An archaeo-seismological study of the Nîmes Roman aqueduct, France: indirect evidence for an M > 6 seismic event?

Abstract

This paper presents a synthesis of a multidisciplinary study carried out along the Nîmes Roman aqueduct, located in the southeast part of France. The study was motivated by archaeologists attempting to explain the partial destructions of only one aerial bridge of the aqueduct (Pont de la Lône). Given its close proximity to the Nîmes seismically active fault, a possible seismic origin for the destructions was invoked. Seismologists and structural engineers thus carried out a variety of field and numerical investigations to test the seismic hypothesis. Supporting field evidence was found first along the aerial bridge section of the aqueduct: broken stalactites, arch warping, cracks and destruction of the bridge just above a breast wall shortly after its construction. Secondly, the underground part of the canal was analysed: irregularity in the thickness of calcite deposits of the canal walls, presence of numerous cracks, horizontal shift of the otherwise linear structure of the canal and presence of calcite twins in the deposits, found only where P. Volant Á O. Scotti (&) IRSN/DEI/SARG/BERSSIN, the canal crosses the Nîmes fault system. Numerical modelling and experimental results show that (1) only the Pont de la Lône would have suffered serious damage under seismic solicitation (assuming an M6 earthquake at 10 km distance). The second aerial bridge, the three levels arches Pont du Gard, an historical monument still standing today, would have suffered less damage, due to its very different fundamental frequency of around 0.4 Hz, far from the amplified seismic near-field spectrum; (2) the numerical models also show that a fall of the canal wall would require a higher magnitude event (M [ 6); (3) the presence of calcite twinning requires a differential static stress of 4 MPa or greater, which can only be achieved very close to the fault rupture of a M [ 6 earthquake; (4) finally, local surface rupturing of such a fault would also corroborate the hypothesis that the observed offset of the canal may be partly seismically induced. Although other possible origins for each individual evidence may not be excluded, the observed spatiotemporal concentration of architectural/geological anomalies together with the numerical results allow us to support a possible co-seismic origin for these disorders, indirectly attesting to the potential seismic activity (M [ 6) of the nearby Nîmes fault. Furthermore, following the conclusions of archaeological studies, the disorders occurred between 250 and 350 year AD, thus providing timing for this possible seismic event, an essential parameter in seismic hazard analysis. These results validate the archaeo-seismological approach as a tool that may help improve the knowledge of major infrequent earthquakes in areas of moderate seismic activity.

Laminated carbonate deposits in Roman aqueducts: Origin, processes and implications

“…Trace‐element data of calcareous sinter deposits were successfully applied to determine the provenance of water for the aqueducts of Fréjus and Pompeii, both of which were fed by more than one source (Ohlig, ; Guendon et al ., ; Bobée et al ., ). Finally, a number of previous studies tried to use calcareous sinter deposits in aqueducts to obtain information on palaeoseismicity (Meghraoui et al ., ; Volant et al ., ; Galli et al ., ; Sbeinati et al ., ). Although these studies addressed the problem of layering in aqueduct calcareous sinter to some extent, this article aims at a more thorough investigation of the nature of layering by a combination of analytical techniques and a discussion of factors that may have contributed to its formation.…”

Distinguishing damages from two earthquakes—Archaeoseismology of a Crusader castle (Al-Marqab citadel, Syria)

“…If the frequency of earth vibrations is close to the frequency of resonance of the building, excitation will occur, damage will be pervasive, and the building will collapse. If frequencies widely differ, the building will survive, possibly intact (for the spectacular example of the Pont du Gard in France, see Volant et al, 2009). Likely, this is the primary cause why halls with lower proportions, e.g., the Main Hall, collapsed (Major et al, 2010), while tall, stout buildings like the donjon survived each earthquake for 800 yr.…”

The Uzege (Southeastern France) 22 March 1186 Earthquake Reappraised

“…This thumbnail sketch of Gard seismicity would not be complete without at least mentioning that a recent study of the Uzès-Nîmes Roman aqueduct, Volant et al (2009), has hypothesized a connection between some archaeological evidence of damage undergone by a section of this artifact and the possible occurrence of one or maybe even two major earthquakes (M ≥ 6, type of magnitude undefined) in the 3rd or 4th century AD. Even if this assumption is not supported-so far-by documentary evidence nor by the findings of the only extensive overview of historical earthquakes available for the period (Guidoboni 1989), it reminds us that the current picture of generally low regional seismicity is a target for improvement.…”