SCEC Award Number 20202 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Effect of pore pressure on rupture dynamics: Laboratory study of the off-fault plasticity and dilatant hardening coupling
Name Organization
Taka Kanaya University of Maryland Wen-lu Zhu University of Maryland
Other Participants Zach Zega (Graduate Student in Zhu group)
SCEC Priorities 2c, 3d, 3f SCEC Groups FARM, SDOT, CXM
Report Due Date 03/15/2021 Date Report Submitted 05/10/2021
Project Abstract
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.
Intellectual Merit Assessing the role of dilatant hardening on fault stability is a major goal of (Q3) Role of evolving fault zone structure and physical property on shear resistance to seismic and aseismic slip. Our results provided key laboratory constraints on the mechanics of fault stabilization by dilatant hardening.
Broader Impacts The effect of pore pressure on faulting investigated by our group - for which SCEC awards provided partial support - involved experiments conducted by Dashaun Horshaw (a senior student with an URM background) under the supervision of the PIs.
Exemplary Figure Fig. 2. Time series of acoustic and strain measurements, showing that dilatant hardening produces a spectrum of fault slip behavior. Strain gage measures stress drop at MHz, while both acoustic sensors record teleseismic signals. With increasing confining and pore pressures (at the same differential pressure), fault slip mode changes from (a) earthquake-like, (b) predominantly episodic slow slip, and to (c) low frequency earthquakes (LFE) only. (d) Both slow slip and LFE can also occur simultaneously.