Reconciling laboratory, small and large fault frictional properties

Laboratory experiments on fault rocks mostly show static friction coefficients in between 0.5 and 0.8 in agreement with the Byerlee’s results (Byerlee, 1978). Several exceptions are reported for rheologically weak rock samples, e.g., phyllosilicates and serpentinites (Tesei et al., 2012; Pozzi et al., 2023), with friction values as low as 0.2. Geodesy and simulations of natural fault slip behaviour return much smaller static friction values, often lower than 0.1 (Zaccagnino et al., 2024 and references therein), while slightly higher coefficients characterize immature fracture zones, but mostly still incompatible with lab measurements. Several hypotheses have been proposed to explain the weakness of fault friction: the presence of fluids (Wintsch et al., 1995) and peculiar rheology/fabrics (Collettini et al., 2009) are among the most popular. However, the maintenance of high fluid pressure needs special conditions, weak minerals are not abundant enough to justify observations and, surprisingly, weak fault mechanics, e.g., creep, is also highlighted is avowed strong lithologies (e.g., Del Sole et al., 2024). At last, weak fabrics friction coefficients are still significantly higher than large fault friction. Here, we propose a theoretical approach to investigate this topic. We suggest that static friction decreases with fault size also depending on a few physical properties (e.g., faulting fractal dimension), while dynamic coefficients are not affected by the spatial scale. Mathematical derivations are based on hypotheses validated using a simple model for earthquake occurrence compatible with fracture mechanics and able to reproduce the crucial statistical properties of seismicity. References Bedford J. D., Faulkner, D. R. and Lapusta, N.; 2022: Fault rock heterogeneity can produce fault weakness and reduce fault stability. Nat. Commun., 13(1), 326. Byerlee J.; 1978: Friction of rocks. Rock friction and earthquake prediction, 615-626. Collettini C., Niemeijer A., Viti C. and Marone C.; 2009: Fault zone fabric and fault weakness. Nature, 462(7275), 907-910. Del Sole L., Mazzoli S., Carafa M. M., Toffol G., Pennacchioni G., Giuli G., ... and Tondi E.; 2024: Interseismic creep of carbonate-hosted seismogenic normal faults: Insights from central Italy. Bulletin, 136(3-4), 1605-1627. Pozzi G., Collettini C., Scuderi M. M., Tesei T., Marone C., Amodio A. and Cocco M.; 2023: Fabric controls fault stability in serpentinite gouges. Geophys. J. Intern., 235(2), 1778-1797. Tesei T., Collettini C., Carpenter B. M., Viti C. and Marone, C.; 2012: Frictional strength and healing behavior of phyllosilicate‐rich faults. J. Geophys. Res., 117(B9). Wintsch R. P., Christoffersen R. and Kronenberg A. K.; 1995: Fluid‐rock reaction weakening of fault zones. J. Geophys. Res., 100(B7), 13021-13032. Zaccagnino D., Bruno O. and Doglioni, C; Spatial scale dependence of fault physical parameters and its implications for the analysis of earthquake dynamics from the lab to fault systems, 29 October 2024, PREPRINT (Version 2) available at Research Square [https://doi.org/10.21203/rs.3.rs-4616332/v2]