A new publication led by Jonathan Simpson appeared in Geophysical Research Letters, this month.  Laser ultrasonic measurements show that the wave speed in Alpine Fault rocks decreases for waves with a larger amplitude, as shown below. The closer to the fault, the more the wave speed decreases with amplitude. The observation of “rock weakening” contributes to the bigger topic of when and how earthquakes happen, in particular to so-called “triggered” earthquakes.