NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION | www.nature.com/naturegeoscience 1 news & views I t does not take much to trigger an earthquake — even strong rainfall can do the job 1 . A variety of human activities can trigger earthquakes as well 2,3 . Numerous examples of seismicity triggered by the impoundment of reservoir lakes, hydrocarbon extraction, quarrying and deep well injections have been documented over the years 3 . If the stress perturbations that triggered those earthquakes were known exactly, they could help us understand the mechanisms that govern the initiation, rupture and arrest of earthquakes, and also help with seismic hazard mitigation 4 . On 11 May 2011, a M w  5.1 earthquake struck southern Spain, killing nine people and seriously damaging buildings in the nearby town of Lorca (Fig. 1). e earthquake was surprisingly destructive, given its modest magnitude. Writing in Nature Geoscience, González et al. 5 show that known human- induced stress changes related to groundwater extraction probably triggered the Lorca earthquake and caused its shallow depth. ere are a number of unsolved questions that relate to triggered earthquakes 6 . For example, the duration and magnitude of the stress perturbation required to trigger an earthquake is unknown. It is also unclear whether the magnitude of the stress perturbation also affects the size of the earthquake, or whether the rupture evolves independently of how it was initiated. Furthermore, it is not clear whether earthquakes triggered by human activities are different from natural earthquakes or are simply prematurely induced natural earthquakes that would have happened anyway at a later time once enough natural stress had built up. More generally, a better understanding of triggered quakes will not only help mitigate against human- induced tremors, but could also improve our understanding of earthquakes that are naturally triggered by crustal stresses induced by past earthquakes, tectonic deformation or movements of magma 4 . To better characterize the source of the 2011 Lorca earthquake, González et al. 5 used interferometric synthetic aperture radar (InSAR) to measure the ground deformation caused by the quake. In spite of relatively strong atmospheric noise in the data set, they were able to measure the small amounts of deformation produced by the earthquake and model the data to identify its source. ey found that the earthquake resulted from slip on the Alhama de Murcia fault, which bounds the Alto Guadalentin Basin just south of Lorca. eir model shows that most of the seismic moment was released by about 20 cm of slip on a compact fault patch, about 2 by 3 km 2 in size, located at a surprisingly shallow depth of about 3 km. Another, much smaller fault patch, located at only 1 km depth, slipped just a few centimetres. EARTHQUAKES Human-induced shaking In 2011, a modest earthquake in southern Spain seriously damaged the city of Lorca. Analysis of surface deformation suggests that the quake was caused by rupture of a shallow fault patch brought closer to failure by the pumping of water from a nearby aquifer. Jean-Philippe Avouac + * Lorca Dropping water table a b Figure 1 | The May 2011 M w  5.1 Lorca earthquake. a, González et al. 5 suggest that water extraction from an aquifer could have triggered and influenced the pattern of rupture propagation of the Lorca earthquake. The earthquake ruptured a fault near the city of Lorca that runs north of the Guadalentin Basin. This fault was in horizontal compression owing to regional tectonics (red arrows). Water extraction caused a drop in the water table and unloaded an area of crust close to the fault. The unloading caused elastic rebound of the crust that induced an additional horizontal compression on the fault (because the fault lies at the edge of the unloaded area), bringing it closer to failure. The star indicates the hypocentre where the rupture initiated. b, The earthquake caused extensive damage to buildings, including the Church of Santiago in the city of Lorca. © JUAN CARLOS CARDENAS/EPA/CORBIS © 2012 Macmillan Publishers Limited. All rights reserved