= Solution
\b[Support for the left curve comes from high laboratory friction and substantial in situ <differential stress>; support for the right curve comes from low inferred fault traction, weak fault materials or high <pore pressure>.] No single observation directly measures the whole plotted envelope.
For the \b[left, stronger shallow-crust model]:
* Many friction experiments on ordinary <rocks> give a <coefficient of friction> around 0.6–0.85. Together with approximately <hydrostatic pressure>, these values predict increasing <frictional strength> with depth.
* Deep-borehole measurements of <stress> and <pore pressure> in some continental settings indicate a crust close to frictional failure at appreciable <differential stress>. Jointly with appropriate <fault> orientations, these support a strong frictional upper <crust>.
For the \b[right, weak shallow-layer model]:
* On well-studied plate-boundary <faults>, low resolved <shear stress> inferred from the orientation of principal <stresses> relative to the active <fault plane> can support low effective traction.
* The absence of the expected localized long-term <frictional heating> anomaly can support weak sliding. Directly measured high <pore pressure> or very low-friction <fault gouge> would give a physical explanation. The heat-flow and stress evidence is particularly developed for the <San Andreas Fault>; it is an analogue argument, not an observation that the entire Cascadia upper <crust> follows that curve.<Stress orientations at intermediate angles to the San Andreas Fault, California by Jeanne L. Hardebeck and Andrew J. Michael>[2004, introduction and discussion of strong and weak fault models]{ref}
\b[To discriminate, measure the strength-controlling quantities through depth and across both faults and their surroundings.] Obtain borehole <stress>, <pore pressure> and <temperature> profiles where feasible; use <borehole breakouts> and hydraulic tests to constrain <effective stress>. Recover fault and host <rocks> and test their <friction> and <material creep> at relevant fluid conditions. The right curve specifically predicts an end to the shallow weakening near its sharp rise: investigate changes in <permeability of a porous medium>, alteration and <fault gouge> with depth, rather than testing only a surface sample.
Combine those measurements with <heat flux> transects, <magnetotelluric sounding> for conductive fluids, accurate <earthquake> depths and <focal mechanisms>, and <GNSS> or <InSAR> observations of deformation. The latter can help constrain the depth distribution of locking and <postseismic relaxation>, but neither locking nor <seismicity> alone gives absolute strength. <Earthquake stress drop> is a change in <stress>, not the pre-event <stress>. Low <heat flux> can also reflect heat transport or transient weakening; high electrical conductivity can have causes other than free <water>. A joint explanation of all these data is needed to choose between the models locally.
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