Friction, overpressure and fault normal compression

More than twenty-five years ago Miller and Low reported the existence of a threshold pore pressure gradient below which water would not flow through clay. Recent experimental observations of the shear strength of structured water on biotite surfaces have provided a physical basis for understanding this threshold gradient. The existence of this phenomenon has profound implications for the rheological properties of mature fault zones, such as the San Andreas, that contain large thickness of fault gouge. For example, a clay-filled fault zone about 1 km wide at the base of the surface could support core fluid pressure equal to the maximum principal stress over the entire seismogenic zone. As a result, the fault would have near-zero strength and the maximum principal stress measured on the flanks of the fault, would be oriented normal to the fault surface. Another consequence of the threshold gradient is that normal hydrostatic fluid pressures outside the fault zone could coexist with near-lithostatic fluid pressures in the interior of the fault zone without the need for continual replenishment of the overpressured fluid. In addition, the pore pressure at any point should never exceed the local minimum principal stress so that hydrofracture will not occur.

[1]  A. Sylvester Strike-slip faults , 1988 .

[2]  R. Chapman,et al.  Abnormal formation pressures , 1977 .

[3]  Patricia McGuiggan,et al.  Measurements of and Relation Between the Adhesion and Friction of Two Surfaces Separated by Molecularly Thin Liquid Films , 1989 .

[4]  S. T. Harding,et al.  Shallow structure and deformation along the San Andreas fault in Cholame Valley, California, based on high-resolution reflection profiling , 1990 .

[5]  L. Ping MEASURING EXTREMELY LOW FLOW VELOCITY OF WATER IN CLAYS , 1963 .

[6]  P. F. Low,et al.  Threshold Gradient for Water Flow in Clay Systems , 1963 .

[7]  M. Zoback,et al.  New Evidence on the State of Stress of the San Andreas Fault System , 1987, Science.

[8]  J. D. Bredehoeft,et al.  On the Maintenance of Anomalous Fluid Pressures: II. Source Layer at Depth , 1968 .

[9]  N. L. Watts,et al.  Theoretical aspects of cap-rock and fault seals for single- and two-phase hydrocarbon columns , 1987 .

[10]  J. Israelachvili,et al.  Dynamic Properties of Molecularly Thin Liquid Films , 1988, Science.

[11]  W. D. Kemper,et al.  Intrinsic Permeability of Clay as Affected by Clay-Water Interaction , 1959 .

[12]  An example of slip instability resulting from displacement-varying strength , 1990 .

[13]  J. Israelachvili,et al.  Measurements of Static and Dynamic Interactions of Molecularly Thin Liquid Films Between Solid Surfaces , 1988 .

[14]  D. Swartzendruber Non‐Darcy flow behavior in liquid‐saturated porous media , 1962 .