Acoustic triggering of shear instabilities in dry and wet granular fault gouges

Laboratory studies of granular friction have emerged as a powerful tool for investigating seismic fault slip [1], including dynamic triggering of earthquakes and landslides [2,3]. However, physical origins of triggering by small strain amplitude from a large remote earthquake still remain unclear.

Here we report the experimental investigation of quasi-static sliding in dry and wet granular gouges layers under constant pressure, monitored with passive (acoustic emission: AE) and active acoustic detections (wave velocity and coda correlation). Both avalanche-like dynamics and quasi-periodic stick-slip behaviour are observed, illustrating a ductile-brittle like transition induced by the cohesion. These phenomena are associated with by distinct statistics of AEs (labquakes) and specific granular flow patterns. A decrease of the acoustic velocity and an increase of AE rate (precursors) are also detected before mainshocks or mainslips.

Moreover, we have investigated the dynamic triggering of the mainslip associated with strong stress drop by applying relatively high-amplitude ultrasound (of the order of 10 nm) in the steady sliding state. This dynamically triggered stress drops appear as slower (lab) earthquakes than the (quasistatic) shear-induced fault slip. We show that such acoustic triggering of macroscopic shear instability originates from the reduction of shear contact stiffness and interparticle friction between grains by the acoustic lubrication [4,5], via microslips.

[1] C. Marone, Laboratory-derived friction laws and their application to seismic faulting, Ann. Revs. Earth & Plan. Sci. 26, 643 (1998); C.H. Scholz, The Mechanics of Earthquake and Faulting (3rd edition, Cambridge University Press, 2018)

[2] P. Johnson and X. Jia, Nonlinear dynamics, granular media and dynamic earthquake triggering, Nature 437, 871 (2005)

[3] V. Durand et al, Repetitive small seismicity coupled with rainfall can trigger large slope instabilities on metastable volcanic edifices, Communications Earth & Environment 4, 383 (2023)

[4] X. Jia, T. Brunet, and J. Laurent, Elastic weakening of a dense granular pack by acoustic fluidization: Slipping, compaction, and agingPhys. Rev E 84, 020301(R) (2011)

[5] J. Léopoldès, X. Jia, A. Tourin, and A. Mangeney, Triggering granular avalanches with ultrasound, Phys. Rev. E 102, 042901 (2020)