The role of pore-fluid pressure on fault behavior at the base of the seismogenic zone

Brown University, Providence RI

Investigators

Abstract

To characterize earthquake rupture, localization of deformation, stress and rheology at the base of the seismogenic zone we will conduct an experimental study on quartz-rich rocks. 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. Our study will provide 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. 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. Experiments will be conducted on low porosity quartzite and sandstone at T = 700-1100oC, strain rates from 10-3/s to 5x10-7/s, and P = 100 MPa to 1 GPa. The results of our study will be 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 (resulting in low effective stresses), and frictional properties near the fault 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.