Effects of Mixing Brittle and Ductile Material on The Slip Behavior of Finite Thickness Fault Zone
Abstract
Whether a fault generates earthquakes or creeps aseismically is believed to depend on the sign of the friction change resulting from an increase in slip velocity, a-b. Faults that exhibit a decrease in friction are velocity-weakening and may generate earthquakes; those that exhibit an increase in friction are strengthening and predominantly creep aseismically. The sign and magnitude of a-b are determined by laboratory experiments on fault zone rocks. Most models of slip behavior assume that a-b varies mostly with temperature. However geological observations show that fault zone composition varies and often accommodates a mixture of brittle and ductile deformation. It has become clear that the nature of this mixture may play an important role in determining whether the fault creeps steadily, or slips in slow slip events (SSEs) and/or fast earthquakes. Using numerical experiments, we explore how the ratio of brittle to ductile material affects slip behavior in brittle-ductile mixtures of variable composition and amplitude of a-b. We treat brittle material as Mohr-Coulomb elastoplastic and ductile material as Maxwell viscoelastic. We simulate velocity-weakening (a-b<0) behavior in the brittle part of the mixture and velocity-strengthening (a-b≥0) behavior in the ductile part using a rate-and-state formulation dependent on plastic strain accumulation. We show that: (1) mixtures can exhibit multiple slip behaviors including earthquakes and slow slip, (2) highly brittle mixtures do not tend to generate slow slip events while weakly brittle mixtures can generate slow slip over a wider range of compositions, (3) structural features formed during simulated creep, slow slip events and earthquakes share notable similarities with structures observed in natural fault zones. We find that slip-synchronous viscous dissipation in the ductile portion of the mixture controls whether rupture propagates as a fast or slow earthquake. Our results confirm that fault core composition plays a critical role in determining slip behavior.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T22B..03L
- Keywords:
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- 8010 Fractures and faults;
- STRUCTURAL GEOLOGY;
- 8045 Role of fluids;
- STRUCTURAL GEOLOGY;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS