout (spike at one point usually at
one hour breaks 2/24/01)
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Dropout (spike at one point usually at
one hour breaks Extracted seismogram) 2/24/01
Amplitude -2000
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Dropout (after median filter 2/24/01)
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Increasing epicenter-to-station distance
Seismograms D (D 9.30o), E (D 14.68o) and F
(D 19.39o).
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Increasing epicenter-to-station distance
Seismograms G (D 24.10o), H (D 29.97o) and I
(D 42.04o).
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Increasing epicenter-to-station distance
Seismograms J (D 51.92o), K (D 61.17o) and L
(D 67.70o).
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Same distance (30o), different magnitudes
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Same distance (30o), different magnitudes
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Same magnitude (6.7), different distance
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Same magnitude (6.7), different distance
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Increasing depth of focus, same distance (65o)
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Increasing depth of focus, same distance (65o)
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Mystery events