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.

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