摘要：

The dynamic friction of faults during earthquake slip is a critical control on earthquake ruptures in the crust. Extrapolation of brittle crack theories to natural earthquakes(1-3) has led to the commonly held view that fault friction reduces during rapid earthquake slip, a process known as slip-weakening(1). High-velocity gouge experiments(4,5) and recent analyses of thermal pressurization(6,7) and frictional melting(8) support such a notion; however, these studies dealt with constant rates of slip along faults. Here we present the results of experiments aimed at understanding the frictional behaviour of fault zone materials under variable slip rates-conditions that are more representative of natural earthquakes. Our results show that faults undergo a sequence of strengthening, weakening and healing during acceleration and deceleration of slip. Such a sequence may be explained by the extrapolation of rate-and-state frictional behaviour(9,10) at low slip velocities to more realistic slip rates, but involving different physical mechanisms and a different scale. The initial strengthening should impose a barrier for rupture growth into large earthquakes. The healing on decelerating fault motion may lead to pulse-like earthquake ruptures(11-14) and static stress drops that are low in comparison with the dynamic stress changes(15).

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