We have undertaken double-direct-shear friction experiments with floating ice sheets. These tests have been done at the ice basins at HSVA, Hamburg and the Helsinki University of Technology (HUT). Floating ice blocks are pushed past each other, with slip displacement, stress and acoustic emissions monitored at the interfaces. We have therefore measured the effect of frictional sliding on simulated faults. Mostly the sliding took a stick-slip form. The velocity and shear stress are non-uniform in space as well as in time. Measurements of the same stick-slip cycle, at several positions along the fault, allow us to identify a nucleation zone, which begins to slip before the rest of the fault and to relate this spatial variation of velocity and the temporal variation of shear stress; the slipping region, therefore, behaves like a wave-packet, propagating away from the nucleation zone.
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