Laboratory earthquakes decipher control and stability of rupture speeds

Earthquakes are destructive natural hazards with damage capacity dictated by rupture speeds. Traditional dynamic rupture models predict that earthquake ruptures gradually accelerate to the Rayleigh wave speed with some of them further jumping to stable supershear speeds above the Eshelby speed (similar to root 2 times S wave speed). However, the 2018 M-w 7.5 Palu earthquake, among several others, significantly challenges such a viewpoint. Here we generate spontaneous shear ruptures on laboratory faults to confirm that ruptures can indeed attain steady subRayleigh or supershear propagation speeds immediately following nucleation. A self-similar analysis of dynamic rupture confirms our observation, leading to a simple model where the rupture speed is uniquely dependent on a driving load. Our results reproduce and explain a number of enigmatic field observations on earthquake speeds, including the existence of stable subEshelby supershear ruptures, early onset of supershear ruptures, and the correlation between the rupture speed and the driving load.

Automated summary

Earthquakes are destructive natural hazards whose damage capacity is determined by rupture speeds. Traditional dynamic rupture models predict that earthquakes accelerate gradually to the Rayleigh wave speed, with some of them reaching stable supershear speeds. However, the 2018 Palu earthquake challenges this viewpoint by demonstrating subRayleigh or supershear propagation speeds immediately after nucleation. Laboratory experiments on shear ruptures and self-similar analysis confirm this observation, leading to a model where rupture speed is solely dependent on driving load. These findings explain various field observations on earthquake speeds.