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Stress field producing tensile earthquakes

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

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Stress field producing tensile earthquakes

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  1. Stress field producing tensile earthquakes Tomáš Fischer Institute of Geophysics, Czech Acad. Sci. Faculty of Science, Charles University in Prague

  2. Motivation • fluid injections often show pure shear fractures • hydraulic fractures are often non parallel to s1 • some authors explain by combined shear-tensile fracture • this presentation shows why tensile fractures are so rare

  3. Tensile earthquakes in focal mechanisms tensile events – opening => normal stress must have been negative, σneff<0 => tensile events occupy limited area in Mohr diagrams σneff<0  σneff= σn− p < 0  p > σn (Vavryčuk, 2002)

  4. σ1 45° 60° n 30°  τ 2  σ1 σ3 σ3 σ Mohr circle, 2D => n and acting at any plane lie on the Mohr circle

  5. σ Material strength • Coulomb (1773) – linear strengthf = c + mn shear failure strength cohesion friction • Mohr (1880) – strength envelope is non-linear ! shear fracture shear + tensile fracture τ c 2 tensile (Hydraulic) fracture

  6. Empirical strength envelopes • granites • limestones(Parry, 1995) tensile strength << compression strength 5..10 MPa 50..300 MPa

  7. σ1 σ - p  σ3 Intact rock failure (high σ1 - σ3) τ Big radius = σ1 - σ3 => Mohr circle touches the strength envelope at σ>0 • ISO = 0 • DC = 100% • high t • 45° <  < ~50° c 2 σ3 σ1 p

  8. σ-p Intact rock failure (small σ1 - σ3) τ Small radius = σ1 - σ3 => Mohr circle touches the strength envelope at σ<0 • ISO  0 • DC >0 • small t • ~50° <   90° c p pure tension= HF σ1  σ3  not well constrained, weak dependance of σn on the angle

  9. 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)

  10. How to achieve s < 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

  11. 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

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