Laboratory observations of fault-normal vibrations during stick slip

We report laboratory observations of interface separation waves during stic k slip on a fault in a uniform polymer material. Our observations, made at stress levels expected at midcrustal depths, share many macroscopic propert ies with ruptures of faults in rocks. We observed a drop in fault-normal st ress shortly before the onset of, and during, stick slip at points along th e fault during a rupture. We suggest that P wave energy in front of the pro pagating rupture tip is responsible for the drop in normal stress. We also interpret that stick slip took place within a traveling slip pulse, and we suggest that the dynamic stress drop within the slipping patch exceeded the overall static stress drop by a factor of at least 5 within a few millimet ers of the fault. Our experiments did not resolve whether the fault surface s actually separate or if fault-normal stress is just greatly reduced. In e ither case the net result is that fault slip is permitted to take place wit h much less frictional resistance than that expected from the applied load. Our observations provide laboratory evidence that fault-normal vibrations may play an important role in sustaining a rupture by facilitating the prop agation of a transient instability. Faults may appear weak in part because they are dynamically weakened as they slip during rupture while retaining t heir strength during the interseismic period.