We are all wondering: if most specialists assure us that an earthquake of magnitude average (5.1) is not so strange in these parts, what has happened? Spain registers about 2,500 earthquakes per year, of which about thirty are felt by the population, but the worst is already here, right under our noses. Whether we like it or not, we are entering the 21st century, as the modern country that we are, with a dozen fatalities from earthquakes. There have been before: yes, in the nineteenth century, also in the twentieth century, on April 20, 1956 in Granada (twelve deaths) and on February 28, 1969 in Huelva (four deaths due to heart attacks after the earthquake). So how is it possible that the damages in plenary session of the Executive Council 2011 and with such a strict construction rules and regulations are so great?

It would be necessary to dissect the answer in two parts, one that concerns Nature itself: the capacity to generate earthquakes in the most seismic area of the peninsula (known to all) in addition to the specific characteristics of the subsoil that has suffered the shock (no longer so well known); and the other would be regarding the subject of damaged buildings and the application of the successive and historical earthquake-resistant standards in the design of the buildings.

Regarding the first one, the Lorca area (on a regional scale) is immersed in a known seismic activity zone. The amount of daily earthquakes (and yes, I mean daily) registered is proportional to the issue of observatories that have been installed during the scientific and technological development of our country. Each time the sensitivity of these devices is greater. Conclusion: we register more and more movements, that is, we can assume that they have always been there, even when we did not detect them; we live on a quite dynamic bull skin: the Canary Islands for its volcanic activity, the Levant, the Pyrenees and the Southeast, take the medals for being on the podium that gives the clash between the African and Iberian plates.

The generation of earthquakes at this scale can be likened to a group of dominoes lying horizontally on the table, close together, touching, tangent to each other. If we move one of them, a train of pushes is generated until they all realign in a few moments. Something so superficial is analogous to the superficiality of the segments that between faults and fractures have generated the Lorca catastrophe.

But specifically in the basin where Lorca is located, which subject soils receive and host these impacts? For the uninitiated in Geotechnics it may sound strange: Miocene marls and gypsum. For specialists it says enough. It is the well-known Zone IV of the guide of planning of programs of study geotechnical for Building of the Ministry of Public Works of the region of Murcia.
The marls with gypsum, given its slightly cohesive and soluble nature, generate voids at the scale of the rock massif (we know it as karst phenomena) but also these voids are present in the most intimate and microscopic part of the soil that by weathering these materials generate, so we talk about potentially collapsible soils or semi-saturated soils, whose cohesion is variable so we are going to see.

These collapsible soils are usually firm soils, with a bearing capacity average, although it is only recently (the CTE came into force in 2007) that the relevant measures have been taken to detect them, to testify to them and point the finger at them as "potentially dangerous". In themselves they are not, they are innocent, it only takes an exogenous force to disrupt this internal or endogenous cohesion and everything falls apart like a house of cards. The same guide quoted says it: the greatest danger is collapse and collapse (BORM November 3, 2001). And those forces sooner or later come in the form of pipe bursts, vibrations due to heavy traffic and the worst of all: an earthquake (or two) like the ones that have occurred.

As for the design of the buildings, the most affected and spectacular are prior to the seismic standards, the fallen cornices that have caused the deaths, wrecked vehicles and the collapses of the buildings are, in the absence of a study that will be necessary in the future, mostly of brick masonry.

This second part remains open. There is no doubt that seismic-resistant standards, especially the latest NCSR-02, are complied with almost to the letter, especially in hospitals. However, in this bullfighting world in which we live, our history has also been sculpted by picaresque practices in the service of easy profit. It would be very sad if some of this dark psychology had played a leading role in one of the fiercest and cruelest natural disasters in our recent history.