Ubiquitous weakening of faults due to thermal pressurization

Robert C. Viesca, & Dmitry I. Garagash

Published October 5, 2015, SCEC Contribution #6004

Laboratory simulations of earthquakes show that at high slip rates, faults can weaken significantly, aiding rupture. Various mechanisms, such as thermal pressurisation and flash heating, have been proposed to cause this weakening during laboratory experiments, 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 energy. Here we compile new and existing 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 elasto-dynamic numerical model of earthquake rupture to explore possible mechanisms. We find that thermal pressurisation 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 pressurisation is a widespread and prominent process for fault weakening.

Viesca, R. C., & Garagash, D. I. (2015). Ubiquitous weakening of faults due to thermal pressurization. Nature Geoscience,. doi: doi:10.1038/ngeo2554.