The Ground Movement in Naples Area

First of all, in the scopes of this chapter it is important to put an emphasis on the fact that the volcanic hazard on the particular territory (Naples area) is a very important factor for the engineering geologists due to the fact that the earthquakes have their effects on the buildings, constructed on a particular territory and, consequently, on the safety of life of those people, who are living in this area.

Fritzalas and Papadopulous (1988) have started their research from the eruptive history of the post-Mionan period, this history is dedicated to the investigation of the Santorini island report, related to the current volcanic risk in the region. The qualitative type zonation is offered by the authors and in addition, the two areas of high risk are determined (the areas, which are exposed to the acid gases and the costs, exposed to tsunami). Authors have proven the fact that the cycles of the Santorini eruption in the timeframes of last two decades follow within the period of 15 years and the seismic energy is mainly associated with the large subcrustal shocks.

The next issue to be discussed in the scopes of this chapter is the Mount Vesuvius in Naples activity. Majority of scholars consider it as one of the most dangerous volcanoes in the world. One of the most exacerbating facts is that the population of the region reaches 3,000,000. While referring to the historical background it is important to put an emphasis on the fact that in AD 79, the deadly power of Vesuvius has been demonstrated dramatically. At that time, the Roman cities of Pompeii and Herculaneum have been ruined by the monumental eruption.

The signs of the eruption have been shown by the Vesuvius for several years before the great eruption has taken its place. Lomax et al (2001) have proven a fact that in majority of cases, the volcanoes and earthquakes occur simultaneously. According to the general principles of geology, both volcanic eruptions and earthquakes are caused by the Earth’s tectonic plates’ movement. Naples lies on the intersection of North American and the Eurasian tectonic plates.

The movement may be towards each other or away from each other and in such case the pressure is caused and the inside part of the volcano is build up. After that, the magma, located within the volcano starts moving. This phenomenon is considered by Guidoboni et al (2006) as the release of the pressure and the final outcome of this process may be eruptions and/or earthquakes.

The temporal evolution of the eruption may be summarised in the following phases, which are typical of those of major large scale volcanic eruptions.

The first phase is characterized the development of a high, sustained column, where the mixture of juvenile gases and pyroclasts takes place, and it mixes with the atmospheric air turbulently. This leads to convective rise of the cloud into the stratosphere which can reach an estimated maximum height of 32 km.

In the second phase, there is collapse of the eruptive column and this is emplaced by the pyroclastic flows which surge and are capable of destroying every settlement within a diameter of 20-30 km from the volcano.

The magma chamber collapses, and water ingresses into the feeding system. The magma water interaction leads to a final phreato-magmatic activity. After the eruption, the ashes and pumice and remobilized by rain water.

These aforementioned phases can be identified by the typical deposits seen near Mount Vesuvius. The first phase can be proved by the fall deposits that consist of a lower part of well sorted white pumice and an upper part of gray pumice that is dispersed to the southeast of the volcano, and can be traced on land to a distance of more than 70 km.

The second phase consists of the surge deposits that were made of layers of thin, poorly sorted ash with cross bedding and dune structure, which alternate with deposits of pyroclastic flow deposits. The deposits of the third phase consist of silty sand bed with abundant accretion lapilli. Angular lava and carbonate blocks supported by ash matrix with minor pumices are seen.

Even while talking in account the fact that the interrelation between the eruptions and earthquakes is obvious to the local population, information about the volcanic activity perpetual nature in the region of Naples and warnings, related to that issue have been unheeded by the people, living in the region.

In the scopes of this chapter additional attention would be paid to the eruptions of Vesuvius which have taken their place in the timeframes of last 75 years (20-th century and beginning of 21st century). The last eruption has taken its place in 1944 (March), which has destroyed the following villages: Ottaviano, Massa di Somma, San Sebastiano al Vesuvio and some part of San Giorgio a Cremano. In the timeframes March 18 to 23 (1944), the flows of lava have appeared within the rim. In addition, there have been outflows. Also, the small explosions then occurred in March 18, 1944.

It is important to put an emphasis on the fact that after the Naples Sub has been constructed, the set of archaeological and geotechnical investigation s have allowed to recognize the fact that the shoreline of the city have experienced the significant changes in the timeframes of Late Holocene high stand, when the core impact on the coastal evolution has been made by the vertical movements of ground. As an evident example of such investigations, it is possible o consider the fact that in the tree sites, which have been investigated, the quality and the density of the information has provided the Auger et al with an option of relative sea level change (RSLC) graphs reconstruction and to carry out the comparison analysis with the Tyrrhenian Sea coeval eustatic changes curves. Later, this information has been transformed into data, related to the local tectonics Late Holocene history.

After the scholars have carried out the integration of the obtained information and its deep analysis, they have made a conclusion that Naples’ coastal strip has experienced the rate of subsidence 1.3 mm/a in the timeframes from about 5000 BP to the XII century. Scientists have made a conclusion about the consistence of the subsidence trend with the regional tectonics, even while taking in account the fact that the rate of mean has been less than in other points of the investigated Naples area. This trend may be partly caused by the fact that the area of study lies among the synthetic and parallel faults, which in turn, form the NW depression margin and are not located on the roper hanging-wall block. The minor episodes have probably interrupted the subsiding trend, which is inherent to the particular region.