Velocity-frequency-amplitude-dependent frictional resistance of planar joints under dynamic normal load (DNL) conditions

The problem of how dynamic friction of faults influences the observed earthquakes remains largely unsolved. Our paper presents results of experiments aimed to understand the complex frictional behavior of faults under dynamic normal load (DNL) conditions under consideration of horizontal slip velocity, dynamic normal force amplitude and normal impact frequency using a dynamic shear box device. Data were obtained by continuous measurements of shear and normal forces as well as corresponding displacements. We identified a phase shift between peak normal force and peak shear force with peak shear force lagging. Consequently, the dynamic friction coefficient shows cyclic behavior and the minimum value of the dynamic friction coefficient increases with increasing of slip velocity which is in conflict to the traditional views under constant normal load (CNL) conditions. We found, that dynamic fault friction coefficient during earthquakes may increase or decrease depending on velocity-frequency (VF), velocity-amplitude (VA) and velocity-frequency-amplitude (VFA) parameter. These findings have significant implications for seismic hazard assessment and reliable forecasting of earthquakes.