Lorca's earthquake. What went wrong?

I finished my last article for CBN on earthquakes on the second week of April and less than one month later on May the 11th it happened. Two earth quakes occurred on the monumental city of Lorca. The first with a magnitude of 4.4 on the Richter scale, it took place at about 17:05, and the second at 18.47 hour with a magnitude of 5.1 of the same scale. Nine people lost their lives.


Now we must ask ourselves: What went wrong?


The last earthquakes that happened in Spain with intensities higher than 5 occurred on the 11th of April of 2010 at Granada with an intensity of 6.3 grades and centred at about 25 km southeast of the city , and at a depth of 616.7 kilometres without any casualties.

 

When the floor start moving under your feet….

In Murcia we are accustomed to earthquakes but when the floor start moving under your feet, the same degree of impotence are felt by children and grown ups alike. Murcia together with Granada and Alicante are the provinces of Spain which are likely to be threaten with earthquakes of scale higher than 5.

 

As I explained in my last three articles on earthquakes, architects are required to design their buildings to comply with the earthquake-resistant standards (Ncsr-02)” but I must confess that the earthquake at Lorca was somewhat paradoxical.

 

With 5.2 degrees, was not even remotely, the highest magnitude recorded in Spain, we had earthquake of 7 degrees with no victims, but why was this one was so destructive? The epicentre was very superficial and its centre very focused.

 

Well to begin with, the epicentre was very superficial and its centre very focused. One of the factors that influenced the earthquake hazard is the depth at which occur. According to the protocol, earthquakes are dangerous from a magnitude of 5.5 onwards. An intensity of 5.2 it is not expected to produce the damage that it had cause.

 

The first earthquake had a magnitude of 4.8, and a peak acceleration of 0.275 g. (each g is the acceleration due to gravity, of 9.8 meters / second) In the second, with an intensity of 5.2, the acceleration climbed up to 0.367g as it shown in a seismograph located in the heart of Lorca. A few miles away in Alhama de Murcia, the nearest seismograph measured a ground acceleration of 0.012 g, 30 times less than in Lorca, This helps to explain why the damage was so much focused on Lorca.

 

Although it seem incongruous only one building collapsed in Lorca and I hope it is investigated and that we obtain the results soon, (I will keep you informed) although many cornices and parapet walls fell on the ground, buildings cracked and inclined balconies were seen in a lot of edifices. I am not taking in consideration what happened to monumental buildings as they are not governed by the Spanish seismic norms and I will explain below why.

 

The (Ncsr-02) norms are mandatory and consist, basically, in reinforcing foundations, columns, beams and trusses, to avoid the progressive collapse effect known as “sandwich collapse”. Looking at it from a strict structural viewpoint, I believed that the majority of buildings have resisted well Lorca’s quake, however architectural elements attached to these buildings failed frightfully.

 

Taken into account important factors such as the depth at which these two earthquakes occurred, the distance from the epicentre, the intensity of the damage it is comparable to a 7 grade quake, which means an earthquake able to produce damage to most concrete structures. Let me remind you however, that the general philosophy of the earthquake norm is not to avoid damage to buildings but to stop them from collapsing during an earthquake allowing people to escape to open spaces.

 

Monumental buildings are excluded from the seismic legislation due to the impossibility of making a masonry or stone building stand up to seismic tremors of intensities superior to 5 or 6 grades although some measures are taken generally to protect them.

 

Masonry and part of roofs fell to the ground because its construction is too rigid and unable to allow any torsion from the tremors. Unfortunately some churches, monasteries etc suffered large structural damages, although as far as I know none of these buildings will have to be demolished but some of them will require expensive rehabilitation work.

 

I was called to inspect a large factory.

I was called to inspect a large factory (about 3.500m2) situated at Lorca’s Industrial Polygon which suffered the effect of the quake mainly on the office and entrance zones, changing rooms etc. Management closed the factory down waiting for an expert to declare it safe. After a thorough check, I found that the steel structure throughout the factory had behave impeccably, nor cracks on the support plates, columns and beams were completely straight and parallel to their axis and all due to the excellent elasticity of the steel used, however a number of brick partitions had suffered severe cracking due to the characteristic stiffness of the material. These very same characteristic of brick walls were responsible for most of the deaths at Lorca. Unfortunately there are no regulations to cover these elements and they were the major cause of injuries and casualties.

 

Surely national authorities and we architects should learn from this experience and produce a set of new norms which will impede these calamities to take place in future earthquakes.

 

Another important aspect which could have saved lives at Lorca is earthquake drills. In Japan, school children and adult alike are given earthquake training and are perfectly prepared to react in and during a quake, here in Spain there are fire drills but no training whatsoever are given to children or adults at their place of work. If people had waited a few seconds inside the buildings before rushing out into the streets a good number of those poor people who are dead, could now be alive. 

 

Escribir comentario

Comentarios: 0