2011-07-24 13:24:13
The story that touches the heart
“Humble contribution from Lorca. Thank you for brightening up these last few hard nights…” Bartolomé Gómez Navarro wrote this phrase, a grateful listener who, after suffering the consequences of the earthquake that shook the city of Lorca on May 11, told us the following: “I honestly wouldn’t know how to pay for that night in which my 7-year-old son, staying in a “strange” house, was oblivious to all the drama we were experiencing and was happily arguing with his 42-year-old uncle… “No matter how you feel, The water is blue and if you see it as transparent it is because there is very little in that glass… it is blue because a scientist said so on my dad’s cell phone…
There are many expressions of gratitude that we receive, thanks to them we continue, against all odds, this effort to disseminate science but, on this occasion, the words reached the depths of our hearts. We knew then that there are many ways to help in difficult situations and, although we could not be there, without knowing it, we were doing it. We thank Bartolomé and his son.
Days have passed and life goes on in Lorca, but that earthquake has taught lessons, some of which we present in today’s program thanks to one of the scientists who dedicate their lives to the study of seismic movements. We spoke with Mr. José Manuel Martínez Solares, Head of the Geophysics Area at the National Geographic Institute, whom we had the opportunity to listen to at the Summer Courses at the Complutense University of El Escorial, where he gave a conference entitled “Earthquake of Lorca: reality or myth”
History
Let’s do a little history for those who, because they live far from Spain, are unaware of the data. On May 11, 2011, at five minutes past three in the afternoon, a magnitude 4.5 earthquake shook the city of Lorca, in Murcia, southeast of Spain. The tremor shook the nerves of the Lorca population, but the worst was yet to come, one hour and 40 minutes later, the ground shook again with more force and, although its magnitude was only 5.1, eight hundred thousand times weaker than the one that had devastated the northern region of Japan just two months before, it caused considerable destruction that has led to it being assigned an intensity of grade VII (below we explain the difference between “Magnitude” and “Intensity”).
Nine people lost their lives, most due to the detachment of cornices and other façade elements, many buildings have suffered structural damage and the historical heritage, the medieval castle, churches and bell towers have been seriously damaged or destroyed.
So much destruction was surprising in the face of a medium-magnitude earthquake and this raises a series of questions: What was the destructive effect due to? What buildings and what part of them are the most vulnerable in an earthquake of this type? Why is there greater destruction in the low areas as can be seen in the photographs? What lessons has the Lorca earthquake taught us?
Magnitude and intensity
We are used to playing with numbers and that habit sometimes leads us down wrong paths, this is what has happened to many when assessing the destructive capacity of the Lorca earthquake. If we are told that the magnitude of the earthquake was 5.1, while another very recent earthquake, that in Japan, reached magnitude 9, we tend to think that the one in Lorca was nothing more than a slight tremor. It’s not like that, we are letting ourselves be fooled by the numbers.
When seismologists talk about “magnitude” they are giving us a measurement of the Total energy released by the earthquake (known as the seismological moment magnitude scale, Mw). The scale is also logarithmic, meaning that a magnitude 6 earthquake is actually 30 times larger, in terms of energy released, than a magnitude 5 one.
Now, if we confuse the energy with the destructive effect We will be making a fundamental mistake. I give an example. A hand bomb, when exploded, releases a specific amount of energy, X, that energy is the same regardless of where the explosion occurs. However, we all know that it is not the same for the bomb to explode a meter away from us as it is a kilometer away. In the first case it will destroy us and in the second we will barely hear a distant detonation, therein lies the difference.
In the interview, José Manuel Martínez Solares gives us an eloquent example. The Lorca earthquake, with a magnitude of 5.1, caused the aforementioned damage, however, in 1954 a magnitude 7 earthquake occurred in Granada and almost no one noticed it. The difference is that the first was very superficial and close to the city, the second, however, occurred 650 kilometers deep. So, let’s not be fooled by the numbers.
To overcome the problems of interpretation given by the seismological moment magnitude scale is used the intensity scale based on the destructive power of an earthquake. The Lorca earthquake had intensity VII (the intensity is given in Roman numerals).
These and other things Mr. José Manuel Martínez Solares tells us in today’s interview.
REFERENCES
Earthquake series NE Lorca (Murcia) 05/11/2011 Edited by the National Geographic Institute
National Geographic Institute IGN
Seismological moment magnitude scale
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