Revision 1

Superseded · first revision · record

Title: Why do mature faults slip at low stress without measurable heating?
Plain: Rock friction measured in the lab predicts that a sliding fault like the San Andreas should heat the ground around it. No such heat is seen, so the fault must be weaker than lab rocks.
Precise: The absence of a frictional heat-flow anomaly over the San Andreas fault implies mean shear stress below about $2e8\,\mathrm{dyn}/\mathrm{cm}^{2}$, while Byerlee friction (coefficient 0.6 to 0.85) at seismogenic depth predicts about $1e9\,\mathrm{dyn}/\mathrm{cm}^{2}$. 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.
Settled by: Deep borehole stress, pore-pressure and temperature measurements on mature faults combined with friction tests on recovered fault rock.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: partially-resolved
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.
Posed since: 1969