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Back to España, tierra de seísmos
Antonio Aretxabala Díez, Professor of Geotechnical Engineering, School of Engineering of Building
Spain, land of earthquakes
More than a week has passed, Lorca continues to appear as one of the most shocking news of recent times, more in the sector of Building. More than a few builders have been pointed the finger, now we ask ourselves what we have done wrong, but with what we are beginning to know, can we talk about culprits?
The catastrophe has already been analyzed from several points of view: purely seismic, constructive, historical, also that of compliance with seismic-resistant standards.
All of them coincide:
1- The region of Murcia is in one of the three most seismic zones of the peninsula, close to a plate edge with associated active faults very superficial
2- Lorca is located just above several of these fractures (they have already generated more than 130 aftershocks), the stresses are compressive NW-SE and distensive NE-SW.
3- The lands that received the tremors under Lorca are loose soils, susceptible to collapse and subsidence. Deformations are transmitted directly to the buildings by the foundations. They do not dampen deformations, sometimes they amplify them.
4- Structurally damaged buildings generally predate, but there are also those that post-date seismic-resistant standards. Some of the demolished buildings were 8 years old.
Lorca is mapped with a contribution or amplification coefficient k=1 and 0.12g (these would be the "whiplashes" that lasted 5 seconds, smashing buildings); g is the acceleration of gravity.
The accelerographs of high school Geographic in Lorca detected 0.28g for the first movement of 4.4 Degrees and no less than 0.37g for the second movement of 5.1. This acceleration catapults the movement, locally to Degree VIII, that for the United States Geological Survey (g = 0.34-0.65) the qualification is "Destructive".
For the moment we clear certain doubts about supposed deficient constructive works, still fulfilling the rule, the requirements fell 300% short.
In Alhama de Murcia, the accelerograph measured an acceleration of 0.012 g, 3,000% less than in Lorca, according to Emilio Carreño, director of the network Sismica Nacional. This shows the weakness of the horizontally distributed earthquake.
Conclusion: Lorca has been the center of the target, the worst is to be on an active fault, future seismic-resistant standards should also be thought for "small" but superficial shocks.
Would there be other localities in similar conditions, could they suffer short movements in time with acceleration peaks also 3 times greater than what the standards still in force predict? The highest coefficient is applied in Granada: 0.24g. Below 0.04g no special measures are mandatory.
It would not be bad if the inevitable revision of the seismic map that Lorca has just made obsolete, would also analyze in depth the geometry and distribution of other fractures or active faults, could affect populations in similar arrangement in: Guipuzcoa, Navarra, Northern Aragon, Catalonia, Valencia and Alicante, Murcia, Albacete, Andalusia, Southern Extremadura and Eastern Galicia, and the Canary Islands, just in case.