Luongo, Giuseppe

This paper analyses data regarding the seismicity and ground deformations of the island of Ischia. The goal is to describe these phenomena as a space–time process, exploring the mechanism as a whole in order to answer questions concerning why and how historical earthquakes occurred and the geological constrains at the time of the earthquake of 21 August 2017. According to our analysis, the genesis of earthquakes in the island may be due either to regional tensile tectonics or to increased pressure in a shallow magma source. The eruptive rest following the 1302 Arso lava flow, the seismic silence from 1883 and the ground sinking recorded at least from the end of the nineteenth century suggest a decline of pressure in the shallow magma source and in the action of the stress field generated by the spreading of the Tyrrhenian Basin. The mechanism that can generate the observed ground deformation field and seismicity is the gravitational and thermal loading of the island, because the load of the massif of Mt. Epomeo onto its hot substratum may induce the gravitational spreading that would have produced its sliding southwards and its rotation northwards. The surface of detachment plunges towards the south and emerges at a high angle at the northern edge of Mt. Epomeo, where coseismic ground effects and the greatest damage have been observed. This mechanism that generated the 21 August 2017 earthquake could be extended to historical seismicity.

The island of Ischia has a land area of 46.3 square kilometers and a population of 64.115 inhabitants by 2019. The territorial density is 1,385 inhabitants per square kilometer. The environmental sustainability of Ischia is about 22 thousand inhabitants. In the year 2017 there were about 2.3 million tourists. The remarkable expansion of historical settlements on the Island in recent decades, without adequate planning that would pay attention to the geological hazard, has produced an exponential increase in risk, as it was shown by the disastrous earthquake that occurred in Casamicciola on 21 August 2017. The underestimation of volcanic, seismic and hydrogeological risks on the Island is incomprehensible due to the catastrophes that occurred with the historic eruption of 1302, the strong earthquakes of 1881 and 1883 and the disastrous flood of 1910.

Geologists, geophysicists and volcanologists have long drawn on historical and archaeological data concerning earthquakes, tsunamis, volcanic eruptions, landslides and floods. These are important data, which help scientists to gain further knowledge about natural events, their evolution, and their effects on the built environment. Such studies - in which “time” is the dimension informing investigations of the dynamics of extreme events and their impact on the environment - stand at intersections among different disciplines.

The earliest sources on the seismicity of the island of Ischia date back to the Greek colonization of the Island in the eighth century BC. Until the eruption of 1302, seismicity was widespread across the whole Island, while thereafter it was mainly concentrated in its northern part, at the foot of the Monte Epomeo massif. One of the characteristics of earthquakes on Ischia is that they occur very near the surface, since at depths of more than 3 km, due to their high temperatures (> 400 °C), the rock has a ductile as opposed to a fragile behavior, and hence do not fracture and do not gene rate earthquakes. Earthquakes are due to the moving of adjacent blocks separated by a surface of discontinuity known as a fault, along which the relative movement of the two blocks occurs when tectonic forces become stronger than the friction that impeded their movement. After an earthquake, the system needs to reload for the friction to be again overcome and for another earthquake to be thus generated.

La Baia di Napoli si sviluppa lungo la fascia costiera del Tirreno nella regione denominata Piana Campana che si estende per un’area di 5000 km2 tra la catena appenninica e il bacino tirrenico, dal Monte Massico, a nord, alla Penisola Sorrentina, a sud. L’antico nome di quest’area è Campania Felix. Questo territorio è uno straordinario monumento geologico caratterizzato da attività vulcanica, terremoti, tsunami, movimenti verticali del suolo (bradisismo) che hanno generato miti, legende, reperti archeologici e documenti storici. La storia eruttiva dei vulcani napoletani e l’evoluzione dell’Appennino Meridionale e della Piana Campana caratterizzano questo territorio come una struttura a diversi rischi geologici. Perciò quest’area può essere considerata un ‘laboratorio’ per la valutazione dei rischi naturali dalla quale trarre elementi da utilizzare nella pianificazione di luoghi di grande pregio paesaggistico e culturale ad elevato rischio. Lo scopo di questo lavoro è di fornire una descrizione del territorio come un monumento fisico che si è costruito in seguito al verificarsi di processi geologici complessi, e in particolare vulcanici e tettonici che hanno agito negli ultimi 10 Ma. Per raggiungere questo obiettivo è necessario conoscere composizione e proprietà fisiche delle rocce della crosta superiore, la morfologia del territorio e la sua evoluzione, in quanto il paesaggio terrestre è costruito per l’interazione di questi elementi.

With the Industrial Revolution the laws of physics were introduced to explain natural phenomena. At that time the Vesuvian Observatory emerged as the first volcanological observatory in the world to monitor the activity of Vesuvius on a permanent basis. Naples became an attractor for scholars, who were to analyze volcanic phenomena by developing relationships between the science of laws and those of processes. After World War I interest in Naples-based volcanology further increases, as attested by the founding in the city of Immanuel Friedlaender’s International Institute of Volcanology. Following the twenty-year Fascist period, Italy had two objectives: to reconstruct the network of science laboratories and rebuild the approach to studying Earth Sciences through comparison with more advanced countries. Significant and original contributions were made regarding the new theory of global tectonics and the mitigation of natural risks.

