Study links groundwater pumping to earthquake mechanics

A study published online on October 21, 2012, by Pablo J. González's team suggests that agricultural pumping did not cause the 2011 Lorca earthquake but may have influenced the location and manner in which the fault slipped. The research focused on how groundwater removal might have altered the stress on the Earth's crust near the earthquake's fault line.

Subsidence in the aquifer area

The study, titled to indicate a controlled slip distribution caused by the unloading of the Earth's crust related to groundwater, notes that the water table in the Alto Guadalentín aquifer has dropped by approximately 250 meters since 1960, more than the height of the Montparnasse Tower. This subsidence, measured at up to 10 cm per year by satellite, may have affected the stress on the nearby Alhama de Murcia fault, which was the site of the 2011 earthquake.

Details of the 2011 Lorca earthquake

The 2011 earthquake, of magnitude Mw 5.1, struck approximately 2 km northeast of the city center, causing 9 deaths and nearly 300 injuries. It is believed to have slipped mainly between 2 and 4 km below the surface, with a slip of 5 to 15 cm. The study used radar images from the Envisat satellite and GPS measurements to reconstruct ground movements and deduce a model of the fault and the area that slipped.

Role of groundwater extraction in stress changes

The researchers calculated the stresses created in the crust by the withdrawal of groundwater and found that the area that slipped corresponded to where these stresses pushed the fault to break. The authors conclude that pumping may have controlled the distribution of the slip, but the earthquake's energy release was primarily due to the natural tectonic forces from the approach of Africa and Europe, which had accumulated constraints over centuries.

Human influence on earthquake dynamics

González estimates that even without pumping, the earthquake was expected, as the energy released far exceeded that which water extraction could have added. Jean-Philippe Avouac, in a comment published with the study, notes that the extraction probably accelerated a natural process of stress accumulation, without triggering the earthquake on its own.

Limitations of the study's findings

The study's conclusions suggest that human activity can influence the way and timing of an earthquake, but the scope of the result remains limited. González himself judges that these conclusions probably do not apply to other seismic regions where water is also pumped. The Murcia region continues to draw its water from an underground that sinks several centimeters per year in certain areas of the basin.