  We perform physical and numerical experiments to study the stick-slip
response of a stack of slabs in contact through dry frictional interfaces
driven in quasistatic shear. The ratio between the drive's stiffness and the
slab's shear stiffness controls the presence or absence of slip
synchronization. A sufficiently high stiffness ratio leads to synchronization,
comprising periodic slip events in which all interfaces slip simultaneously. A
lower stiffness ratio leads to asynchronous slips and, experimentally, to the
stick-slip amplitude becoming broadly distributed as the number of layers in
the stack increases. We interpret this broadening in light of the combined
effect of complex loading paths due to the asynchronous slips and creep.
Consequently, the aging rate of the interfaces can be readily extracted from
the stick-slip cycles, and it is found to be of the same order of magnitude as
existing experimental results on a similar material. Finally, we discuss the
emergence of slow slips and an increase in aging-rate variations when more
slabs are added to the stack.
