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? 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 Earth’s Surface Seismograph Earthquake Source Time Function and Mechanism Earth’s 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!
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)
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
Noisy day… (9/29-30/04; same gain setting) Noise Examples… Quiet day… (7/1-2/04) Noisy day… (9/29-30/04; same gain setting)
Noise Comparison – Quiet Day vs Noise Comparison – Quiet Day vs. Noisy Day (Same scale; 10 minute seismograms)
Microseismic Noise (8/12-13/05) Wind Noise (4/2-3/05) Microseismic Noise (8/12-13/05) Closeup ~ 6 s Period
Hurricane Ivan (9/18-19/04) Electronic Noise
Foot Steps (rectangle; 5/27/05) 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)
Amplitude ~ -2000 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