An international group of researchers has identifiedin the most recent history of the Alhama de Murcia fault, six large earthquakes of magnitude greater than 7. This fault is a ground fracture plane that runs through the entire earth’s crust, so to identify prehistoric earthquakes in its walls, scientists made surface excavations perpendicular to the fault – trenches between 20 and 30 meters long and four meters deep – which has allowed an exceptionally long paleoseismic record to be obtained.
“Large earthquakes, when they exceed magnitude 6, tend to break on the surface and, therefore, we have been able to identify these breaks in their walls,” José J. Martínez Díaz, a researcher at the Complutense University of Madrid and co-author of the study published by the journalGeological Society of America Bulletin. The dating of these tectonic deformations was carried out using carbon-14 and infrared-stimulated luminescence techniques.
To understand the behavior pattern of the Alhama de Murcia fault, researchers have to go back in time hundreds of thousands of years “much longer than Americans or Japanese, who can already identify them from data of 10,000 years”, he adds. Martínez Díaz. This is becausein Spain the faults are slower and, therefore, the recurrence time between large earthquakes is much longer-on the order of thousands of years- than those with much faster faults such as San Andrés (California). According to his estimates, lThe Alhama fault would have been generated more than 9 million years agoand would cause earthquakes from the beginning, thereby controlling the landscape of the region.
By studying the seismic activity of the Quaternary (those earthquakes that are less than 1.8 million years old), scientists have identified a minimum of six earthquakes of great magnitude during the period studied (more than 300,000 years), but they suspect that the real number of great events must have been greater.
More seismic risk
Another novelty is that, according to the article, a larger earthquake could occur in the area than previously thought . “The faults do not break along their entire length at the same time during earthquakes, but are segmented. We have verified that this fault can break its two western segments in one go, from Góñar (Almería) to Totana (Murcia), which It would imply an earthquake with a magnitude greater than 7 “, explains Martínez Díaz, who adds that it is essential that it be taken into account in the calculations of danger and construction regulations in the area. Until now, its danger was underestimated because, until now, the estimates were based on the catalog of historical seismicity with the events that occurred in the last 2000 years.
Regarding the possibility of predicting the next large-magnitude seismic movement, the authors emphasize that it is possible to say its maximum magnitude and where it may occur, but today there is no way to predict the exact moment of the earthquake, which is a geological phenomenon. complex governed by nonlinear physical processes. “What we can know, because it has been modeled and we have published it in previous studies, is that earthquakes such as the one in Lorca and previous ones cause stress changes in the fault, with increases in certain parts of it. In these areas it is The next one is more likely to happen. Now, estimating when it is impossible, “concludes Martínez Díaz.
