OCEAN/ESS 410
12. Faulting and Earthquake
Focal Mechanisms
William Wilcock
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 beachball 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
Normal Fault Extension
Reverse Fault or
Thrust Fault Compression
Strike-Slip Fault Horizontal Shear
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
Two options for defining
unambiguous strike & dip directions
1. Define Strike so that fault dips to your right
when you are facing the direction of the
strike (e.g., strike = 220°; dip = 55°)
2. 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.
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
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
C
C
D
Compressional
(upward) first
motion
Two orthogonal fault planes known as “focal planes”
will fit the first motions
D
D
C
C
C
C
D
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
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.
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:
1. Looking straight down on it and drawing what we
see (upper hemisphere projection)
or
2. 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
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
Wulff Sterographic Projection - Upper Hemisphere Projection
1
3
2
Flip upside down
for lower
hemisphere
projection
4
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)
Strike-Slip
Focal
Mechanism
Normal Faulting
Cross Section
Cross Section
Thrust (Reverse) Fault
Cross Section
Cross Section
Confused?
Do the labs starting with
Lab 12. Determining a Focal Mechanisms
Background Reading
A draft primer on focal mechanism solutions for
geologists by Vince Cronin
http://serc.carleton.edu/files/NAGTWorkshops/str
ucture04/Focal_mechanism_primer.pdf