stress field producing tensile earthquakes
DESCRIPTION
Stress field producing tensile earthquakes. Tomáš Fischer Institute of Geophysics, Czech Acad. Sci. Faculty of Science, Charles University in Prague. Motivation. fluid injections often show pure shear fractures hydraulic fractures are often non parallel to s 1 - PowerPoint PPT PresentationTRANSCRIPT
Motivation
fluid injections often show pure shear fractures
hydraulic fractures are often non parallel to 1
some authors explain by combined shear-tensile fracture
this presentation shows why tensile fractures are so rare
Tensile earthquakes in focal mechanisms
tensile events – opening
=> normal stress must have been negative, σneff<0
=> tensile events occupy limited area in Mohr diagrams
(Vavryčuk, 2002)
σneff<0
σn
eff= σn− p < 0
p > σn
Mohr circle, 2D
=> n and acting at any plane lie on the Mohr circle
τ
σσ1σ3
2
60°45°
30°
2 221 3
1
4n s
Material strength
Coulomb (1773) – linear strength f = c + n
shear failure strength cohesion friction
Mohr (1880) – strength envelope is non-linear !
τ
σ2
tensile (Hydraulic) fracture
c
shear fracture
Empirical strength
envelopes granites
limestones
(Parry, 1995)
tensile strength << compression strength 5..10 MPa 50..300 MPa
Intact rock failure (high σ1
- σ3)Big radius = σ1 - σ3
=> Mohr circle touches the strength envelope at σ>0
ISO = 0 DC = 100% high 45° < < ~50°
σ - p
c
p2
σ3
σ1
σ3 σ1
τ
σ-p
c
p
τ
Intact rock failure (small σ1 - σ3)
Small radius = σ1 - σ3
=> Mohr circle touches the strength envelope at σ<0
ISO 0 DC > 0 small ~50° < 90°
σ3
σ1
not well constrained, weak dependance of σn on the angle
pure tension= HF
Depth dependance of σ1-σ3
KTB, Soultz: SH ~ 2 Sh
SH – Sh = 2/3 Sv
tensile fractures possible at < 1-2 km depths
(Brudy et al., 1997)
depth SH-Sh
100m 1.8 MPa
1 km 18 MPa
10 km 180 MPa
How to achieve < 0
high pore pressure – fluid injections stimulations in gas fields, < 3 km
geothermal reservoir stimulations, < 5 km
earthquake swarms, < 10 km
very small depths (small lithostatic pressure) land/rock slides – extensional regime
=> monitoring of ISO component helpful
Conclusions
strength envelope is non-linear !!
tensile earthquakes occur only if deviatoric stress σ1-σ3 is small enough preexisting fractures strike close to σ1
any tensile fracture shows some shear component
tensile frac occurrence decreases with depth