Precursory changes in seismic velocity for the spectrum of earthquake failure modes

Temporal changes in seismic velocity during the earthquake cycle have the potential to illuminate physical processes associated with faultweakening and connections between the range of fault slip behaviours including slow earthquakes, tremor and low-frequency earthquakes(1). Laboratory and theoretical studies predict changes in seismic velocity before earthquake failure(2); however, tectonic faults fail in a spectrum of modes and little is known about precursors for those modes(3). Here we show that precursory changes of wave speed occur in laboratory faults for the complete spectrum of failure modes observed for tectonic faults. We systematically altered the stiffness of the loading system to reproduce the transition from slow to fast stick-slip and monitored ultrasonic wave speed during frictional sliding. We find systematic variations of elastic properties during the seismic cycle for both slow and fast earthquakes indicating similar physical mechanisms during rupture nucleation. Our data show that accelerated fault creep causes reduction of seismic velocity and elastic moduli during the preparatory phase preceding failure, which suggests that real-time monitoring of active faults may be a means to detect earthquake precursors.