Fluids, faulting and earthquakes in the brittle crust: recent advances and new challenges

Abstract

Interactions between fluids and deformation are widespread in the brittle crust. As experimentally shown, a high pore fluid pressure p f can fracture intact rocks or reactivate pre-existing fractures. The preference of reactivation over the formation of a new fracture depends on the orientation of the pre-existing fracture with respect to the stress axes and on p f . In nature, the predominant reactivation of misoriented pre-existing faults rather than the formation of new faults with more favorable orientations suggests that pressurized fluids are present in the brittle crust. There is a large body of evidence indicating that supra-hydrostatic p f contributes to the reactivation of lowangle thrust faults or normal faults. Conversely, supra-hydrostatic p f values are less common along vertical or steeply dipping plate boundary transform faults or intra-continental strike-slip faults. If these faults are severely misoriented with respect to the ambient stress field, their reactivation may not be due to supra-hydrostatic p f but to other mechanisms such as shear-enhanced compaction or thermal pressurization. Supra-hydrostatic p f also plays a role in the nucleation or propagation of seismic ruptures in the continental or oceanic crust, and in subducting slabs in convergent margins, as reported for aftershocks, swarms, slow earthquakes, and to a lesser extent for major earthquakes. Lastly, increase or decrease of p f in depth due to human activities such as hydrocarbon extraction, dam impoundment, gas storage or geothermal energy production result in many cases in the inception or enhancement of seismic activiy, adding clues in favor of a relationship between fluids and earthquakes.