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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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