The San Andreas fault (SAF) in California is known from surface heat-flow data to be far weaker than ordinary rock friction would predict, with an apparent coefficient of friction below about 0.2 along much of its length. d'Alessio, Williams and Bürgmann (2006) asked whether the ~150-km-long creeping section — the segment that slips almost entirely by aseismic creep rather than large earthquakes — is an exception, and concluded from numerical thermal models that it is not: models with an apparent friction around 0.1 fit best, and a frictionally strong creeping section should have produced a detectable heat-flow anomaly that is not observed. What remains open is the completion of that program: a robust, quantified determination of the creeping section's apparent friction in the face of uncertain background heat flow, thermal refraction, groundwater advection, poorly known fault geometry and creep-initiation epoch. The identity of the persistent weakening mechanism (weak minerals such as saponite and talc versus sustained elevated pore pressure) at seismogenic depths remains motivating open context: direct sampling at one locality (the SAFOD borehole) strongly implicates weak clay minerals, but no fault-wide discrimination exists, and a 2024 stochastic reanalysis argues that surface heat flow alone may not be a suitable quantity to constrain SAF friction.

Background

The San Andreas fault (SAF) is the transform boundary in California between the Pacific and North American plates. Most rock materials tested in the laboratory slide with a coefficient of friction (shear stress divided by normal stress on the fault) of roughly 0.6–0.85, an empirical rule known as Byerlee friction. If the SAF obeyed Byerlee friction at the depths where earthquakes occur (the seismogenic zone, roughly the upper 10–15 km), frictional resistance to the fault's slip rate of a few centimeters per year would generate measurable frictional heat; that heat conducts upward and should appear as an elongated peak in surface heat flow (the outward flow of geothermal energy per unit area, measured in boreholes) centered on the fault trace. Along much of the SAF no such anomaly exists, which was shown by Lachenbruch and Sass (1992) to limit the fault's depth-averaged apparent friction — conventionally denoted μapp\mu_{\mathrm{app}} — to below about 0.2, far below Byerlee values; this is the classic "weak San Andreas fault" observation. Local studies near Parkfield, at the southern end of the creeping section, reinforce the picture: re-evaluated heat flow with full three-dimensional terrain corrections (Fulton et al., 2004) and the 2.2-km-deep SAFOD pilot hole (Williams et al., 2004) found no localized frictional heat anomaly and a conductive thermal regime.

The central ~150 km of the SAF is unusual: it accommodates plate motion almost entirely by aseismic creep — steady, stable sliding that radiates almost no seismic energy — rather than by great earthquakes. In the thermal sense this creeping section acts as a frictional "asperity", a patch of the fault with its own frictional strength embedded in the surrounding fault system: if it were substantially stronger than the adjacent locked segments, its frictional heat should stand out against the regional heat-flow background. d'Alessio, Williams and Bürgmann (2006) tested this with numerical models of heat conduction around finite-length, spatially and temporally heterogeneous strong patches, including the lateral offset of rock produced by strike-slip faulting over geologic time. Their conclusion was twofold: models with μapp≈0.1\mu_{\mathrm{app}}\approx 0.1 fit the sparse existing heat-flow data best, and a patch with μapp>0.2\mu_{\mathrm{app}}>0.2 should have been detectable even with the sparse data for most scenarios — so the creeping section is as weak as the rest of the SAF, and because it never slips in large earthquakes, its weakness cannot come from dynamic weakening processes triggered by fast seismic slip rates; it must be persistent. The authors explicitly identified what their conclusion rested on: background heat flow in the California Coast Ranges varies spatially in ways not uniquely resolved by the data; the conclusions depend on the assumed thermal history, in particular the epoch when present-day creep began (shorter-lived strong patches generate smaller anomalies); their models included heat conduction and the slow lateral transport of heat by moving crustal blocks but not advective transport by flowing groundwater; real three-dimensional fault geometry and the activation history of multiple fault strands were idealized; and the data could not distinguish which persistent weakening mechanism — serpentinization (alteration of ultramafic rock to weak serpentine minerals), clay-rich gouge, sustained elevated fluid pressure in the fault zone, or low-permeability compartments that trap such pressure — actually keeps the fault weak. Serpentine-derived minerals matter here because reaction of serpentinite with the surrounding quartzofeldspathic wall rock can produce saponite, a magnesium-rich smectite clay that is among the weakest known phyllosilicates, and talc, another extremely weak mineral; materials whose friction increases with slip velocity ("velocity-strengthening") deform by stable sliding rather than stick-slip earthquakes. Two later developments frame the surviving problem. First, the San Andreas Fault Observatory at Depth (SAFOD), a scientific borehole near Parkfield drilled through the actively creeping fault at about 2.7 km depth, recovered fault gouge whose laboratory-measured friction is approximately 0.15 owing to saponite (Lockner et al., 2011), and whose active-strand material is weak, velocity-strengthening, and shows no frictional healing — the restoration of strength between slip episodes (Carpenter et al., 2011). These are point measurements at one locality and shallow depth, and later work still describes the underlying cause of aseismic creep as unsettled. Second, systematic reexamination of the heat-flow route has clarified the scatter sources: thermal refraction (the bending of conductive heat flow across contrasts in thermal conductivity) is a plausible explanation for the observed heat-flow scatter, but refraction effects cannot mask frictional heat generation in a manner consistent with observations, and the data remain most consistent with a fault producing little to no frictional heat (Fulton and Saffer, 2009); advection of heat by topographically driven groundwater flow through heterogeneous permeability is another scatter source (Popek and Saffer, 2011; Fulton et al., 2010); and stochastic models of regional heat-flow variability (Ziebarth and von Specht, 2024) show that scatter and non-uniqueness in the Coast Ranges data are large enough that the identification of a frictional heat anomaly, and hence of μapp\mu_{\mathrm{app}}, from heat flow alone remains genuinely underdetermined.

Problem Statement

Determine the frictional strength of the creeping section of the San Andreas fault in the sense established by d'Alessio, Williams and Bürgmann (2006), who tested the hypothesis that the creeping section has a substantially higher or lower μapp\mu_{\mathrm{app}} than adjacent sections of the SAF. Concretely: determine the depth-averaged apparent coefficient of friction μapp\mu_{\mathrm{app}} of the ~150-km creeping section — or, with an explicitly stated conversion convention (the effective normal-stress profile, the seismogenic depth range over which the average is taken, the along-strike slip-rate distribution, and the fault area), the equivalent total frictional power of the fault — with a quantified uncertainty, from surface heat flow and complementary observations, with the background Coast Ranges heat-flow field, thermal refraction and terrain effects, advective heat transport by groundwater, the three-dimensional geometry and slip history of the fault strands, and the epoch at which present-day creep began treated as uncertain inputs whose uncertainty is propagated rather than assumed away. Explain quantitatively why the creeping section, given the determined apparent friction, produces little or no frictional heat and slips stably.

References

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