Experimental and Numerical Perspectives on The Role of Localization and Heating in Small-Magnitude Earthquakes

Small-magnitude earthquakes (M < 4) have received escalating attention in recent years owing to the global surge in human-induced seismicity. Subsurface activities such as hydrocarbon production can lead to triggering of faults cutting the reservoir system, sometimes bringing significant societal unrest. To evaluate the risk of induced events, it is important to understand the strength and behavior of the fault during fast movement (i.e. slip velocity > 1 m/s). This requires a full characterization of the process of where slip occurs and how faults strength breaks down due to frictional heating from rupture nucleation to propagation. This thesis focuses on Europe's largest gas field, the Groningen gas field in the Netherlands, as a case study for small-magnitude earthquakes under upper crustal conditions. Against this background, I performed laboratory experiments on sandstone-derived fault gouge materials, coupled with numerical modeling to simulate Groningen earthquakes under (near) in-situ conditions. A key finding is the pivotal role of pore fluid in weakening a gouge-filled fault during seismic slip, attributed to thermal expansion of pore fluid. This weakening process operates at various scales, from millimeter-scale shear-bands to micrometer-scale grain-to-grain contacts, leading to distinct stages of weakening. Pore fluid properties and host-rock materials influence the efficiency of heating and weakening, with microstructural analyses revealing the impact of initial gouge microstructure and slip localization development. The results provide better constraints on slip-weakening parameters enhancing geomechanical models of seismicity in the Groningen reservoir system but can also provide input for feasibility studies of future projects in the subsurface.