The frictional properties and deformation mechanisms of faults in near-surface, poorly lithified sediments: implications for rupture propagation in the shallow crust

Abstract:

To improve our understanding of the factors controlling rupture propagation to the surface it is critical to constrain the frictional properties and deformation mechanisms of shallow crustal fault zones in poorly lithified sediments.

We performed a set of rotary shear friction experiments on clay-rich, carbonate synthetic gouges, from the seismogenic Masada (Dead Sea) fault zone in Israel. The experiments were run at low- (0-130 μm/s) and high-velocity (1.3 m/s), at normal stress of 1-18 MPa (equivalent to a depth range of 0.05-1 km), and under room-humidity, water-saturated and brine-saturated conditions.

During rate and state, low-velocity experiments at 1 MPa, all gouges behave in a velocity-strengthening manner, particularly when fluid-saturated. During high-velocity experiments at 1 MPa, all gouges have large slip weakening distances of >16 m. In both cases, the dominant deformation mechanism is distributed particulate flow. During low-velocity experiments at 18 MPa, a transition to velocity-weakening behaviour is observed in the room-humidity and water-saturated gouges at displacements > 15 cm, corresponding to shear localization. However, the brine-saturated gouge remains velocity-strengthening throughout. The dominant deformation mechanism in these gouges is distributed cataclasis. During high-velocity experiments at 18 MPa, all gouges exhibit small slip-weakening distances of <1.7 m for dry gouges, and <<0.1 m for fluid saturated gouges, which also exhibit low fracture energy. The dominant deformation mechanism is localized cataclasis in the dry gouge, with evidence for frictional heating, and distributed particulate flow in the fluid-saturated gouges, with no evidence of significant frictional heating.

Our results show that the lack of fracturing during seismic faulting in saturated, clay-rich sediments will result in earthquakes rupturing the faults having very low fracture energy, thus greatly facilitating rupture propagation to the surface. The fact that fluid-saturated gouges, at low normal stress, deform via the same mechanism (particulate flow) at both sub-seismic and seismic slip velocities, suggests that it is not possible to use microstructural observations alone to differentiate seismic from aseismic fault gouges in shallow-crustal, fluid-saturated fault zones.