Strain localization and the onset of dynamic weakening in granular fault gouge

1. Strain localization and the onset of dynamic weakening in granular fault gouge Steven Smith1, Stefan Nielsen1, Giulio Di Toro1,2 INGV, Rome1; University of Padova2 This research is funded by the European Research Council http://erc.europa.eu/

2. Talk outline • Exploring the rheology of granular fault gouges during rapid (earthquake) shear – motivations • Experimentally reproducing “earthquake-like” slip pulses in fault gouges • Feedback between strain localization and weakening in fault gouges at coseismic slip rates • Conclusions

3. Motivation: In mature fault zones, large displacements (including coseismically) are often localized in granular fault rocks (e.g. gouges) Punchbowl Fault incohesive cataclasites cohesive cataclasites How does dynamic stress evolve in the presence of granular fault rocks during coseismic slip? One approach: experimentally deform granular fault rocks using combined high slip rates and high normal stresses Localized slip zone 5 mm 20 m Chester & Goldsby, SCEC Ann. Rpt., 2003Chester & Chester, Tectonophys. 1998

4. SHIVA, Italy, July 2009 SHIVA apparatus at INGV, Rome Axial Load Rotary Motion Future experimental work I The machine at INGV Standard “solid” samples sn< 60 MPa Slip velocity< 6.5 m s-1 5 cm

5. Purpose-built sample holder for use with incohesive gouges Outer ring Inner ring 65 mm Incohesive gouge (5 g of calcite gouge, <250 µm)

6. Tested up to 30 MPa σn and 3 m s-1 slip velocity. Ongoing tests with various gouges: calcite, dolomite, quartz, clays Stationary side (normal load) Rotary side 50 mm

7. High velocity “slide-hold-slide” experiments: feedback between localization and strength in fault gouges 1 m s-1, 8.5 MPa SLIDE 1 SLIDE 2 1 min. HOLD 10 Slide 1: Prolonged strengthening phase Slide 2: Much faster weakening as in solid rocks Slip velocity 1 Shear stress (MPa) Slip velocity (m s-1) 5 Shear stress 0 0 0 1 2 2 3 4 SOLID CYLINDERS OF CALCITE MARBLE Slip (m)

8. Strengthening phase during Slide 1 accounts in some cases for >50% of experimental “fracture energy” Strengthening phase 10 1 m s-1, 8.5 MPa Onset of dynamic weakening Weakening phase Shear stress (MPa) 5 G1 G2 G2 0 0.2 m 0 1 2 Slip (m)

9. Strengthening phase is shorter at higher normal stress (acceleration the same in each experiment) 2 mm thick gouge layers 1.5 mm thick gouge layers Slide 2 Strengthening phase (m) Solid cylinders of calcite marble Normal Stress (MPa)

10. Velocity at which weakening initiates (Vcrit) during Slide 1 is relatively high Slide 2 Weakening Slide 1 Strengthening Weakening Vcrit: 0.8 m s-1 Vcrit: 0.1 m s-1

11. Starting material: incohesive calcite gouge derived from Carrara marble (<250 µm size fraction) 55 mm 1 mm

12. s491: stopped at end of strengthening phase (0.08 m slip) 8.5 MPa, 1 m s-1 Strengthening s491

13. Incipient localization to boundary-parallel shear band 100 – 200 µm wide precedes weakening Fine-grained shear band 100 µm 1 mm Laterally continuous shear band parallel to layer boundaries (Y-shear) Minimal grain size reduction

14. s631: stopped during weakening (0.2 m slip) 8.5 MPa, 1 m s-1 s631

15. Within 100 µm-wide shear band, multiple short and anastomosing slip surfaces have formed, flanked by “welded” layers of calcite grains (local heating within shear band...?). 10 µm Cohesive, “welded” layer of calcite grains around micro-slip surface Small pores related to CO2 degassing 100 µm wide shear band 1 µm

16. s492: stopped at the end of weakening (0.35 m slip) 8.5 MPa, 1 m s-1 s492

17. Prominent, single slip surface 2-3 µm wide surrounded by a zone of dynamically recrystallized calcite – this slip surface is stable with increasing slip and is reactivated following hold period Slip surface 0.5 mm Zone of dynamically recrystallized and indurated calcite (Smith et al 2013, Geology)

18. Conclusions • Confined calcite gouges show significant early phase of strengthening during acceleration to high velocity (influenced by normal stress, layer thickness, grain size...) • Weakening initiates by formation of a locally hot shear band. Continued localization during weakening leads to a single, discrete slip surface that is reactivated during Slide 2. • In nature, fresh gouge is probably generated during every rupture event by e.g. dynamic fracturing and wear processes. Therefore, localization is expected to be an important process in the dynamic strength evolution of faults.