By Shannon Jones
Images produced by author unless stated otherwise
Introduction
Modern Rome is a beautiful amalgamation of buildings dating back three thousand years. In no other city can you see such juxtaposition as a brand new McDonalds next door to a church built in 300 BC.
A city so old though has endured a plethora of geographic as well as social changes. In any conventionally old city, there are likely to be ruins from wars, past regimes, or damage from perhaps one paramount natural disaster. Rome, however, has withstood and repaired from all of the above, as well as at least 32 seismic events cataloged back to the Imperial times (Harries, 1996). There is a relatively sparse official reporting of seismic activity in much of ancient Rome, likely due to religious customs, which will be further discussed later on.
Walk twenty feet in Rome and you’ll find an example of buildings that have been built upon or altered several times from antiquity to the modern age. Structure recycling and alteration is a natural result of a population continuously inhabiting the same region. When a building sustained damage from human interference, natural decay, or geologic event, the Romans simply patched it up, demonstrating an incredibly deep knowledge of the engineering of their buildings.
Figures 1 and 2, the Theatre of Marcellus and the Portico of Octavia demonstrate the extent to which Romans built on top of or next to existing structures.
Ancient Rome is also quite famous for the load-distributing capabilities of the arch. Not only did this enable them to create massive structures with large open indoor spaces; buildings with domed roofs proved to also be ideal for distribution of seismic waves, leading to further longevity of the structure. The paramount example of this was the Pantheon; built more than 2,000 years ago, remaining the largest unreinforced concrete dome in the world.
Figure 3 of the Pantheon built between 25 and 27 AD is an example of one of the best-preserved structures in Roman history. The unique dome shape is not only an engineering marvel, but also, luckily, is incredibly seismically resistant, as it is able to transmit stress equally throughout.
This paper will investigate the geologic environment Rome was built upon and has evolved with. I will seek to understand if Ancient Roman construction was built to withstand seismic activity, or if remaining buildings are simply here because of their massive size.
1. Geologic Background
I have a strong personal background in earth sciences, so coming into my study abroad in Rome, I was excited to learn more about processes that shaped the city and how the empire expanded in conjunction with geologic events over its several thousand year reign. Rome is unique in this context because it’s been around long enough that it’s witnessed and worked around literally centuries of seismic and volcanic activity, while most other cities we see today merely exist in the aftermath of major events.
1.1 Timeline
In order to get an understanding of the geologic processes that have shaped current landscapes and events, it’s important to have an idea of the chronology of the region. Though the city of Rome is famous for its measly several thousand year reign, a geologic time scale is needed to understand even recent geologic processes in the region. Although all of human existence has been within the Holocene Epoch, this paper will look as far back as the Paleozoic Age.
1.2 Tectonic History
The Italian Peninsula we see today has actually been around for a significant portion of earth history; the ‘boot’ was first seen in the Paleozoic. Rome itself is situated on the southern portion of the Eurasian tectonic plate, roughly 400 miles north of the border with the African plate. Since the late Cretaceous, the African plate has been subducting under the Eurasian plate, a process resulting in the uplift of the Alps of south central Europe hundreds of millions of years ago. (“Volcano Hotspot”, 2022)
Modern geological formations seen today were likely formed in conjunction with the uplift of the Apennine mountain range in central Italy, the cause of which remains somewhat of an enigma. Based on the structure of the mountain range, geologists have concluded that they were formed in a subduction event where the Adriatic sea floor moved west underneath the Italian landmass. The issue here, though, is that the Dinaric mountain range on the other side of the Adriatic was definitely formed via the subduction of the Adriatic sea floor east under what is now Bosnia and Herzegovina. This presents a mystery, as there is no sign of seafloor spreading in the Adriatic, but somehow it moved in two opposite directions (Ollier et al, 2009).
These tectonic processes created a vast network of faults under Rome and all of Italy, some of which are mapped and some which remain elusive. Below is a map of current faults that are known to be active. Due to insufficient historical documentation, though, there is a high likelihood that there are more active faults in the area that simply have not been recorded recently.
1.4 Formation of Rome
Ask any engineer why Roman buildings have stood the test of time and chances are they will either tell you about arches or the volcanic tuff found in both construction blocks and concrete mix. My follow-up question to this was – how did the tuff get there? Some further investigation from an article by Boris Behncke revealed the source to be two Pleistocene-era volcanic complexes; the Sabatini Complex and the Colli Albani Complex, straddling the city to the north and south respectively (Behncke, 1996).The eruptions of these volcanoes took place over 300,000 to 500,000 years, and literally laid the groundwork for the Romans to build upon. The discharge of massive amounts of incredibly durable building materials directly at and around the site the Roman empire was expanding on was hugely fortuitous in terms of supply procurement and management. Think of trying to build a log cabin in Wisconsin versus Kansas. The Romans had copious material on hand for all their construction wants and needs, which not only fueled expansion, but also meant a more intimate familiarity with the medium.
