In their article, Prof. Reinoud Vissers, Utrecht University, and Bart Meijninger, Fugro, the Netherlands, explain how this moderate earthquake could cause so much devastation and even casualties. Earthquakes are quite common in this region, but rarely cause much damage.

These serious effects most likely were the result of the shallow depth of the hypocentre (1-2 km) and proximity to the city of Lorca. The focal mechanisms of the main earthquake as well as a M 4.5 foreshock agree very well with a recent structural analysis by the same authors.

Vissers, R. L. M. and Meijninger, B. M. L.: The 11 May 2011 earthquake at Lorca (SE Spain) viewed in a structural-tectonic context, Solid Earth, 2, 199-204, doi:10.5194/se-2-199-2011, 2011.

Abstract
The Lorca earthquake of 11 May 2011 in the Betic Cordillera of SE Spain occurred almost exactly on the Alhama de Murcia fault, a marked fault that forms part of a NE-SW trending belt of faults and thrusts. The fault belt is reminiscent of a strike-slip corridor, but recent structural studies have provided clear evidence for reverse motions on these faults. Focal mechanisms of the main earthquake, but also of a foreshock, are strikingly consistent with structural observations on the Alhama de Murcia fault. This strengthens the conclusion that, rather than a strike-slip fault, the fault is at present a contractional fault with an oblique reverse sense of motion, presumably in response to the NW-directed motion of Africa with respect to Europe.