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Shear (double couple) mechanisms

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Types of shear faulting

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Stress fields associated with shear faulting

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Polarities of first motions

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Shear faulting mechanisms

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Fault motions in Cartesian coordinate system

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P-wave radiation

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S-wave radiation

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P- & S-wave

radiation patterns

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The focal sphere

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Plotting perpendicular planes on a stereonet

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“Pressure” & “Tension” (P & T) axes

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Focal mechanisms

from waveforms

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Focal mechanisms of earthquakes at spreading ridges and transforms

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Problems •  Violations •  “best fit” result not obtained only a suite of

possible answers •  The double-couple assumption is controlling

the answer

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Non-double-couple sources

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3D mechanisms

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Non-shear geothermal earthquakes

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Varying crack/fault geometry

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The shapes of nodal lines cannot

usually be determined from polarities alone

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Must use P and/or S

amplitudes, P:S amplitude

ratios, or waveforms

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Displaying ratios that may go to zero or infinity

•  Unit-length arrow

•  4-quadrants ���(A & B have signs)

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P:SH amplitude ratios can

constrain nodal surfaces

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P:SH amplitude ratios can

constrain nodal surfaces

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The 9 force couples that comprise the

seismic moment tensor

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Equivalent body forces

for:������

single force���single couple���double couple

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Explosions: triple force dipoles

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Shear fault���Explosion���

Tensile fault���CLVD ���

(Compensated Linear Vector Dipole)

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Radiation patterns

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A theoretical crack

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Decomposing moment tensors

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A crack can be decomposed into an isotropic component and a CLVD

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A CLVD can be decomposed into two orthogonal double couples

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CLVD mechanisms at Long Valley, California

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Long Valley CLVD

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Example of CLVD-type mechanisms: Bardarbunga, Iceland

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Where fault/crack combinations plot Source-type plot

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Coso Weapons Test Site

Coso

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N20˚E dip 75˚

MTs + relative relocations

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34-9RD2

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Successful ground truthing

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Mining disaster

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Societal application