Ubiquitous weakening of faults due to thermal pressurization

Abstract

Faults weaken during earthquakes. Analysis of the amount of energy released during earthquakes globally suggests that heat-induced pressurization of pore fluids can weaken faults during earthquakes of all sizes. Laboratory simulations of earthquakes show that at high slip rates, faults can weaken significantly, aiding rupture1,2,3. Various mechanisms, such as thermal pressurization and flash heating, have been proposed to cause this weakening during laboratory experiments1,4,5,6, yet the processes that aid fault slip in nature remain unknown. Measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening, known as an event’s fracture energy7,8,9. Here we compile new and existing8,9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes. We observe a distinct transition in how fracture energy scales with event size, which implies that faults weaken differently during small and large earthquakes, and earthquakes are not self-similar. We use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms. We find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes, over seven orders of fault slip magnitude. We conclude that thermal pressurization is a widespread and prominent process for fault weakening.