Strain localization and the onset of dynamic weakening in granular fault gouge Steven Smith 1, Stefan Nielsen 1, Giulio Di Toro 1,2 INGV, Rome 1 ; University of Padova 2 This research is funded by the European Research Council

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

Chester & Goldsby, SCEC Ann. Rpt., 2003 Chester & Chester, Tectonophys Motivation: In mature fault zones, large displacements (including coseismically) are often localized in granular fault rocks (e.g. gouges) cohesive cataclasites incohesive cataclasites 20 m Localized slip zone 5 mm Punchbowl Fault 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

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

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

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

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

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

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 Solid cylinders of calcite marble Slide 2 Strengthening phase (m) Normal Stress (MPa)

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

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

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

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

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

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...?). 100 µm wide shear band 10 µm Cohesive, “welded” layer of calcite grains around micro-slip surface 1 µm Small pores related to CO 2 degassing

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

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)

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.