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

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Presentation transcript:

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

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

(increase of P f as considered here) Is fluid pressure (P f ) 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.

Lab experiment indicates sealing in the rocks Blanpied et. al., 1992, Nature Sealing of the faultPore pressure on the fault apparent friction Mechanism for sealing: 1, precipitation 2, compaction

Compaction/dilatation according to Sleep and Blanpied (1992) Loading Creep results in compaction Compaction leads to high P f High P f leads to weak shear strength and earthquake Earthquake (modeled as instantaneous stress drop and slip) results in dilation Dilation leads to lower P f * Model of porosity: f 1 = f p + f c fp: pore porosity fc: crack porosity Details in Sleep, 1988, JGR

Model setup

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

Helpful parameters: Creep rate: Max earthquake circle time:

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. ?