Numerical modelling of earthquake sequences involving valving and pumping of fluids

SUMMARY Faults in the upper crust are sometimes thought to act as self-sealing valves, episodically releasing highly overpressured fluids trapped at greater depth during earthquakes. They are also often considered to be capable of actively pumping fluids into or out of faults in response to coseismic volumetric strain, thermal pressurization or other mechanisms. In this study, we investigate how these different types of behaviour (i.e. valve versus pump) are manifested in earthquakes. We do this using a 2-D plane strain model where frictional sliding on a thrust fault that is fed by a fluid source at its base is coupled to porous flow, thermal pressurization and strong variations in permeability. Our results show that thermal pressurization leads to dramatic dynamic weakening that produces earthquakes that propagate as slip pulses producing large stress drop, large slip and high slip velocities. On the other hand, valve-type behaviour typically produces smaller, less energetic earthquakes that commonly arrest before rupturing the entire fault. In some valve models, we observe complex compound ruptures and swarm activity, which is linked to the ascent of a propagating fluid pressure pulse driven by a large increase in permeability during sliding. Both pump and valve mechanisms can produce anomalously weak faults, though they are each associated with distinctly different fluid pressure and strength evolution over the seismic cycle and during rupture. Our models highlight the complex way in which fluids may interact with earthquakes, especially if valve and pump models coexist.