Rough fault with dilation. Triggering of full sliding

The presentation proposes a 2D model of sliding over a rough fault of finite length. Sliding over such a fault involves interaction between asperities of its opposite sides. The interaction proceeds in two stages. At Stage 1 (the initial stage) the interaction involves climbing asperities over the asperities on the opposite side of the fault, causing dilation. As a result, additional resistance to sliding is induced working as friction such that the friction angle becomes the sum of the material friction angle representing friction between the asperity contacts and the average angle of inclination of the tangential line of the asperity contacts. With the increase of shear stress above the critical value of the friction stress shear over the fault is initiated. This corresponds to the conventional interpretation of sliding over rough fault.

This conventional sliding however only proceeds until it reaches the half asperity length, that is when the corresponding local dilation (fault opening) reaches its maximum, which is the asperity height. Under uniform shear stress this is obviously reached at the fault centre. At this point Stage 2 commences. At Stage 2 the asperities which produced maximum dilation (the size of two asperity height) start reducing the dilation creating the effect of local negative friction since the pressure at this stage drives sliding. Subsequently, in the fault zone sliding in Stage 2 the average friction angle drops to that of the material friction angle. As a result, the central zone slides under reduced friction. It was found that there exists a critical magnitude of shear stress, which triggers self-sustained extension of the zone with reduced friction.

Sliding of this type involves creation of loci of negative friction, which can affect the microseismic signals. The presented model provides an additional mechanism of fault sliding and the associated induced seismicity.