1. Interpreting Seismograms
L Braile, 12/28/2006 (revised 9/14/08)

2. Interpreting Seismograms - A Tutorial for the AS-1 Seismograph

3. 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: …

4. AmaSeis 24-hour Screen Image

5. AmaSeis
Extracted
Seismogram

6. What factors affect the seismogram that you see on the screen?

7. 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

8. 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!

9. M7.5 Oaxaca earthquake seismogram, Earth structure and approximate P-wave raypath

10. P-wave raypaths and wavefronts through the Earth (after Gutenberg)

11. What are the distinctive characteristics
of a seismogram?
(7/26/05 Earthquake)

12. 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!)

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

14. What are some commonly recorded Noise Sources?
Wind
Microseisms
Hurricanes (large microseisms)
Local noise (trucks, machinery, walking)
Electronic
Spikes
Dropouts

15. 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)

16. Noise Comparison – Quiet Day vs
Noise Comparison – Quiet Day vs. Noisy Day (Same scale; 10 minute seismograms)

17. Microseismic Noise (8/12-13/05)
Wind Noise (4/2-3/05)
Microseismic Noise (8/12-13/05)
Closeup
~ 6 s Period

18. Hurricane Ivan (9/18-19/04)
Electronic Noise

19. Foot Steps (rectangle; 5/27/05)
Spike Noise (2/15-16/00)
Foot Steps (rectangle; 5/27/05)

20. Foot Steps (Close-up)

21. Dropout (spike at one point usually at
one hour breaks; 2/24/01)

22. Amplitude ~ -2000 Dropout (spike at one point usually at
one hour breaks; Extracted seismogram) 2/24/01
Amplitude ~ -2000

23. Dropout (after median filter; 2/24/01)

24. Increasing epicenter-to-station distance
Seismograms D (D = 9.30o), E (D = 14.68o) and F (D = 19.39o).

25. Increasing epicenter-to-station distance
Seismograms G (D = 24.10o), H (D = 29.97o) and I (D = 42.04o).

26. Increasing epicenter-to-station distance
Seismograms J (D = 51.92o), K (D = 61.17o) and L (D = 67.70o).

27. Same distance (~30o), different magnitudes

28. Same distance (~30o), different magnitudes

29. Same magnitude (~6.7), different distance

30. Same magnitude (~6.7), different distance

31. Increasing depth of focus, same distance (~65o)

32. Increasing depth of focus, same distance (~65o)

33. Mystery events