Ongoing and Recent Projects
Geophysical observations indicate that high fluid pressures occur in regions of subduction zones characterized by slow slip, which is behavior intermediate between standard seismicity and steady creep. Seismic hazards associated with slow slip include (1) triggered seismicity along the updip seismogenic portion of subduction zones and (2) downdip and landward propagation of a seismic slip event. I use experimental methods to characterize the effects of pore fluid pressure on slip processes in subduction zone rocks. I particularly interested in the interactions between fracture, friction, and dissolution precipitation processes in the phyllosilicate-rich rocks that include antigorite-rich serpentinite and metamorphosed sediments.
Seismic Hazards along the San Andreas Fault
I study clay-rich fault gouge recovered from 2.7 km depth during the San Andreas Fault Observatory at Depth (SAFOD) program to constrain the likelihood that seismic slip could initiate on a locked segment of the San Andreas Fault and propagate into the creeping segment. Experiments were conducted at co-seismic slip-rates in a rotary shear apparatus located at the Kochi Core Center of the Japanese Agency of Marine Science and Technology. We show that the microstructural and thermal evolution of the fault rock during shearing causes it to weaken dramatically at co-seismic slip-speeds, and the magnitude of weakening could promote rupture propagation along the central segment of the San Andreas Fault.
San Andreas Fault Creep
I study the same clay-rich fault gouge from Central Deforming Zone (CDZ) at SAFOD using the stress-relaxation technique to understand why the central segment of the San Andreas Fault currently creeps aseismically. This technique allows us to achieve strain rates within an order of magnitude of in-situ rates. My co-authors and I show that the creep behavior can be explained by intergranular sliding between clay particles and crystal plastic deformation of the clay at geometric irregularities in the rock.
Fluid Flow Through Fractured Rock
Fluid transport in low-porosity rocks in fault damage zones and sedimentary basins is believed to be largely controlled by the transmissivity of interconnected fractures, but not all fractures are equally transmissive. I investigate the effects of deformation and stress state on the permeability and porosity of fractured low-porosity (7 %) and low permeability (~10-19 m2) arkosic sandstone. In contrast to low-porosity crystalline rocks, micro fracture damage associated with fracture nucleation has a negligible effect on the bulk permeability, whereas a through-going fracture increases the permeability by several orders of magnitude. The tabular fractures that develop are at least 2 orders of magnitude more permeable than the host rock at mean stresses up to 90 MPa and the fractures dilate as the stress state approaches the friction envelope resulting in up to a 3 order of magnitude increase in fracture permeability. As a result, the enhanced permeability within fault damage zones composed of low-porosity polyphase sedimentary rocks is expected to be limited to a zone of enhanced macrofractures, not the broader zone of enhanced microfractures.