12.520 Lecture Notes 16
Interseismic: Slip below depth D
� xy
-3
X/D
0
Strain
3
-15 -10 -5
5 10 15
X/D y
Displacement
Figure 16.1
Figure by MIT OCW.
Coseismic: Region above D “catches up”
� xy
-3 0
X/D
Figure 16.2
Figure by MIT OCW.
-15 -10 -5
5 10 15
X/D y
3

Consider a strike-slip fault with displacement S, independent of depth – A screw
“dislocation” – i.e., a slip discontinuity. Original ring (dashed) becomes helix (solid).
1
2
Fault displacement S
D x
3
S/2
S/2
Sur face faul t
Dis loc atio axi s n x
2 x
1
Figure 16.3. Section across a mathematical model of a transcurrent fault.
Figure by MIT OCW.
“Dislocations” are used to describe defects in crystals, as well as fault motions. A crystal disrupted by a screw dislocation is shown in the figure below.

6
5
7
4
8
3 2
9
10
1 b *
Types:
Figure by MIT OCW.
Figure 16.4. A screw dislocation in a cubic lattice constitutes a deformation that is out of the plane of atoms illustrated. The two atoms denotes by solid circles are essentially part of a second plane. The Burgers circuit indicated by the numbered steps naturally moves into this second plane. Therefore in order to close the circuit the Burgers vector b* must be perpendicular to the plane of atoms shown.
Edge
Screw
Figure 16.5
Figure by MIT OCW.

10 b *
9
8
1
10
9
8
1 2
7
2
7
6
3
6
3
4
5
4
5
Figure 16.6 Side view of an edge dislocation in a cubic lattice.
Figure by MIT OCW.
Dislocation motion helps crystals deform (don’t all have to slip at one time). Also helps
Earth deform!
C
D
E
S
A
Figure 16.7 A dislocation.
Figure by MIT OCW.
B

E
C b
B
A
E
C
A
B
E
E b
S
S
S S
Figure 16.8 Upper group: Slip by propagation of an edge dislocation EE. Lower group: Slip by propagation of a screw dislocation SS.
Figure by MIT OCW.

2
1
0
0 2 4 6
Distance from fault (km) NE
8 10
Figure 16.9 Displacement as a function of distance from a transcurrent fault.
Figure by MIT OCW.