A cycle of memory creation, erasure, and phase transitions in granular assemblages sheared by natural faults

A hallmark of many natural hazards (i.e., avalanches, landslides, and earthquakes) is the phase transition that frictional granular assemblages undergo, from being structurally arrested to creeping to flowing. Shear jamming, in conjunction with several other granular physics theories, predicts that the phase transitions are governed by the assemblage's temperature, disorder, volume fraction, and the shear rate, and makes several phenomenological predictions that would benefit from being tested in natural granular assemblages and in three dimensions. Here, I use x-ray microtomographic image analyses to assess controls on if and how the shear jamming phenomenologies of memory creation, memory erasure, and phase transitions are recorded in the three-dimensional re-arrangement and fracturing of minimally disturbed fault zone grains that experience slow versus fast tectonic shearing. The samples are from Pallett Creek and Ferrum, within the southern San Andreas fault zone (sSAFZ). The analyses reveal that the fault zone assemblages transition between one or more interrelated but geometrically localized and invisible to the naked eye (mesoscopic) deformation phases that change the stability of the assemblages during the seismic cycle. These phases include (1) bulk grain re-arrangement, (2) localized grain re-arrangements, (3) individual grain breakages, (4) bulk fracturing of the assemblage, and (5) localized zones of grain fracturing. One fault core hosts gas bubbles or nematode burrows that record 300 years of tectonic deformation history after the ~Mw 7 ca. 1726 sSAFZ earthquake. Grain re-arrangement is mostly a global versus local feature. Grain size, which represents one level of disorder, exerts control over the phases of breakage and re-arrangements. Higher shear rates lead to greater numbers of fractured grain, erasure of previous memories of slow shear-rate induced preferred azimuthal grain re-arrangement in the direction of fault slip, and introduce shear-hardened zones with grain preferred inclinations near the slip surface instead. Memory creation and erasure are thus controlled by shear rate and elastoplasticity, and thus vary with the distance from a fault. These findings imply that sheared fault zone grains experience a cycle of memory creation and erasure accompanied by varying deformation phase transitions (creeping, flowing, and structural arrest).

Speaker: Vashan Wright, UC San Diego