Dilatant strengthening as a mechanism for slow slip events

Paul Segall, Allan Mattathias Rubin, Andrew M. Bradley, James R. Rice

Research output: Contribution to journalArticle

163 Citations (Scopus)

Abstract

The mechanics of slow slip events (SSE) in subduction zones remain unresolved. We suggest that SSE nucleate in areas of unstable friction under drained conditions, but as slip accelerates dilatancy reduces pore pressure p quenching instability. Competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast. We model SSE with 2-D elasticity, rate-state friction, and a dilatancy law where porosity φ evolves toward steady state φss over distance dc and φss = φ0 + ε ln(v/v0); v is slip speed. We consider two diffusion models. Membrane diffusion (MD) is approximated by -(p - p)/tf where p and p are shear zone and remote pore pressure and tf is a characteristic diffusion time. Homogeneous diffusion (HD) accurately models fault-normal flow with diffusivity chyd. For MD, linearized analysis defines a boundary ε = 1 - a/b between slow and fast slip, where ε ≡ f 0ε/βb(σ - p), f0, a, and b are friction parameters and β is compressibility. When ε < 1 - a/b slip accelerates to instability for sufficiently large faults, whereas for ε > 1 - a/b slip speeds remain quasi-static. For HD, Ep εh/( β(σ - p)√v/c hyddc) defines dilatancy efficiency, where h is shear zone thickness and v is plate velocity. SSE are favored by large εh and low effective stress. The ratio Ep to thermal pressurization efficiency scales with 1/(σ - p), so high p favors SSE, consistent with seismic observations. For Ep ∼ 10-3 transient slip rates, repeat times, average slip, and stress drops are comparable to field observations. Model updip propagation speeds are comparable to those observed along-strike. Many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress. Model SSE accommodate only a fraction of plate motion; the remaining deficit must be accommodated during coseismic or postseismic slip.

Original languageEnglish (US)
Article numberB12305
JournalJournal of Geophysical Research: Solid Earth
Volume115
Issue number12
DOIs
StatePublished - Dec 1 2010

Fingerprint

slip
friction
Pressurization
Pore pressure
shear stress
Friction
dilatancy
drained conditions
heat
compressibility
Membranes
elasticity (mechanics)
mechanics
diffusivity
Strengthening (metal)
porosity
Compressibility
pore pressure
shear zone
Quenching

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Cite this

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title = "Dilatant strengthening as a mechanism for slow slip events",
abstract = "The mechanics of slow slip events (SSE) in subduction zones remain unresolved. We suggest that SSE nucleate in areas of unstable friction under drained conditions, but as slip accelerates dilatancy reduces pore pressure p quenching instability. Competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast. We model SSE with 2-D elasticity, rate-state friction, and a dilatancy law where porosity φ evolves toward steady state φss over distance dc and φss = φ0 + ε ln(v/v0); v is slip speed. We consider two diffusion models. Membrane diffusion (MD) is approximated by -(p - p∞)/tf where p and p∞ are shear zone and remote pore pressure and tf is a characteristic diffusion time. Homogeneous diffusion (HD) accurately models fault-normal flow with diffusivity chyd. For MD, linearized analysis defines a boundary ε = 1 - a/b between slow and fast slip, where ε ≡ f 0ε/βb(σ - p∞), f0, a, and b are friction parameters and β is compressibility. When ε < 1 - a/b slip accelerates to instability for sufficiently large faults, whereas for ε > 1 - a/b slip speeds remain quasi-static. For HD, Ep εh/( β(σ - p∞)√v∞/c hyddc) defines dilatancy efficiency, where h is shear zone thickness and v∞ is plate velocity. SSE are favored by large εh and low effective stress. The ratio Ep to thermal pressurization efficiency scales with 1/(σ - p∞), so high p∞ favors SSE, consistent with seismic observations. For Ep ∼ 10-3 transient slip rates, repeat times, average slip, and stress drops are comparable to field observations. Model updip propagation speeds are comparable to those observed along-strike. Many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress. Model SSE accommodate only a fraction of plate motion; the remaining deficit must be accommodated during coseismic or postseismic slip.",
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Dilatant strengthening as a mechanism for slow slip events. / Segall, Paul; Rubin, Allan Mattathias; Bradley, Andrew M.; Rice, James R.

