Project Abstract
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Dilatant hardening is thought to be an important mechanism underling the family of slow earthquakes observed in plate-boundary faults. Along a propagating rupture, if dilatancy occurs faster than fluid flow into newly created pore spaces, a reduction in fault pore pressure (i.e., an increase in the effective pressure) retards further rupture propagation. However, the laboratory demonstration of this well-known process remains scarce. The SCEC grant provided supplementary support for our continuing investigation of the effect of dilatant hardening on faulting. Here our experiments show that dilatant hardening results in two distinct timescales of fault stabilization. The early stage of failure is characterized by a prolonged period of aseismic, quasistatic slip over a timescale comfortable with that of imposed displacement rates. Unexpectedly, the later stage of failure also shows stabilization at a timescale of dynamic rupture: the early stable failure eventually becomes unstable, but with a much reduced rupture speed and seismicity. Contrary to previously thought, we observe long-term stabilization in low bulk diffusivity rocks deformed under low strain rates, in which pore fluid is drained during axial loading, but becomes undrained during failure. In contrast, the short-term stabilization occurs in rocks with any bulk diffusivities at all strain rates tested at high pressures. With increasing pressure, the mode of fault slip changes from earthquake-like, episodic slow slip, and to low-frequency earthquakes. Our results support the hypothesis that dilatant hardening causes the spectrum of slow earthquakes observed in nature. |
