This is the surprising experiment with coffee and sand that explains the earthquakes in Spain

The approach of the tectonic plates causes earthquakes like those of Granada, but at the other end of the Peninsula it is the fractures and fluids of the earth’s crust that generate seismic movements.

Comparison between the main tectonic structures and depth variations of the 'brittle-ductile transition zone' in the northwest of the Peninsula (left) and what the model shows (right).

The earthquakes that have recently occurred in the south of Spain and in the east of Japan have their origin in the collision of the tectonic plates, of Eurasia and Africa in the first case, and of the Philippines and the Pacific in the second.

However, in the northwest of the Iberian Peninsulawhich is far from the edge of the plates, from time to time seismic movements also occur, which have exceeded magnitude 5 in towns such as Puebla de Sanabria (Zamora) in 1961 and Lugo in 1997, with damage and alarm between the population.

Behind these earthquakes are the fractures that were formed and reactivated in the earth’s crust during the Alpine orogeny (the same one that raised the Pyrenean, Cantabrian and Betic mountain ranges), but the exact mechanisms that trigger them are not at all clear.

To solve the problem, geologists from the universities of León, Rey Juan Carlos and Complutense of Madrid have devised an original model that simulates what has happened under that territory with the help of a ‘box’ in which they deposit three elements: silicone to represent the lower and middle crust (20 km=2 cm), white sand colored by layers as upper bark (15 km= 1.5 cm) and coffee particles sprinkled on top to follow their movements. One of the walls of this sandbox it is mobile.

“As we generate a deformation in the box like the one that occurred during the Alpine orogeny, we record everything that happens using a laser scanner, and thus obtain a digital model to analyze the topography and the changes that occur in the relief”, explains one of the authors, Javier Fernandez Lozanofrom the University of Leon.

The team, who publish their study in the journal tectonics, uses a mathematical algorithm to measure the displacements generated by the fractures on the surface of the model. In this way it is possible to observe where the deformations are concentrated and, therefore, where earthquakes are most likely to occur.

According to the study, its origin lies in the presence of fluids that circulate at great depths and with high thermal gradients, which facilitates the breakage of fractures in the crust and thus increases seismic activity in certain areas of the peninsular northwest.

“This phenomenon would explain the significant variations in seismicity observed at the western end of the Cantabrian Mountains and the Galician-Leonese MountainsIn the call brittle-ductile transition zone (where the rocks of the earth’s crust go from being harder and more fragile to more malleable as the temperature rises with depth)”, explains Fernández Lozano, “and in that area, the increase in the pressure of the pores facilitates the opening of the fractures and circulation of hot fluids, reducing the resistance of the crust and the depth at which earthquakes occur”.

gold deposits

In addition, the geologist points out that this study has important implications for the formation of gold deposits in northwestern Spain: “Fractures act as escape valves for hot fluids, and when their pressure exceeds a certain threshold, the rock breaks and circulate towards shallow areas where mineral elements of great interest precipitate, such as gold, tin and tungsten”.

“Therefore –he concludes–, with the new earthquakes the possibility is opened that a new deposit may be developing under those areas of Castilla y León and Galicia, that is, primary deposits (where the ore is formed in the fractures of the parent rock) as important as those that later, by transport and sedimentation, have given rise to the secondary the marrows (ancient Roman gold mines) could be in formation today throughout the northwest of the Peninsula”.

Posted on