The 2011 Lorca earthquake slip distribution controlled by groundwater crustal unloading

Earthquake initiation, propagation and arrest are influenced by fault frictional properties(1,2) and preseismic stress(3,4). Studies of triggered and induced seismicity(5-7) can provide unique insights into this influence. However, measurements of near-field, surface ground deformation(8,9) and pre-earthquake stress conditions necessary for such studies are rare. Here, we use geodetic data to determine surface deformation associated with the M-w 5.1 earthquake that occurred in Lorca, southeast Spain, on 11 May 2011. We use an elastic dislocation model to show that earthquake nucleation and the area of main fault slip occurred at very shallow depths of 2-4 km, on a rupture plane along the Alhama de Murcia Fault. Slip extended towards the surface, across fault segments with frictional properties that changed from unstable to stable. The area of fault slip correlates well with the pattern of positive Coulomb stress change that we calculate to result from the extraction of groundwater in a nearby basin aquifer. We therefore suggest that the distribution of shallow slip during the Lorca earthquake could be controlled by crustal unloading stresses at the upper frictional transition of the seismogenic layer, induced by groundwater extraction. Our results imply that anthropogenic activities could influence how and when earthquakes occur.