1 Shear (double couple) mechanisms 2 Types of shear faulting 3 Stress fields associated with shear faulting
1
Shear (double couple) mechanisms
2
Types of shear faulting
3
Stress fields associated with shear faulting
2
4
Polarities of first motions
5
Shear faulting mechanisms
6
Fault motions in Cartesian coordinate system
3
7
P-wave radiation
8
S-wave radiation
9
P- & S-wave
radiation patterns
4
10
The focal sphere
11
Plotting perpendicular planes on a stereonet
12
“Pressure” & “Tension” (P & T) axes
5
13
Focal mechanisms
from waveforms
14
Focal mechanisms of earthquakes at spreading ridges and transforms
15
Problems • Violations • “best fit” result not obtained only a suite of
possible answers • The double-couple assumption is controlling
the answer
6
Non-double-couple sources
17
3D mechanisms
18
Non-shear geothermal earthquakes
7
19
Varying crack/fault geometry
20
The shapes of nodal lines cannot
usually be determined from polarities alone
21
Must use P and/or S
amplitudes, P:S amplitude
ratios, or waveforms
8
22
Displaying ratios that may go to zero or infinity
• Unit-length arrow
• 4-quadrants ���(A & B have signs)
23
P:SH amplitude ratios can
constrain nodal surfaces
24
P:SH amplitude ratios can
constrain nodal surfaces
9
25
The 9 force couples that comprise the
seismic moment tensor
26
Equivalent body forces
for:������
single force���single couple���double couple
27
Explosions: triple force dipoles
10
Shear fault���Explosion���
Tensile fault���CLVD ���
(Compensated Linear Vector Dipole)
29
Radiation patterns
30
A theoretical crack
11
Decomposing moment tensors
32
A crack can be decomposed into an isotropic component and a CLVD
33
A CLVD can be decomposed into two orthogonal double couples
12
34
CLVD mechanisms at Long Valley, California
35
Long Valley CLVD
36
Example of CLVD-type mechanisms: Bardarbunga, Iceland
13
37
Where fault/crack combinations plot Source-type plot
38
Coso Weapons Test Site
Coso
39
14
40
41
N20˚E dip 75˚
MTs + relative relocations
42
34-9RD2
15
43
Successful ground truthing
44
Mining disaster
45
Societal application