Earthquake driven mechanical alteration of fault core material and its effect on post-seismic fluid-rock interaction

Earthquakes release vast quantities of energy over very short timescales. At shallow depths, a portion of this energy is used to fracture, crush, and grind fault hosting rocks, resulting in reduced particle size and mineral crystallinity; frictional heating; mass movement of pore-fluid; and overall extreme but transient conditions. These seismic  processes partially control mineral alteration reactions that often take place within fault gouges. The mineralogy and therefore mechanical and chemical properties of fault core material will influence the style of future slip on faults. Many studies have shown that mineralogical differences within fault cores result from inter-seismic alteration by pore fluid, but have neglected co-seismic processes. Here we highlight the role of co-seismic mechanically and mechanochemically influenced mineral reactions. These reactions enhance fluid driven alteration and affect the frictional properties of fault rocks.

Transient co-seismic conditions cannot be studied in the field, so earthquakes were simulated in the lab using a high velocity rotary shear apparatus and silicate based synthetic fault material to enable control of experimental inputs. We found significant frictional differences in reworked gouge after having experienced a high velocity (seismic) event, particularity in healing capabilities. Our investigations indicate this is due to generation of “shocked” material that has undergone dehydration and dehydroxylation of hydrated minerals, amorphisation, and  grain comminution; all equating to a more reactive gouge. In natural post-seismic settings, this ‘shocked’ material sits in contact with pore-fluid that is at least partially externally derived. Due to the increased reactivity of this gouge, it is more prone to rapid post-seismic fluid-rock alteration, producing clay abundant retrograde authigenic minerals and reducing fault strength.

Using experiments to simulate this process, we show that synthetic post-seismic gouge exhibited increased fluid-rock interaction and enhanced precipitation of authigenic material relative to unsheared gouge. This was traced by analysing pore-fluid chemistry after prolonged contact with the gouge, close examination of the clay sized fraction using SEM techniques, and detailed XRD of shear inputs and outputs. Our work highlights the key role that co-seismic processes play in 1) the initial post-seismic change of frictional properties; 2) accelerated retrograde mineral evolution due to increased gouge reactivity; 3) and the associated reduction of fault strength and friction coefficient of fault core material post alteration.