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A stochastic prediction of in situ stress magnitudes from the distributions of rock strength and breakout width at IODP Hole C0002A in Nankai accretionary prism, SW Japan
April 14, 2015
Insun Song1, Chandong Chang2, and Hikweon Lee1
(1) Korea Institute of Geoscience and Mineral Resources(2) Chungnam National University, Korea
Location of study area and tectonic setting
Geologic profile
Breakouts and in situ stress
shmin
shmin
shmax shmax
IODP EXP 314 NanTroSEIZE report
0
5
10
15
0 60 120 180 240 300 360
s qq (
MPa
)
(q º)sH
rockstrengthb b
Strength vs. sonic velocity
Depth
(m
bsf
)
VP (m/s) UCS (MPa)
Counts
VP (m/s)
C0002A 200-230 mbsfAve.=1784.4m/s
(Chang et al., 2009)
1.5
2.0
3.5
4.0
0 60 120 180 240 300 360
s qq (
MPa
)
(q º)sH
bB
Heig
ht
(m)
B
C0002A (200-230mbsf)
Counts
UCS (MPa)
C0002A 200-230 mbsf
Determination of far-field stress magnitudes
b hmax
breakout
B
Average strength vs. average breakout width
Str
ess
(M
Pa)
q (o)
Average strength
Aver
age
wid
th
Strength distribution vs. breakout width distribution
Str
ess
(M
Pa)
q (o)
In situ stress determination algorithmC
ounts
UCS (MPa)
For Sh = 26 MPaDSH = 0.05 MPa
Inte
gra
tion
of
all
mis
fits
SH =
28
.5 M
Pa
B
For Sh = 26 MPaDSH = 0.05 MPa
Cum
ula
ted
den
sity
in h
eig
ht
(m)
Cum
ula
ted
pro
bability
• Assume Gaussian distribution of UCS(P) and mi = 0.6 (Song et al., 2010)
• Using the M-C failure criterion with probability, calculate the probability function of breakout width for given far-field stresses (Shmax and Shmin)
• Compare the probability with breakout density in height
• Select the far-field stress with the minimum misfit
Examples of grid searching method
335 mbsf484 mbsf
1295 mbsf1172 mbsf
845 mbsf
S hmin fr
om L
OT
875 mbsf
Stress profiles
ms =0.6
hyd
rosta
tic
Lithostatic
(Chang et al., 2009)
unconformity
LOP
Stress polygon
C0002A
LOP
Conclusions
• Using the distributions of rock strength and breakout width instead of their averages, we were able to determine both Shmin and SHmax simultaneously.
• In normal faulting stress regime, Shmin is insensitive to error and becomes more sensitive as it goes to the strike-slip faulting stress regime.
• In Kumano forearc basin in situ stress is very close to the limit equilibrium condition for normal faulting.
• Underneath the unconformity the stress condition becomes stable and goes to the strike-slip fault stress regime with increasing depth.
THANK YOU