SCEC Award Number 12153 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title The role of pore-fluid pressure on fault behavior near the base of the seismogenic zone
Investigator(s)
Name Organization
Greg Hirth Brown University Nick Beeler United States Geological Survey
Other Participants Terry Tullis, Brown University Collaborator
David Goldsby, Brown University Collaborator
Taka Kanaya, Brown University Grad Student
SCEC Priorities 1b, 5c, 3a SCEC Groups FARM, SDOT, Geology
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 To characterize earthquake rupture, degree of localization, stress and rheology in the fault slip stability transition zone at the base of the seismogenic zone we are conducting an experimental study on the rheological properties of quartz-rich rocks at conditions scaled to be appropriate for the brittle-plastic transition. The project is focused on the mechanical role of pore fluids and how the mechanical properties of fluid-rock systems respond to variations in temperature and strain rate. The role of fluids on the processes responsible for the brittle-plastic transition in quartz-rich rocks has not been explored at experimental conditions where the kinetic competition between microcracking and viscous flow is similar to that expected in the Earth. Our initial analysis of this competition between these brittle and ductile processes suggests that the effective pressure law for fracture and sliding friction should not work as efficiently near the brittle-plastic transition (BPT) as it does at shallow conditions. Our experimental data obtained using both solid-medium Griggs apparatus and gas-medium Paterson apparatus are consistent with the hypothesis that the effective pressure law evolves rapidly when the temperature-dependence of rock strength results in a lower effective viscosity.
Intellectual Merit An understanding of possible changes to the effective pressure law is directly relevant for understanding many critical scientific problems related to seismicity and the rheological behavior of plate-boundary faults. For example (1) The long term strength of faults depends critically on pore-fluid pressure, thus investigating where the long-term strength faults is actually controlled by frictional properties rather than ductile creep – and how fault strength evolves during the seismic cycle – remains a key problem. (2) The presence of fluids (low effective stresses), frictional properties near the slip stability transition and a “fully effective”, effective pressure law are invoked in almost all models for the generation of non-volcanic tremor. However, the interactions between crystal plastic processes and pore-fluid pressure are not well constrained at these conditions. (3) A key initial condition to understanding the evolution of fault resistance during seismic slip and the maximum depth of seismic faulting is the stress state and scale of strain localization at the base of the seismogenic zone during interseismic periods. The experiments we conducting are designed to help constrain these properties.
Broader Impacts Our project provides new data relevant for understanding the evolution of seismic hazards by concentrating on the links between long-term tectonic and earthquake processes. We are pursuing a relatively unstudied area that may turn out to be critical for applying geophysical data to constrain a wide range of fault zone processes that limit the depth extent of earthquake rupture. In addition, our goals are relevant to understanding processes responsible for drilling-induced seismicity – in particular in deeper geothermal systems where higher temperatures may promote previously unanticipated changes in the effective pressure law.
Exemplary Figure Figure 1

Figure 1: Strength-depth diagram showing transition from a frictional strength controlled by an effective stress (with a hydrostatic pore-fluid pressure) to a lithostatic pressure gradient. The transition occurs owing to a decrease in alpha (α) with increasing temperature (shown by blue curve). The yield stress of frictional asperities is shown by the black curve. The yield stress and dislocation creep stress were calculated with a strain rate of 1e-12/s and lambda = 0.4.