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No such heat is seen, so the fault must be weaker than lab rocks.","posed_since":"1969","precise":"The absence of a frictional heat-flow anomaly over the San Andreas fault implies mean shear stress below about $2 \\times 10^{8}\\,\\mathrm{dyn}/\\mathrm{cm}^{2}$, while Byerlee friction (coefficient 0.6 to 0.85) at seismogenic depth predicts about $10^{9}\\,\\mathrm{dyn}/\\mathrm{cm}^{2}$. Determine whether mature faults are statically weak (weak clay minerals, high pore pressure) or only dynamically weak during slip. An answer is a measured in situ shear stress and friction profile consistent with heat flow and stress orientation data. The residual question concerns locked, seismogenic fault segments.","problem_ref":"5e6fca620806a66a8dd548a606382dfcdb3cafffcedb0a2a32209d80e7cf600b","references":"","settled_by":"Deep borehole stress, pore-pressure and temperature measurements on mature faults combined with friction tests on recovered fault rock.","status_note":"SAFOD core showed the creeping San Andreas segment is statically weak through saponite-bearing gouge with friction coefficient below about 0.15 (Lockner and coauthors, Nature 2011, https://doi.org/10.1038/nature09927); whether locked seismogenic segments are statically or only dynamically weak remains open.","title":"Why do mature faults slip at low stress without measurable 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