The model was further investigated by Abe and Kato (2013) employing a different expression for the friction law and assuming a velocity-weakening behaviour for both blocks.
These results confirm that water-assisted, thermally activated quartz deformation is a key process in the velocity-weakening behaviour at intermediate temperatures in wet illite/quartz gouges and support the existing microphysical model.
The difference in the extent of velocity-weakening of wet illite gouge with 6% to 7% quartz versus wet 65/35 illite/quartz gouges, at least at 350°C, suggests that an increase in quartz content promotes velocity-weakening behaviour.
In addition, at 350°C, illite samples show near-neutral to slightly velocity-weakening behaviour, as opposed to mainly velocity-weakening for the illite/quartz mixtures.
Our conclusions are as follows: 1. Wet (near- pure illite gouge shows similar temperature-depenear- puree-regilliteip stability behaviour to that previously found for wet 65/35 illite/quartz gouge, but with near-neutral to showstly velocity-weakening behaviour at 350°C insimilarf velocitemperature-dependent
Wet (near- pure illite gouge shows similar temperature-depenear- puree-regilliteip stability behaviour to that previously found for wet 65/35 illite/quartz gouge, but with near-neutral to showstly velocity-weakening behaviour at 350°C insimilarf velocitemperature-dependent
In the present study, wet illite gouge exhibited similar three-regime behaviour, but with velocity-neutral rather than velocity-weakening behaviour at the intermediate temperatures.
This result supports the proposed key role of quartz in generating velocity-weakening behaviour in wet mixtures of illite and quartz (Den Hartog and Spiers 2014).
A decrease in the RSF parameter (a-b) (approaching velocity-weakening behaviour) with increasing quartz fraction in wet illite/quartz mixtures (Den Hartog and Spiers 2013) does suggest such a role.
Notably, whereas the wet illite/quartz samples showed predominantly velocity-weakening behaviour in Regime 2, the wet illite samples were velocity-neutral to slightly velocity-weakening at 350°C.
This suggests that earthquake nucleation along megathrusts may be caused by the same mechanism that results in velocity-weakening behaviour in the model, and that seismogenesis is in fact controlled by gouge compaction through thermally activated deformation of quartz clasts, most likely by pressure solution, that moderates dilatation.