Abstract – The Campanian Plain: a geological monument of 5.000 km2 from the Tyrrhenian Sea to the Apennines - The Campanian Plain is the region in Southern Italy which extends between the Apennine Chain and Tyrrhenian Sea over an area of 5,000 km2 from Mt. Massico, in the north, to Sorrento Peninsula in the south of the Plain. The ancient name of this area was Campania Felix. This territory is an extraordinary geological monument characterized by volcanoes, earthquakes, tsunamis, slow ground vertical movements (bradiseism) which generated myths, legends, archeological findings, historical documents. The Campanian Plain is affected by extensional tectonics related to the spreading of the Tyrrhenian Basin. The onset of this process occurred between the Mid–Pliocene and the Early Pleistocene, when large morpho-structural depressions began to develop, giving rise to the Bay of Naples. The whole range of phenomena observed, in particular the recent tectonics and chemism of magmas feeding active volcanoes can be accounted for the local upwelling of the Mantle, the lytosphere plate bending and subsequent collapses. The measure of the collapse in the Campanian Plain, due to the stretching of the crust produced by the Mantle migration, is obtained by the thickness of sediments forming Campanian Plain. These data and the time lapse of the process furnish a value of about 1-2 mm/yr for the velocity of the subsidence during the Quaternary. Coeval to the deformation of this morphostructural depression was the onset of intense magmatic activity with the formation of the polygenic apparatus of Mt. Vesuvius and the volcanic fields of Phlegraean Fields and Ischia Island. In the Phlegrean Fields and Ischia caldera collapses, ignimbrite eruptions, and caldera resurgences occurred; instead at Vesuvius numerous Plinian eruptions were recorded. The most recent eruptions in the Neapolitan area occurred in 1302 at Ischia, 1538 at Phlegrean Fields and 1944 at Vesuvius. At present large fumarolic fields and thermal springs outcrop in several sites, while in the Phlegrean Fields slow oscillations of ground are recorded. The eruptive history of neapolitan volcanoes, the seismic history of Southern Appennines as well as the geological features of Campanian Plain characterize this area as a geological multirisk territory. Thus it is a “laboratory” for volcanic, seismic and hidrogeological risk assessment, from wich to draw lessons for integrated planning of pleasant places in risk areas.

The scientific study of earthquakes received a critical stimulus in 1755, when a disastrous earthquake struck Lisbona on November 1. A significant contribution to this evolution come from Italy, where by the mid-nineteenth century many scintists operated to record, localize and classify the intensity of the earthquakes.

Ground deformations in active volcanoes are considered precursors of eruptions according to the most tested models; therefore monitoring networks of ground deformations are installed on inhabited dangerous volcanoes. Direct measurements of such deformations are carried out since 1861 when Luigi Palmieri monitored the eruption at Mt. Vesuvius with levelings along the shoreline near the town of Torre del Greco. Relative sea level changes were measured at Serapeo in Pozzuoli in the middle of 19th century to record soil uplifts which are locally known as bradyseism. To enlarge the time series of data on these phenomena it is necessary to utilize historical and prehistorical informations on the location of shore-line of human settlements. As regards the regions of active volcanoes as the Neapolitan one three processes contribute to sea level changes as eustatism, regional tectonics and local intrusive and effusive phenomena. Therefore at the same time the relative sea level should be different at far-away places only few kilometres according to the volcanic activity. In fact eustatic and tectonic processes contribute to sea level changes with very lesser rates than volcanic activity. The Neapolitan region for its geological history is an excellent laboratory for testing the validation of new paradigms for some natural phenomena.

Riassunto “Una brusca interruzione e un profondo sconvolgimento sofferse la mia vita familiare per il terremoto di Casamicciola del 1883, nel quale perdetti i miei genitori e la mia unica sorella, e rimasi io stesso sepolto per parecchie ore sotto le macerie e fracassato in più parti del corpo”. Così Benedetto Croce ricorderà questo terremoto che diverrà un laboratorio per la crescita delle conoscenze sui fenomeni sismo-vulcanici. Il terremoto colpì un territorio denso di memoria storica, di paesaggi straordinari, di acque salutari, un luogo di vacanze per una clientela di élite. Il sisma accade in un momento di fervore scientifico per lo studio dei terremoti, con l’Italia capofila mondiale, e di ripensamento del ruolo del paese a livello internazionale a poco più di un decennio dall’Unità Nazionale con Roma capitale. La rinomanza dell’evento a livello europeo impegnò il Governo a un intervento massiccio per i soccorsi. Numerose personalità e famosi scienziati intervennero sulla scena del disastro; nel volume sono ricordati quelli che hanno lasciato le tracce più significative: Francesco Genala, ministro dei Lavori Pubblici; Henry James Johnston-Lavis, medico, con una grande passione e competenza per i vulcani; Luigi Palmieri, direttore dell’Osservatorio Vesuviano; Michele Stefano de Rossi, il primo a introdurre in Italia le scale di intensità per i terremoti e a realizzare una rete sismica; Giuseppe Mercalli, che propose una nuova scala delle intensità e fu direttore dell’Osservatorio Vesuviano; Giulio Grablovitz, fondatore e direttore dell’Osservatorio Geodinamico di Casamicciola.