The 2011 Lorca earthquake slip distribution controlled by groundwater crustal unloading

Abstract

Detailed studies of earthquakes triggered by a known source of stress change can shed light on the influence of fault frictional properties and preseismic stress on the initiation, propagation and arrest of seismic ruptures. Triggered and induced seismicity can provide unique opportunities to understand this problem. However, direct evidence is rare due to the absence of e.g., near-field surface ground deformation observations and unknown pre-earthquake stress conditions. Here, we collect geodetic data recording the coseismic effects of the Mw 5.1, 11 May 2011 Lorca (SE Spain) moderate earthquake. Elastic modelling results suggest that the nucleation process and main slip area occurred at very shallow depths (2-4 km) on the rupture plane along the Alhama de Murcia fault. Slip extends towards the surface from unstable to stable friction fault segments. We find that the slip area matches well a pattern of positive Coulomb stress change due to groundwater extraction in a nearby basin aquifer. These results indicate that the shallow slip distribution during the earthquake could be controlled by groundwater induced unloading stresses at the upper frictional transition of the seismogenic layer. The relationship between known crustal stress changes (e.g., groundwater extraction) and coseismic slip distribution could help, in general, to understand where and how earthquakes tend to occur.