Stress and Strain in Subduction Earthquake Cycles

1
Stress and Strain inSubduction Earthquake
Cycles

• Kelin Wang
• Pacific Geoscience Centre, Geological Survey of
  Canada
• School of EarthOcean Sciences, University of
  Victoria
• Acknowledgements
• Dragert, He, Hyndman, James, Konomi, Melosh,
  Mulder, Rogers, Sagiya, Suyehiro, Wells, Zhao,
  etc.

2
Stress and Strain inSubduction Earthquake
Cycles

• Weakness of subduction faults
• Stress and stress change
• Interseismic deformation

3
Summary of Stresses

• focal mechanisms, in situ measurements, and most
  geological evidence show consistent stress pattern

• maximum compression is margin-parallel

• margin-normal stress is similar to vertical
  stress ( lithostatic)

4
Summary of Stresses
5
(No Transcript)
6
Nankai Forearc Stresses and geodetic strain
rates are similar to Cascadia
7

• Why is margin-parallel compression large?
• Why is margin-normal stress small?
• Why is geodetic contraction margin-normal?

• Local tectonic environment
• Fundamental process
• Interseismic deformation

8
Why margin-parallel compression?
PA-NA plate motion B R spreading Oblique
subduction Northward forearc motion Canadian
buttress
9
(No Transcript)
10
Why is margin-normal stress small?

• Margin-normal stress controlled by two competing
  factors
• Gravity induces horizontal tension in forearc
• Plate coupling causes compression

11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
Why is geodetic contraction margin-normal?
Geodetic measurements do not detect absolute
elastic stress. They detect changes in elastic
stress.
GREAT earthquake cycles cause small
perturbations to forearc stress.
18
A Stretched Elastic Band
Time 1 Tension
Time 2 Less tension
Contraction
19
(No Transcript)
20
Margin-normal stress perturbation
Margin-parallel compression
21
Margin-normal stress perturbation
Margin-parallel compression
22
Viscoelastic 2-D finite element
23
(No Transcript)
24
(No Transcript)
25
Rupture
After-slip
Stress relaxation
Silent slip
Stress relaxation
26
Viscoelastic deformation model for Cascadia (3-D
spherical finite element)
Model by J. He
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
Cascadia
Nankai
Interseismic strain rates are much larger at
Nankai
31
GPS data by GFZ, Potsdam, Germany Model by Yan Hu
32
Effective transition zone (Wang et al., JGR,
2003)
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
(Courtesy of Dragert and Rogers, PGC) See SSA
abstract by Dragert and Rogers, IUGG abstract by
Dragert, Rogers, and Wang.
37
Summary

• Subduction faults are weak (m lt 0.1)
• Small margin-normal stress
• Low frictional heating
• Great earthquakes cause small stress perturbation
• Interseismic margin-normal contraction
• May modulate forearc seismicity
• Interseismic deformation is time dependent
• Strain rates decrease with time
• Mantle rheology (viscoelastic) vs. fault
  behavior

38
(No Transcript)
39
(No Transcript)
40
Aug. 1999 Transient Displacements vs Long-term
Deformation Motions (both wrt DRAO)