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A map of the UA shows the seismic danger in this province and in the region of Valencia

 

Inten_Sism_Esperada

 

Alicante, 12th May 2011

The province of Alicante and in particular, the Vega Baja, are the areas of Valencia with more seismic hazard, according to a detailed map, resulted from a research recently conducted by the University of Alicante for the special action before the the seismic risk in thisd Region. This map shows that this danger is increased in this part from north to south, although this is not regular.

The research work, called Estudio de Peligrosidad Sísmica de la Comunidad Valenciana (Study on Seismic Hazard in the Region of Valencia), was jointly conducted by José Juan Giner Caturla, lecturer in Earth Sciences and Director of the UA Seismic Network, and the Provincial Fire Brigades Consortium. This study has been performed by bringing together both historical data of earthquakes from 1396 and a classification of the entire territory of the Region of Valencian by its land features.

The study has been performed on a scale of earthquake "intensity", other than "magnitude", which is the most used. The intensity scale is subjective, that is, it assesses the effects of the earthquake and how it is perceived by people: for example, how many people feel the shock, if the objects move or if there is damage to buildings. José Juan Giner explained that the reason is that the magnitude data are relatively recent, coming from the instrumental records since the equipments were installed, and are, therefore, not comparable with those obtained from historical documentary sources, which only provide information about the effects observed, ie. the intensity. We have used the European intensity scale, ranging from Grade I to XII. Grade IV earthquakes have been collected for over 600 years, the first from which slight damage has been produced in vulnerable buildings, and higher; whereas in the magnitude scale (with no intensity) Grade VI would represent major destruction.

In the Region of Valencia, the earliest documented earthquake was in Orihuela in 1048, evidenced thanks to the translation of Arab testimoniesperformed by the also UA lecturer Mike Epalza. However, the project began with the earthquake of 1396 Valldigna Tavern because it is thereafter when the catalogue used in these episodes is homogeneous. The sources used to compile the catalogue are from the local archives of the parishes, private correspondence, etc.

In order to conduct the study,  the territory of the Region of Valencia has not been the only one taken into account, but also adjacent regions at a distance of 100 kilometers and higher. José Juan Giner gives the example that an earthquake of intensity IX in Murcia can be perfectly perceived in Torrevieja as intensity VII.

In total, the project collects data from 115 seismic levels VI to X, the latter corresponding to the earthquake occurred in Torrevieja in 1829, although in fact the worst hitted was Almoradí with nearly 200 dead people. This population, says José Juan Giner, learned so much from this tragedy that after the reconstruction it has been qualified for many years as the best built-up to cope with an earthquake, with wide streets and low buildings of uniform height.

For mapping seismic hazard map, they have combined two maps: the average intensities expected with the characteristics of the land according to their way of reacting to the vibrations of the earthquake.

A "return period" of 500 years has been taken into account in map of the expected intensities, ie, the statistical probability that an earthquake of grade VI or above occurs in a period of 500 years at each point in the territory .

Also, the map of the land features classifies it into four categories according to their ability to react and transmit seismic waves, the highest of which doubles the lowest. This "site effect" map  has been produced by dividing the land into squares of 500 by 500 meters, not only of the Region of Valencia but of its regional environment. The so-called "site effect" is known since an earthquake in Mexico showed that there were areas that suffered the earthquake intensity even higher though being farther from the epicenter, depending on the type of stones they are made of.

The combination of the two maps reflects the hazard index of the Region of Valencia. There, it is found that the highest values of risk for a period of 500 years are the south of the province of Alicante, while it decreases to be virtually nonexistent by the middle of the province of Valencia, with just one interruption towards the centre of the boundaries between the provinces of Alicante and Valencia, where there is an area of increasing hazard.

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