12. Faulting and Earthquake Focal Mechanisms

William Wilcock

OCEAN/ESS 410

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

Reverse Fault or Thrust Fault - Compression

Normal Fault - Extension

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 mechanismsP-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.

Auxilia

ry P

lane

Fault PlaneCompressional (upward) first motion

Dilatational (downward)

first motion

C C

D

D

= Earthquake

= Seismic wave

Two orthogonal fault planes known as “focal planes”

will fit the first motions

Auxilia

ry P

lane

Fault Plane

C C

D

D

Fault

PlaneAuxiliary Plane

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)or2. 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 2

3 4

Flip upside down for lower hemisphere projection

Vertical Ray

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

Ray taking-off to SE and upwards at 45° (upper hemisphere projection) or downwards at 45° (lower hemisphere projection)

Wulff Steronet with 2° grid

Strike-Slip Focal Mechanism

Normal Faulting

Cross Section

Dilatation

Dilatation

Compression

Compression

Cross Section

Compression

Compression

Dilatation

DilatationCross Section

Thrust (Reverse) Fault

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/structure04/Focal_mechanism_primer.pdf