In: Journal of Geophysical Research: Solid Earth, Vol. 115, No. 12, B12305, 01.12.2010.

Research output: Contribution to journalArticle

TY - JOUR

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AU - Segall, Paul

AU - Rubin, Allan Mattathias

AU - Bradley, Andrew M.

AU - Rice, James R.

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N2 - The mechanics of slow slip events (SSE) in subduction zones remain unresolved. We suggest that SSE nucleate in areas of unstable friction under drained conditions, but as slip accelerates dilatancy reduces pore pressure p quenching instability. Competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast. We model SSE with 2-D elasticity, rate-state friction, and a dilatancy law where porosity φ evolves toward steady state φss over distance dc and φss = φ0 + ε ln(v/v0); v is slip speed. We consider two diffusion models. Membrane diffusion (MD) is approximated by -(p - p∞)/tf where p and p∞ are shear zone and remote pore pressure and tf is a characteristic diffusion time. Homogeneous diffusion (HD) accurately models fault-normal flow with diffusivity chyd. For MD, linearized analysis defines a boundary ε = 1 - a/b between slow and fast slip, where ε ≡ f 0ε/βb(σ - p∞), f0, a, and b are friction parameters and β is compressibility. When ε < 1 - a/b slip accelerates to instability for sufficiently large faults, whereas for ε > 1 - a/b slip speeds remain quasi-static. For HD, Ep εh/( β(σ - p∞)√v∞/c hyddc) defines dilatancy efficiency, where h is shear zone thickness and v∞ is plate velocity. SSE are favored by large εh and low effective stress. The ratio Ep to thermal pressurization efficiency scales with 1/(σ - p∞), so high p∞ favors SSE, consistent with seismic observations. For Ep ∼ 10-3 transient slip rates, repeat times, average slip, and stress drops are comparable to field observations. Model updip propagation speeds are comparable to those observed along-strike. Many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress. Model SSE accommodate only a fraction of plate motion; the remaining deficit must be accommodated during coseismic or postseismic slip.

AB - The mechanics of slow slip events (SSE) in subduction zones remain unresolved. We suggest that SSE nucleate in areas of unstable friction under drained conditions, but as slip accelerates dilatancy reduces pore pressure p quenching instability. Competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast. We model SSE with 2-D elasticity, rate-state friction, and a dilatancy law where porosity φ evolves toward steady state φss over distance dc and φss = φ0 + ε ln(v/v0); v is slip speed. We consider two diffusion models. Membrane diffusion (MD) is approximated by -(p - p∞)/tf where p and p∞ are shear zone and remote pore pressure and tf is a characteristic diffusion time. Homogeneous diffusion (HD) accurately models fault-normal flow with diffusivity chyd. For MD, linearized analysis defines a boundary ε = 1 - a/b between slow and fast slip, where ε ≡ f 0ε/βb(σ - p∞), f0, a, and b are friction parameters and β is compressibility. When ε < 1 - a/b slip accelerates to instability for sufficiently large faults, whereas for ε > 1 - a/b slip speeds remain quasi-static. For HD, Ep εh/( β(σ - p∞)√v∞/c hyddc) defines dilatancy efficiency, where h is shear zone thickness and v∞ is plate velocity. SSE are favored by large εh and low effective stress. The ratio Ep to thermal pressurization efficiency scales with 1/(σ - p∞), so high p∞ favors SSE, consistent with seismic observations. For Ep ∼ 10-3 transient slip rates, repeat times, average slip, and stress drops are comparable to field observations. Model updip propagation speeds are comparable to those observed along-strike. Many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress. Model SSE accommodate only a fraction of plate motion; the remaining deficit must be accommodated during coseismic or postseismic slip.

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