This document provides an overview of topics in seismology, including measuring earthquake faults, fault parameters, focal mechanisms, and waveform modeling. It discusses different types of faults like normal faults, reverse faults, and strike-slip faults. It also covers plotting fault planes based on strike and dip, determining fault types from beach ball representations, and using waveform modeling to better resolve earthquake source parameters and focal mechanisms when first motion data is insufficient.

1. Solid Earth Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Today’s class: Seismology Measuring the Interior Reading: Fowler Chapter 4

2. Earthquake Faults PP. 215-17 of Stein and Wysession, 2003

3. Earthquake Fault Parameters PP. 218 of Stein and Wysession, 2003

4. Directivity of Slip Motion PP. 219 of Stein and Wysession, 2003

5. Earthquake Focal Mechanisms PP. 226 of Stein and Wysession, 2003

6. Earthquake Mechanism Focal mechanism solution for a fault is commonly a lower focal sphere projection. For a dip-slip fault, this projection is equivalent to the compression/dilatation pattern viewed by a bird flying over the earthquake focus . Black = Compression White = Dilatation

7. Cross sections of Dip-Slip Faults The opposite pattern is observed for a normal fault (b) Reverse Fault Normal Fault The focal mechanism solution for a reverse fault (a) has a compression (black) in the inside portion of the circle, surrounded by regions of dilatation (white).

8. Source Parameters Try to write about the parameters: Strike =? Dip of Fault Plane=? Dip of Auxiliary Plane? Inside Portion=? 5 Minutes A=? B1=? B2=? C=?

9. what type of earthquake along what fault orientation is this? Ambiguity: • which is fault plane ? • which is auxiliary plane ? can be either: • right-lateral on EW fault • left-lateral on NS fault 2 Minutes

10. Thrust faults (b): focal mechanism same for both use geological setting to determine most reasonable shaded: compressional (a): W dipping fault (c): E dipping fault also Anderson’s theory : thrusts dip < 45° http://www.seismo.unr.edu/ftp/pub/louie/class/plate/seismology.html

11. Normal faults shaded: compressional (a): W dipping fault (c): E dipping fault (b): focal mechanism same for both use geological setting to determine most reasonable

12. Beach Ball Representation of Typical Faults Fig. 4.2.14 of Stein and Wysession, 2003 Bolt, 1978

13. What type of faulting? A normal faulting mechanism. Which plane do you think is the fault plane ? A reverse faulting mechanism. Which plane do you think is the fault plane ? Strike-slip faulting. Which strike?

14. Fournier and Petit, 2007, JSG www.lgs.jussieu.fr/tectonique/tectonique2/vie_du_labo/pdf/oblique_rifting_JSG_ 2007 .pdf

15. World Stress Map Different colors correspond to different methods of measurement maximum horizontal stress

16. Plotting Planes -1: Strike & Dip Fig. 4.2.10 of Stein and Wysession, 2003

17. To plot a plane striking N45E and dipping 60E, rotate the stereonet (or tracing paper above it) so that the strike is at the top and the dip can be measured along the equator. After plotting the appropriate meridian, rotate the net back to the geographic orientation with north at the top. Plotting Planes : Strike & Dip Strike = N45°E, Dip = 60°, Rake = 70° Fig. 4.2.11 of Stein and Wysession, 2003

18. WAVEFORM MODELING S&W 4.3-11

19. Mechanism has significant dip-slip components but one of other plane has not resolved well . -Additional information is obtained by comparing the observed body and surface waves to theoretical, or synthetic waveforms computed for various source parameters, and finding a model that best fits the data , either by forward modeling or inversion . Why Waveform-Modeling? In brief, waveform analysis also gives information about earthquake depths and rupture processes that can’t be extracted from first motions. It provides about the information about structural parameters as source of faulting and moment magnitude, focal depth. It works also for cased in case FIRST MOTIONS DON’T CONSTRAIN FOCAL MECHANISM due to lack of deployed instruments (say few stations). ? ? ?

20. Regard ground motion recorded on seismogram as a combination of factors: earthquake source earth structure through which the waves propagated Seismometer Create synthetic seismogram as Fourier domain convolution of these effects Synthetic Seismogram: CONVOLUTION