1. 12. Faulting and Earthquake Focal Mechanisms William Wilcock
OCEAN/ESS 410
12. Faulting and Earthquake Focal Mechanisms William Wilcock

2. Lecture/Lab Learning Goals
Know the different styles of faulting and how to specify the orientation and slip direction of a fault.
Understand why the pattern of P-wave first motions divides into 2 compressional and 2 dilitational quadrants.
Understand how we represent the first motion pattern graphically with a beach-ball and how to read beach-ball plots in terms of the two possible fault planes (more practice in LAB 12).
Be able to identify polarities and determine a focal mechanism solution - LAB

3. Normal Fault - Extension
Reverse Fault or Thrust Fault - Compression
Strike-Slip Fault - Horizontal Shear

4. Strike, Dip and Rake
Strike - Direction of line formed by intersection of fault plane and horizontal plane (defined so dip is to right of strike)
Dip - downward inclination of fault plane relative to horizontal
Rake - Direction of motion on fault measured anticlockwise on fault plane from strike direction

5. Two options for defining unambiguous strike & dip directions
Define Strike so that fault dips to your right when you are facing the direction of the strike (e.g., strike = 220°; dip = 55°)
State the dip direction (strike = 040°; dip = 55° to the NW)
You can use either option but the second is probably easiest

6. Body Waves: P-waves
Primary Wave: P wave is a compressional (or longitudinal) wave in which rock (particles) vibrates back and forth parallel to the direction of wave propagation. P-waves are the first arriving wave and have high frequencies but their amplitude tends not to be very large

7. P-wave first motions focal mechanisms
P-waves will radiate in all directions away from a fault. In some directions the first motion of the P-waves will initially be compressional (C) (the earthquake pushes the ground in the direction of motion). In other directions the P-waves will be dilitational (D) (the earthquake pulls the ground away from the direction of wave motion. The dilitational and compressional first motions are divided into quadrants. Seismologist can use this pattern of first motions to infer the orientation of the fault.
= Earthquake
= Seismic wave
Dilatational (downward) first motion D
Fault Plane
Compressional (upward) first motion
Auxiliary Plane C C D

8. Two orthogonal fault planes known as “focal planes” will fit the first motions
Auxiliary Plane
Fault Plane C D
Fault Plane
Auxiliary Plane C D
Fault Plane Solutions obtained from P wave first motions will have this ambiguity. To determine the true fault plane
Use geological understanding to discriminate
Look at aftershocks. They will likely fall on the fault plane
Analyze the full seismic waveform

9. Focal Sphere
An small imaginary sphere surrounding the location where the earthquake first ruptures.
The seismic waves (or rays) traveling from the earthquake to any station will intersect the focal sphere.
The regions of dilitational and compressional motions will divide the focal sphere into four quadrants (orange slices) separated by the fault and auxiliary planes.
You will now get a ping pong ball which you will prepare a visual aid to understand this.

10. Visualizing the focal sphere
You can orient your ping-pong ball to represent the focal sphere
To visualize the focal sphere on a sheet of paper we can imagine:
Looking straight down on it and drawing what we see (upper hemisphere projection) or
Using it as a stamp to make an impression of what is on the bottom half (lower hemisphere projection)
Lower hemisphere projections are more common but you will see both

11. Visualizing Focal Mechanisms
To plot a focal mechanism we use a projection called a Wulff projection. You will be working with these in the exercise

12. Wulff Sterographic Projection - Upper Hemisphere Projection1 2
Flip upside down for lower hemisphere projection 4 3

13. Wulff Steronet with 2° grid
Horizontal Ray in NE direction
Plane dipping down at 50° to the east (upper hemisphere projection) or down at 40° to west (lower hemisphere projection)
Vertical Plane striking north south
Vertical Ray
Ray taking-off to SE and upwards at 45° (upper hemisphere projection) or downwards at 45° (lower hemisphere projection)

14. Strike-Slip Focal Mechanism

15. Normal Faulting
Dilatation
Compression
Cross Section
Cross Section

16. Thrust (Reverse) Fault
Compression
Dilatation
Dilatation
Cross Section
Compression
Cross Section

18. Confused? Do the in lab Lab 10. Determining a Focal Mechanisms
Background Reading
A draft primer on focal mechanism solutions for geologists by Vince Cronin