World Stress Map Conference - International Lithosphere Program ...

Wednesday, October 15 th
Oral Session I: Stress and rock strength measurements – techniques and results
Constraining the magnitude of the maximum horizontal stress
in the Taiwan Chelungpu-fault Drilling Project (TCDP) from
borehole breakout dimensions and rock true triaxial strength
Bezalel Haimson and Haruyuki Oku
University of Wisconsin, Madison, WI, USA
bhaimson@wisc.edu
Weiren Lin
Japan Agency for Marine-Earth Science and Technology, Kochi, Japan
Sheng-Rong Song
Department of Geoscience, National Taiwan University, Taiwan
Borehole breakout traces identified on geophysical logs have been primarily used as an important in
situ stress direction indicator. They provide indispensable data required for the construction of the
World Stress Map. In addition, cross sections of breakout angular spans at vertical borehole perimeters
have been shown to be directly related to the magnitudes of the in situ principal stresses (Song and
Haimson, IJRMMS, 1997). However, the explicit relationship requires knowledge of the rock strength
criterion, because of the reasonable assumption that the points of intersection between breakout and
borehole wall cross section represent the boundary between stable rock on the outside of the breakout
and failed rock on the inside. Equating the correct strength criterion to the state of stress at the points
of intersection, one obtains an equation in terms of the three in situ principal stresses. Two of the in
situ principal stresses are typically determined from the gravitational gradient (σv) and from leak-off
or hydrofracturing tests (σh). Thus, the only unknown variable is the maximum horizontal in situ stress
σH. Vernik and Zoback (JGR, 1992) showed that the commonly used Mohr-Coulomb strength
criterion, which ignores the effect of the intermediate principal stress, was not applicable to borehole
walls, where the local principal stresses are widely differential. Rather, a true triaxial compressive
strength criterion that takes in consideration all three principal stresses is necessary. At the University
of Wisconsin, we had designed and fabricated an apparatus for determining the true triaxial strength of
rectangular prismatic specimens subjected to three unequal principal stresses and used it originally to
obtain a generalized strength criterion for the amphibolite in the KTB scientific ultra deep well,
Germany. Equating the criterion to the state of stress at the points of borehole-breakout intersections
yielded the magnitudes of σH between 3200-6800 m depth (Haimson and Chang, JGR, 2002). The
present paper describes the computation of the σH magnitude in the vicinity of the Chelungpu Fault,
Taiwan, host of the slip zone during the Chi-Chi earthquake (Mw 7.6; 1999). The TCDP scientific
hole B intercepts the Fault at 1136 m. Within the depths of logged breakouts (930-1300 m), the
vertical stress σv as estimated from density logs varied between 22 and 30 MPa. Within the same
depth range, two pairs of leak-off tests, one above and one below the fault, yielded σh magnitudes
between 16 and 24 MPa (Hung et al., Tectonophysics, 2007). A series of true triaxial tests on the
Chinshui siltstone straddling the 30°-east dipping Fault yielded a strength criterion relating the
octahedral shear stress to the mean normal stress on the failure plane:
[ ] 49 . 0
0.
5 ( σ
1 2
2
2
3 ( σ 1 − σ 2)
+ ( σ 2 −σ
3)
+ ( σ 3 −σ
1)
= 5.
66 3 + σ 3)
. The principal stresses σ1>σ2>σ3 at the
breakout-borehole wall intersection are represented by σθ > σz > σr, and these are related to the in situ
principal stresses σH, σh, and σv by the well-known Kirsch solution. By substituting the known σh and
σv at each breakout depth, a non-linear equation was obtained in terms of one unknown: σH. Solving
the equation using Newton’s numerical method yielded estimates of σH varying between 48 and 56
MPa, indicating that the stress condition presently prevailing along the Chelungpu Fault favors strikeslip
faulting, bordering on thrust faulting.
22 3 rd World Stress Map Conference, 15.-17. October 2008 in Potsdam