October 21, 2012

expert reaction to Spanish earthquake linked to groundwater extraction

A study published in Nature Geoscience suggests the earthquake in Lorca, southern Spain, in May 2011 was triggered by groundwater extraction.

Professor Peter Styles, Professor of Applied and Environmental Geophysics at Keele University, said:

“This is a very exciting and stimulating paper, which makes a very strong case for crustal unloading due to excessive groundwater extraction as the trigger for the Mw 5.1 earthquake that occurred in Lorca, southeast Spain, on 11 May 2011.

“Faults in the crust are in a state of equilibrium under complex systems of stress, partly tectonic in this case through the interaction between the North African and Southern European areas but also with the weight of the rock itself.

“Isostatic unloading and the associated elastic response of the crust and lithosphere is well known as a cause of seismicity; in fact much of northwestern Scotland’s recent historic seismicity is associated with glacial unloading from the last ice sheet c 10ka ago.  The Betic Cordillera is one of the most seismically active areas in the Iberian Peninsula reflecting the neotectonics and it is not unexpected that the removal of 250 metres of groundwater since 1960 (a very significant mass change over a relatively short period of time) together with many centimetres of subsidence caused by compaction are sufficient to act as the minor perturbation (the straw which breaks the camel’s back) for a stress system which was probably near to failure.  

“The authors comment on the role which anthropogenic activity can play in stimulating the response of the crust and there will no doubt be speculation as to the implications of this for hydraulic fracturing in the context of shale gas exploration.  Indeed, prior background seismicity of significant magnitude and the presence of active fault systems are two of the prime parameters, which need to be considered before hydrofraccing is permitted; and this region of Spain, even if it were prospective for shale gas, would be categorically excluded because of historic activity.

“The mechanism which has been generally accepted for the Blackpool earthquake sequence is of fluid percolation along bedding planes into a fault and the subsequent reduction of normal shear stress leading to minor slip of a few mm in a relatively weak rock with a consequent minor tremor of 2.4 ML.  This is in no way similar to the isostatic elastic response seen in Lorca.  Calculations of the Coulomb stress changes associated with extraction of shale gas (not groundwater) could be made but they are likely to be orders of magnitude smaller than are seen here.”

‘The 2011 Lorca earthquake slip distribution controlled by groundwater crustal unloading’ by Pablo J. González et al, published in Nature Geoscience on Sunday 21 October 2012.