Interpreting Seismograms
L Braile, 12/28/2006 (revised 9/14/08)
http://web.ics.purdue.edu/~braile
http://web.ics.purdue.edu/~braile/edumod/as1lessons/InterpSeis/InterpSeis.ppt
Interpreting Seismograms - A Tutorial
for the AS-1 Seismograph
http://web.ics.purdue.edu/~braile/edumod/as1lessons/InterpSeis/InterpSeis.htm
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 earthquake’s depth of focus.
Seismic Wave Propagation in the Earth:
Catalog of seismograms at various distances:
Catalog of Seismograms for Different Magnitudes:
…
AmaSeis 24-hour Screen Image
AmaSeis
Extracted
Seismogram
What factors affect the seismogram
that you see on the screen?
What factors affect the seismogram
that you see on the screen?
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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
Earthquake Source
Time Function and
Mechanism
Seismograph
Earth’s Surface
Surface
waves
Propagation
Effects (raypaths
and attenuation)
Nearsurface
layers
Multiple
reflection
P
S
P to S
conversion
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!
M7.5 Oaxaca earthquake
seismogram, Earth
structure and
approximate P-wave
raypath
P-wave raypaths and wavefronts through the Earth
(after Gutenberg)
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 (can’t explain
every wiggle!)
What are some commonly recorded
Noise Sources?
(6/17/05 Earthquake and noise)
What are some commonly recorded
Noise Sources?
 Wind
 Microseisms
 Hurricanes (large microseisms)
 Local noise (trucks, machinery, walking)
 Electronic
 Spikes
 Dropouts
Noise Examples…
Quiet day… (7/1-2/04)
Noisy day… (9/29-30/04; same gain setting)
Noise Comparison – Quiet Day vs. Noisy Day
(Same scale; 10 minute seismograms)
Wind Noise (4/2-3/05)
Microseismic Noise (8/12-13/05)
Closeup
~ 6 s Period
Hurricane Ivan (9/18-19/04)
Electronic Noise
Spike Noise (2/15-16/00)
Foot Steps (rectangle; 5/27/05)
Foot Steps (Close-up)
Dropout (spike at one point usually at
one hour breaks; 2/24/01)
Dropout (spike at one point usually at
one hour breaks; Extracted seismogram) 2/24/01
Amplitude ~ -2000
Dropout (after median filter; 2/24/01)
Increasing epicenter-to-station distance
Seismograms D (D = 9.30o), E (D = 14.68o) and F (D = 19.39o).
Increasing epicenter-to-station distance
Seismograms G (D = 24.10o), H (D = 29.97o) and I (D = 42.04o).
Increasing epicenter-to-station distance
Seismograms J (D = 51.92o), K (D = 61.17o) and L (D = 67.70o).
Same distance (~30o), different magnitudes
Same distance (~30o), different magnitudes
Same magnitude (~6.7), different distance
Same magnitude (~6.7), different distance
Increasing depth of focus, same distance (~65o)
Increasing depth of focus, same distance (~65o)
Mystery events