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Stress field producing tensile earthquakes

Stress field producing tensile earthquakes

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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