<font size="2" face="Arial, Helvetica, sans-serif"> </font> <p style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px" align="justify"><font face="Arial, Helvetica, sans-serif"><font size="2">The dynamics of how two rough frictional interfaces detach is a fundamental question in fields ranging from material science to geophysics. While frictional slip has long been modeled as a dynamic rupture problem, progress has been hampered by the very nature of the rough and often nonuniform interface that separates two sliding bodies. Experiments have demonstrated that the onset of frictional slip is mediated by a number of different rupture modes. These include slowly propagating fronts, ruptures bounded by the Rayleigh wave-speed and ruptures that propagate beyond the shear wave-speed. Similar rupture modes have been reported on natural faults. Both the origin and selection of these modes, however, are poorly understood. Here we show how rupture mode selection depends on stress profiles along the frictional interface, which are generically non-uniform. These experimental measurements reveal a well-defined relation between the different rupture modes and the ratio, prior to rupture, of the local values of the shear stress, τ(x), to the normal stress, σ(x). We further demonstrate that, locally, τ(x)/σ(x) can far exceed the coefficient of static friction, μ<sub>s</sub>, without precipitating slip. The three different rupture modes correspond to distinct regimes of τ(x)/σ(x). This selection is independent of how loads are externally applied. These results indicate the key role of nonuniformity to frictional stability and dynamics with important implications for the prediction, selection and arrest of different modes of earthquakes. </font></font></p> <p style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px" align="justify"><font face="Arial, Helvetica, sans-serif"><font size="2"></font></font> </p> <p style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px" align="justify"><font face="Arial, Helvetica, sans-serif"><font size="2"><font color="#697da9"><span style="FONT-WEIGHT: 700"><font color="#697da9"><a href="http://sciences.hit.ac.il/events/ps/fliers.2010-2011/fineberg.22.feb.2011.pdf" target="_blank">Get the flier...</a></font></span></font></font></font></p>