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Understanding Creep, Compaction, and Weak Rheology of Major Faults: Insights from the San Andreas Fault

This seminar explores the interrelationships between creep, compaction, and the weak rheology of major faults, focusing on evidence from the San Andreas Fault. It discusses the role of fluid pressure (Pf) and shear traction in fault dynamics, the impact of elevated pore pressure during earthquakes, and the mechanisms of sealing in fault zones. Laboratory experiments and models are cited to illustrate the behaviors of faults under stress and the effects of transient and permanent fluid pressures on shear strength. Key insights into fault mechanics and earthquake generation are presented.

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Understanding Creep, Compaction, and Weak Rheology of Major Faults: Insights from the San Andreas Fault

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  1. Seminar for Ge169-277 2012 Jan. Shengji Wei & Nadia Lapusta Creep, compaction and the weak rheology of major faults Sleep & Blanpied, 1992, Nature Segall & Rice, 1995

  2. ? (decrease F or increase Pf) • Evidence for the weak San Andreas Fault, (~20MPa forshear traction) : absence of elevated (high) heat flow. • While lab experiments assumed hydrostatic pore-pressure requires larger shear traction (90~260MPa). Lockner et. al. , 2011, Nature

  3. (increase of Pf as considered here) • Is fluid pressure (Pf) permanently high or temporary high? • Permanently, regionally: (impossible as said in the paper) because other country rock would be weakened by the fluid. • Permanently within the shear zone, maybe. (sealing needs) • Transiently: Yes, such as during the earthquake.

  4. Lab experiment indicates sealing in the rocks Blanpied et. al., 1992, Nature Sealing of the fault Pore pressure on the fault apparent friction Mechanism for sealing: 1, precipitation 2, compaction Experiment condition: Temperature: 600 Pressure: 400MPa Pore pressure: 0~100MPa Assumed : 0.46~0.52

  5. Compaction/dilatation according to Sleep and Blanpied (1992) Shear module : loading rate : Creep rate : elastic length scale : time interval K: Bulk viscosity Pf: Fluid pressure Ps: Mean pressure f1: Porosity: Shear viscosity • Loading • Creep results in compaction • Compaction leads to high Pf • High Pf leads to weak shear strength and earthquake • Earthquake (modeled as instantaneous stress drop and slip) results in dilation • Dilation leads to lower Pf * Model of porosity: f1 = fp + fc fp: pore porosity fc: crack porosity * K and are the function of intrinsic viscosity i Details in Sleep, 1988, JGR

  6. Model setup

  7. Narrower and more viscous Broader and less viscous Upper limit for crack porosity

  8. Helpful parameters: • Creep rate: • Max earthquake circle time: 0.2mm/yr

  9. Important assumptions • Faults that produce earthquakes experience interseismic creep • Shear zones are sealed from the surrounding rocks on the scale >> 100yr but << than geological time • Earthquake result in substantial dilation (produce more cracks, larger pores) • Earthquakes occur when: • Next presentation: Potentially problematic, because the upper equation implies slip but not necessarily unstable (fast) slip. ?

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