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L Braile, 12/28/2006 (revised 9/14/08)
Interpreting Seismograms
http://web.ics.purdue.edu/~brailehttp://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?
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
Site Response
Seismogram
SeismographEarth’s Surface
Near-surface layers
PS
Surface waves
Propagation Effects (raypaths and attenuation)
Instrument Response and
Filtering
Four wave types (P, S, R, L), wave conversions (such as P to S), different paths, and multiple reflections produce complex seismogram!
Multiplereflection
P to S conversion
Sample Teleseismicraypaths
Crust
Lithosphere (~100 km thick)
Ma
ntle Lower
Mantle
Outer Core
Upper Mantle
Asthenosphere
Inner Core
Transition Zone(~670 km depth)
Moho (Crust - Mantle boundary, ~35 km depth)
Core - Mantle boundary(~2885 km depth)
Surface
Center of Earth (6371 km depth)
Outer Core - Inner Core
Boundary(~5155 km depth)
*Earthquake
Seismograph
Raypath
16:36:00 16:38:00 16:40:00 16:42:00 16:44:00 16:46:00
Arrival Time (16:37:34)
Start Time of Seismogram
32o
TimeHR:MN:SS(GMT/UTC)
P-wave S-waveOrigin Time(unknown)
BINY LPZ component, 30 Sept. 1999, M7.5 Oaxaca, MX
Vp~8 km/s
Vp~11 km/s
Vp~13 km/s
Vp~9 km/s
Vp~11 km/s
M7.5 Oaxaca earthquake seismogram, Earth structure and approximate P-wave raypath
Travel Time (minutes)
Distance (degrees)
Source
Mantle
Inner Core
Outer core
Wavefronts
Raypaths PPdiffracted
PKPPKIKP
PKiKP
PKPP
P-wave raypaths and wavefronts through the Earth (after Gutenberg)
What are the distinctive characteristicsof a seismogram?
(7/26/05 Earthquake)
What are the distinctive characteristicsof 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)
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).
Increasing epicenter-to-station distance
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