SCEC Award Number 14183 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title A Dynamic Compaction Mechanism for Fluid Overpressure in the Fault Zone
Investigator(s)
Name Organization
Shuo Ma San Diego State University
Other Participants
SCEC Priorities 3e, 3c, 3d SCEC Groups FARM, CS, USR
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 Triaxial experiments show that samples of fault gouge deform distinctly differently than those from the adjacent fault damage zone (e.g., Chester and Logan, 1986). Rock samples in the damage zone follow a characteristic elastic-brittle behavior, whereas fault gouge readily compacts and deforms in a more ductile manner. We incorporate undrained compaction into a dynamic rupture model of a strike-slip fault with a strongly velocity-weakening friction (in a rate-and-state framework). A 20-cm thick fault gouge layer is modeled by an end-cap failure criterion (e.g. Wong et al., 1997) and experiences compaction and dilatancy, while the remainder of the model domain obeys the standard Mohr-Coulomb criterion. We show that large dynamic stresses associated with rupture propagation cause the gouge layer to compact ahead of the rupture front, leading to rapidly elevated pore pressure in the undrained fault zone and significant dynamic weakening of the principal fault surface. Compared to other dynamic weakening mechanisms such as flash heating and thermal pressurization, this mechanism does not require slip to initiate. Weakening ahead of the rupture front lowers the static friction of the fault, leading to a lower strength drop on the fault. After the passing of the rupture front, strong dilatancy of undrained fault gouge reduces the pore pressure and restrengthens the fault, promoting a more pulse-like rupture. Thus dynamic gouge compaction and dilatancy provides a simple mechanical explanation for weak mature faults and pulse-like earthquake ruptures on these faults.
Intellectual Merit This model addresses the fundamental problems in earthquake physics that why large plate- bounding faults are weak. We have shown that dynamic undrained compaction in the gouge due to S wave and large shear stress increase ahead of crack tip effectively weakens the fault and undrained dilatancy associated with strength drop strengthens the fault, promoting more pulse-like ruptures. This model is self-consistent and has a simple mechanical basis. Compared with other dynamic weakening mechanisms, this model does not require slip to operate and does not involve large strength drop. Strong dilatancy tends to suppress thermal pressurization (e.g., Andrews, 2002), but in this model it promotes slip pulse generation. These mechanisms, however, can operate simultaneously.
Broader Impacts Dynamically elevated pore pressure from compaction provides insight into other fundamental problems in tectonics, such as the occurrences of episode tremor and slow-slip earthquakes, the cause and effect of fluid flow immediately following seismic events, and the conditions leading to aftershocks, earthquake triggering, and induced seismicity.
Exemplary Figure Figures 2 and 3
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