11. Faulting and Earthquake Focal Mechanisms William Wilcock

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11. Faulting and Earthquake Focal Mechanisms
William Wilcock
OCEAN/ESS 410
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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 the LAB).
• Be able to identify polarities and determine a
  focal mechanism solution - LAB

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Normal Fault - Extension
Reverse Fault or Thrust Fault - Compression
Strike-Slip Fault - Horizontal Shear
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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
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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 and may see the first
  in the scientific literature but the second
  option is the easiest for you to use.

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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
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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.

Dilatational (downward) first motion
D
Fault Plane
Compressional (upward) first motion
Auxiliary Plane
C
C
D
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Two orthogonal fault planes known as focal
planes will fit the first motions

• 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

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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 as a visual aid to understand this.

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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

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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

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Wulff Sterographic Projection - Upper Hemisphere
Projection
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Flip upside down for lower hemisphere projection
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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)
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Strike-Slip Focal Mechanism
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Normal Faulting
Cross Section
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Thrust (Reverse) Fault
Compression
Dilatation
Dilatation
Cross Section
Compression
Cross Section
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Confused?

• Do the labs starting with
• Lab 11. Determining a Focal Mechanisms
• Background Reading
• A draft primer on focal mechanism solutions for
  geologists by Vince Cronin
• http//serc.carleton.edu/files/NAGTWorkshops/struc
  ture04/Focal_mechanism_primer.pdf