  The surface morphology of faults controls the spatial anisotropy of their
frictional properties, and hence their mechanical stability. Such anisotropy is
only rarely studied in seismology models of fault slip, although it might be
paramount to understand the seismic rupture in particular areas, notably where
slip occurs in a direction different from that of the main striations of the
fault. To quantify how the anisotropy of fault surfaces affects the friction
coefficient during sliding, we sheared synthetic fault planes made of plaster
of Paris. These fault planes were produced by 3D-printing real striated fault
surfaces whose 3D roughness was measured in the field at spatial scales from
millimeters to meters. Here, we show how the 3D-printing technology can help
for the study of frictional slip. Results show that fault anisotropy controls
the coefficient of static friction, with the friction coefficient along the
striations being three to four times smaller than the friction coefficient
along the direction perpendicular to the striations. This is true both at the
meter and the millimeter scales. The anisotropy in friction and the average
coefficient of static friction are also shown to decrease with the normal
stress applied to the faults, as a result of the increased surface wear under
increased loading.
