Effect of water on the frictional behavior of cohesive rocks during earthquakes

Fluid-rock interactions can control earthquake nucleation and the evolution of earthquake
sequences. Experimental studies of fault frictional properties in the presence of fl uid can provide
unique insights into these interactions. We report the fi rst results from experiments performed
on cohesive silicate-bearing rocks (microgabbro) in the presence of pressurized pore fl uids (H2
O,
drained conditions) at realistic seismic deformation conditions. The experimental data are compared
with those recently obtained from carbonate-bearing rocks (Carrara marble). Contrary
to theoretical arguments, and consistent with the interpretation of some fi eld observations, we
show that frictional melting of a microgabbro develops in the presence of water. In microgabbro,
the initial weakening mechanism (fl ash melting of the asperities) is delayed in the presence of
water; conversely, in calcite marble the weakening mechanism (brittle failure of the asperities)
is favored. This opposite behavior highlights the importance of host-rock composition in controlling
dynamic (frictional) weakening in the presence of water: cohesive carbonate-bearing rocks
are more prone to slip in the presence of water, whereas the presence of water might delay or
inhibit the rupture nucleation and propagation in cohesive silicate-bearing rocks.