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

2. 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 ?

3. (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.

4. 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

5. 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

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:

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. ?