Flow-geomechanics coupling constrains fault geometry in fluid-induced earthquakes

Abstract

Post-mortem analysis of earthquakes induced by fluid extraction or injection is often complicated by the uncertainty in the location and geometry of the causative fault. The 2011 Lorca earthquake in southeast Spain is believed to be triggered by long-term groundwater withdrawal, causing slip along the Alhama de Murcia Fault (AMF) dipping northwest. However, the regional InSAR deformation data can be equally fit by AMF and an unmapped fault located approximately 5 km west of AMF and dipping southeast, which creates an ambiguity in the causative fault that hosted the earthquake. Here, we show that the assumptions of elastic dislocation, undrained deformation, and decoupling between flow and deformation processes contributed to the ambiguity, which can be resolved by conducting a fully coupled analysis that provides additional constraints on the problem. We test that hypothesis and propose that the Lorca earthquake was likely caused by the rupture of a southeast dipping fault plane, which is antithetic to AMF. We build a mechanistic model of groundwater withdrawal over the time period of interest (1960–2010) that includes pressure diffusion, aquifer contraction, crustal unloading, and basement expansion mechanisms. The model identifies the difference in pumping-induced loading of the two faults: poroelastic compression and down-dip shear on AMF vs. tension and up-dip shear on the antithetic fault. We demonstrate that two-way coupling between flow and deformation processes plays a crucial role in the natural selection of the earthquake-inducing fault and holds the potential to detect hidden faults in the case of anthropogenic triggering.