Rupture speed dependence on loading conditions: Insights from glacier and laboratory stick-slip

Most earthquakes rupture at a rate approaching, and occasionally surpassing, the shear wave speed, Cs, of the rocks through which they propagate. Recent data have, however, revealed that faults also fail with rupture velocities significantly below Cs. The processes controlling such slow rupture are poorly understood. Surprisingly, glacial slip events generate a variety of rupture velocities at a single location, therefore velocity is not simply controlled by the local material properties. Laboratory experiments suggest that rupture velocities may be controlled by the stresses imposed along a frictional interface prior to the rupture. Moreover, the onset of slip is not governed by a characteristic stress condition (e.g. a static friction coefficient). We compare data from a large Antarctic ice stream, Whillans Ice Plain (WIP), to analogous laboratory measurements. The WIP undergoes bi-daily stick-slip seismic events that displace an ice mass over 100km long. We demonstrate that in both systems, average rupture velocities increase systematically with the pre-rupture stresses, with local rupture velocities exhibiting large variability that correlates well with local interfacial stresses. This analogous behavior indicates that local pre-stress may control rupture behavior in frictional failure events like earthquakes and suggests that fault failure conditions are not controlled by a well-defined threshold value of applied stress.