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/

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., 2003Chester & Chester, Tectonophys. 1998

Motivation: In mature fault zones, large displacements (including coseismically) are often localized in granular fault rocks (e.g. gouges)

cohesivecataclasites

incohesivecataclasites

20 m

Localizedslip zone5 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 IThe machine at INGV

SHIVA, Italy, July 2009SHIVA apparatus at

INGV, Rome

5 cm

Axial Load

RotaryMotion

n < 60 MPaSlip 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 σn and 3 m s-1 slip velocity.Ongoing tests with various gouges: calcite, dolomite, quartz, clays

Rotary side50 mm

High velocity “slide-hold-slide” experiments: feedback between localization and strength in fault gouges

0 1 2 2 3 4

SLIDE 1 SLIDE 21 min. HOLD

Slip (m)

Shea

r str

ess

(MPa

)

0

10

5

1 m s-1, 8.5 MPa

SOLID CYLINDERS OFCALCITE MARBLE

Slide 2: Much faster weakening

as in solid rocks

0

1

Slip

vel

ocity

(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

Weakeningphase

Onset ofdynamic weakening

0 1 20.2 m

G2G1

Slip (m)

Shea

r str

ess

(MPa

)

0

10

5

1 m s-1, 8.5 MPa

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30

Str

en

gth

en

ing

ph

ase

(m

)

Normal Stress (MPa)

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30

Str

en

gth

en

ing

ph

ase

(m

)

Normal Stress (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

Stre

ngth

enin

g ph

ase

(m)

Normal Stress (MPa)

0

2

4

6

8

10

0 0.2 0.4 0.6 0.8 1

She

ar

stre

ss (

MP

a)

Slip velocity (m s-1)

Velocity at which weakening initiates (Vcrit) during Slide 1 is relatively high

Slide 1

Vcrit:0.8 m s-1

Slide 2

Vcrit:0.1 m s-1

Weakening

WeakeningStrengthening

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

Strengthenings491

Minimal grain size reduction

Incipient localization to boundary-parallel shear band 100 – 200 µm wide precedes weakening

1 mm

100 µm

Laterally continuous shear bandparallel to layer boundaries (Y-shear)

Fine-grainedshear band

s631: stopped during weakening (0.2 m slip)

s631

8.5 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 wideshear band

10 µm

Cohesive, “welded” layer of calcite grains around micro-slip surface

1 µm

Small pores related to CO2 degassing

s492: stopped at the end of weakening (0.35 m slip)

s492

8.5 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 recrystallizedand 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.