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GSA TODAY | MAY 2015 construction and, at the peak of Rome’s development, delta and on parts of its alluvial plane, Rome has frequently been
>80,500 km of roads were stone-paved. The Romans also had a afflicted by floods. Fifteen floods have been described between
large availability of sands, clays, and gravels deposited over time B.C. 414 and B.C. 4 (Bersani and Bencivenga, 2001), although the
by the Tiber River (Heiken et al., 2007). The abundance of these Tiber was generally considered as a calm river with few floods. Up
materials promoted the manufacture and use of bricks, which until Republican Times, flood damage had not been devastating
became very common in Roman buildings. Like Rome, Naples because competent city planners had placed facilities on the flood-
was built on volcanic hills, with the eastern remnants of several plain and most residences on hilltop areas. However, during the
coalescent tuff cones and lava domes forming the Phlaegrean Imperial period (B.C. 27–395 A.D.), settlement was also allowed
Fields (Fig. 1), which were emplaced prior to the violent erup- across the floodplain up to the Tiber’s banks, as a result of which a
tions of the Campanian Ignimbrite (39 ka) and Neapolitan large proportion of the city’s population was exposed to flooding
Yellow Tuff ignimbrites (NYT; 15 ka). The unconsolidated facies (Lamb, 1995).
of the NYT (pozzolana) were also frequently used to produce
hydraulic cements. The NYT represented a solid but soft and On the other hand, flooding has been a regular phenomenon
easily workable ground layer for buildings, and for digging throughout the history of Naples. The city developed in an area
cisterns and sewers. It was the most widely used building stone where torrential rain-fed waters flowed into several rugged
in Neapolitan and regional architecture since Greek times canyons or gullies on the mountains that unified at the back of
(Cardone, 1990; de Gennaro et al., 2000). The urban develop- the city. The first flood records of the city date back to 1010 A.D.,
ment of Naples was strongly determined by the underground but thick alluvial deposits in the central area of the city testify to
exploitation of tuffs that created a network of caves and tunnels the occurrence of floods in pre-Roman time.
(Cardone, 1990). Building materials were one of the most
important economic resources for both Rome and Naples, and Magmatic activity and fault systems are also responsible for
so mining became a very significant industry. elevated toxic gas concentrations in the near-surface environ-
ments of Naples and Rome, with the gases produced at depth
Both Rome and Naples suffered risks associated with frequent migrating upward along highly permeable fault zones. The most
earthquakes, flooding, and CO2 and radon emissions. Seismicity common gases are CO2 and H2S, likely formed by magma cooling
in Rome and Naples was, and still is, mainly associated with two and degassing, and by water-rock interactions. Sudden releases of
distinct seismogenic areas; namely, the Apennines and the CO2 have been documented from antiquity until the present day
volcanic districts. Earthquakes of VII–VIII Mercalli-Cancani- in several areas of the Alban Hills volcanic district. The quantity
Sieberg scale (MCS) maximum intensity have occurred in Rome of CO2 that rises from the depths and subsequently dissolves into
and Naples and were the result of the present extensional regime shallow groundwater has been estimated to be more than 4.2 × 109
related to post-orogenic collapse of the Apennines (D’Agostino mol year −1. In addition, elevated Rn comes from the high content
et al., 2011). The greatest effects of these earthquakes are felt of U and Ra present in both volcanic lavas and tuffs. In the
where there is an alternation of soft, loose alluvial and coastal Phlegraean Fields volcanic area, many hydrothermal phenomena,
sediments with the more rigid bedrock (marine clay and such as fumaroles and hot springs, testify to the still active volca-
volcanic sediments). The alternating soft and rigid sediments nism of this area.
give rise to an anomalous amplification of ground motion
causing buildings to be subjected to long and strong shaking; Finally, both Rome and Naples have been affected by anthropo-
this occurred in both the historical center of Naples (Nunziata et genic hazards. The fast growth of the cities and their populations
al., 2000) and along the Tiber river valley in Rome (Heiken et al., prompted the urbanization of areas subject to natural hazards,
2007). In addition, the low-lying geological setting of Naples has such as river and coastal flood plains previously used for agricul-
been affected by huge natural and artificial geomorphological ture, thereby exposing the population to flood risk and also
changes (Nunziata et al., 2000). Volcanic earthquakes, with reducing the ability of the city to ensure self-sustenance. Another
maximum intensity of VIII MCS, have originated in the volcanic hazard affecting both cities stems from the underground cavities
areas of the Alban Hills since Roman times, while eruptions of that remained after the prolific quarrying for building materials.
Vesuvius have often been accompanied by large earthquakes in The subterranean tunnel networks were so dense and intricate
the Apennines, 50–60 km to the northeast, such as those that that the vaults of the galleries were prone to collapse. In addition,
occurred in 63 A.D. and 64 A.D., before the destructive Vesuvius the lack of a clear separation between water cisterns and sewers
eruption in 79 A.D. The Neapolitan area is also affected by exposed the population of Naples to diseases such as cholera.
bradyseism, the subsidence and uplifting of the ground surface
caused by the volcanic and hydrothermal systems beneath the During late Republican time, continuous construction of new
Phlegrean Fields. buildings, higher consumption of goods, and the generation and
burial of the resultant waste increased the rate at which both cities
Both Rome and Naples developed during long quiescent periods rose above their original geologic foundations. In the Tiber flood
of their volcanoes. The last known eruption of the Alban Hills is plain of Rome, the anthropic debris layers are 10–15 m thick. In
dated ca. 29,000 yr B.P., although more recent phenomena of lahar the historical part of Naples, the same deposits can locally reach
generation from the crater of Albano have been identified thicknesses of 20 m (Nunziata and Panza, 2002). Debris layers
(Funiciello et al., 2003). Similarly, the ancient development of generally have very poor geotechnical properties and so provide
Naples mainly took place during the 3,000 years of quiescence of weak foundations that, in the event of an earthquake, can actually
the Phlegraean Fields and Vesuvius before the 79 A.D. eruption. magnify seismic waves.
Flooding also represented another important hazard both in Urbanization also caused degradation of the hill slopes,
Rome and Naples. Because of its location close to the Tiber River creating unstable geomorphological conditions that can trigger
landslides, especially during the occurrence of seismic tremors.
The earliest evidence of slope instability and mass movement in
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