1
How Deep Below the Seismogenic Zone
do Large Earthquakes Rupture?
Bruce Shaw
Columbia University
2
Faults and Stability of Sliding
[Scholz, 1998]
3
Slip-Length Scaling
L [km]
Aspect ratio, L/W
0
4
8
0.8
2
6
4
D
1
Slip
10
Mean slip, m, interplate
Mean slip, m, intraplate
Slip
12
20
100
200
300
400
500
4
3
0.6
2
[m]
10
0.4
intraplate
interplate
2
0
0
100
200
300
400
Length, km
1
0.2
500
0
0
10
20
L/H
Length / H
Length / H
Earthquakes
Model
30
0
[Shaw and Scholz, 2001]
4
surface slip
surface slip
3D Model
0.4
0.2
0
65
70
75
80
85
90
95
distance along fault / W
vertical exaggeration 8:1
unstable
sliding
slip
peak velocity
initial displacement
stable
sliding
5
Slip-Length Scaling
1.4
1.2
Slip
1
0.8
0.6
0.4
0.2
0
0
20
40
60
80
Length / H
• Blue dashed line fit for W = 3H
[H≡seismogenic depth]
6
Velocities During Slip Events
0.8
velocity
0.6
0.4
0.2
0
−0.2
−0.4
75
80
85
90
95
100
time
• High and low frequencies at seismogenic depths
• Only low frequencies below seismogenic depths
• Coseismic slip at depth
105
7
GPS Inversion Ambiguity
1836
G. C. P. King and S. G. Wesnousky
(a)
(b)
Fault slip
0
1
2
3
4
5
6
7
8
0
2.0
2.0
4.0
4.0
6.0
6.0
8.0
8.0
10.0
10.0
12.0
12.0
14.0
14.0
16.0
16.0
18.0
18.0
20.0
20.0
22.0
22.0
24.0
26.0
Fault slip
0
0
1
2
3
4
5
6
7
8
24.0
Depth
26.0
(c)
Depth
4.0
3.0
2.0
1.0
0
-1.0
-2.0
-3.0
-4.0
-100
-50
0
50
100
Figure 5.
The (a) box and (b) tapered displacement functions produce (c) the same deformation field (circles and lines) at the earth’s
surface. The vertical axis is meters of surface displacement, and the horizontal axis is distance in kilometers from the fault. The downward
pointing arrow indicates the average value of slip for the tapered function.
[King and Wesnousky, 2007]
[Page et al, 2008]
Ambiguous depth resolution
Poor resolution for Parkfield
8
Seismic Inversion Ambiguity
Inversions for slip in 2004 M6.0 Parkfield earthquake
(images from [Mai, 2007] database)
• Big differences even in extremely well networked event
9
Exhumed Rocks: Fast Deep Slip
from Pseudotachylytes Crosscutting Mylonites
[Lin, 2008]
10
Sliding Rate Effect on Lab Friction
[Blanpied et al, 1995]
• Unstable sliding deepens at faster sliding rates
• Potentially big effect
Deep Moment / Seismogenic Moment
11
Moment at Depth
1
0.8
0.6
0.4
0.2
0
0
20
40
60
80
Length / H
• Half of seismogenic moment, or 1/3 total moment at depth
12
Effective Rupture Width: Extrapolate to zero slip
14
12
0.5
Slip
Log10 Slip
1
0
−0.5
10
8
6
−1
4
−1.5
2
−2
0
2
4
depth / H
Logarithmic fit
6
0
0
1
2
3
4
depth / H
Linear fit
5
6
13
Stress Drop
Stress Drop
0.5
0.4
0.3
0.2
0.1
0
0
20
40
60
80
Length / H
• Macroscopic estimates with Effective W (+) matches directly
measured (x) stress drop
14
Magnitude-Area Scaling
[Shaw, 2007]
9
Magnitude
8
7
6
5
1
2
3
4
2
Log10 Area [km2 ]
m = log10 A + 23 log10
!
A
)
H2
(1, A2 ))/2
H β
max(1,
(1+max
5
DATA [Hanks & Bakun, 2007]
[Wells & Coppersmith, 1996]
[Ellsworth "B", 2003]
[Hanks & Bakun, 2002]
[Shaw, 2008]
+ const
• Constant stress drop scaling across whole magnitude range
• Best fit parameters H = 15km, β = 6.9 → W ∼ 35km
15
Conclusions
• Geological, Seismological, Geodetic, Laboratory
Observations allow deep slip, but do not require it
• Theoretical work suggests it may be occuring
• Direct observations needed:
Perhaps subduction zones?