Volcanic rock was so bountiful and has proven so durable that it is still found in many streets of modern Rome. It completely lines many side streets as seen in the first picture, but has been replaced with more forgiving asphalt in more highly trafficked areas, as seen in the second picture.
Figure 8 demonstrates the proximity of the formerly active volcanic complexes to the city of Rome. As opposed to one lone mountain, these complexes were comprised of several calderas over a considerable area. Lakes present in either area are in the craters of the largest calderas.
As mentioned, the bulk of the sediment in the region is middle-Pleistocene, high potassium volcanic deposits. Along the current riverbed lies Plio-Pleistocene sedimentary deposits coming down from the erosion of the Apennines, as well as Holocene-age alluvial fans. Several meters down, the shore of the Tiber is underlain with Oligocene-age flush deposits and meso-Cenozoic silicic-carbonatic deposits, a testament to the amount of flooding the river engaged literally ages before the birth of the city.
2. Documented Earthquakes, Past and Present
As mentioned above, there is a relative lack of documentation of earthquakes throughout the Roman empire. With that being said, it’s possible to piece together a partial timeline of seismic events throughout Rome by means of following recorded damage on major buildings like the Colosseum and cathedrals.
The acute lack of documentation of earthquakes throughout ancient Rome also means a lack of information about fault lines around the city. “Hundreds of kilometers of geologic faults”(Lipuma, 2019) stretch from under the Apennine mountains and underneath Rome, yet there is little documentation or mapping.
This proved deadly in 2016, when the Mount Vettore fault, previously thought to be dormant, ruptured, killing 300 people. At this sudden push, scientists looked into the history of the rupture of this fault, finding that it had “ruptured five other times in the past 9,000 years”(Lipuma, 2019), and concluding a rupture rate of once every 1,500 to 2,100 years.
The Mount Vettore fault situation is a prime example of the lack of knowledge about Rome’s underlying earth structure, and a modern demonstration of the importance of geologic mapping in urban areas. Odds are there are plenty of other fault systems that could rupture at any time, with no effective record keeping to predict them.
Additionally evident from this timeline is the sheer amount of damage the Colosseum has sustained over the centuries. Beginning at most 200 years after its completion, the Colosseum has been contending with the impact of seismic waves its entire life. The sheer number of events it’s faced and withstood is impressive, even though the building we see today is missing most of its southern exterior. According to Adrian Tan’s 2015 dissertation, the 1349 earthquake that caused the fall of the southern exterior section was largely due to the failure of the base the colosseum was constructed upon in the first place. As demonstrated in Figure 10, the southern region of the Colosseum was unwittingly constructed on recent alluvial material; very fine clay, sand, and silt. This material is incredibly susceptible to ground movement, especially in comparison to the hard gravels that the rest of the structure is rooted in.
The relatively fine condition the Colosseum is in today in spite of 2,000 years of seismic activity demonstrates the ingenuity of its construction, which will be covered later on.
Figure 11 illustrates some of the damage the Colosseum sustained in the 1349 earthquake, and demonstrates the way the city has recovered from and preserved seismic damage.
3. Roman Construction
The question of why so many massive Roman buildings are left standing in the aftermath of continual seismic activity does not have a straightforward answer. A sociological response would relate to the emergence of Christianity and the remaining usefulness of the buildings to the Christian empire. From a more natural, engineering point of view though, there are a couple of factors that likely worked in conjunction to keep the buildings standing.
3.1 Over-Construction
Of the structures from ancient Rome that are still standing today, a majority of them share two characteristics; they’re short and they’re stout. You don’t see statues, pillars, or frankly many roofs left standing. The term “over-construction” was used several times throughout this course with respect to the aqueducts and the Colosseum. A somewhat less nuanced hypothesis for the longevity of massive structures is that they lasted merely because they were so massive that no geologic event has been powerful enough to topple them.
As seen in Figure 12, the Aqua Claudia and Aqua Anion Novus are examples of relative over construction, resting on massive travertine blocks. Additional speculation on why the Romans engaged in over-construction may lead one to examine the above ground aqueducts more closely. The great size of the blocks the aqueducts were built upon was helpful for a few reasons. Since there was a large allowance in terms of design strength, a new aqueduct could be built atop older ones with little to no reinforcement of the base. As discussed in class, the construction process of the aqueducts was largely completed by unskilled workers overseen by architects. This meant that there was likely going to be some error in the geometry of the arches and pillars, however again the massive bearing capacity of the blocks allowed some wiggle room in the event of incorrect estimations.
The over-construction of the colosseum has had a couple of additional benefits as well; the massive size of the blocks making up the majority of the structure meant that while the building was scavenged for hundreds of years for material, the formwork is left largely intact. Additionally, even amidst crowds, one can still feel the cooling effect the thick walls have on the interior of the structure; a valuable insulation from a time before air conditioning.
3.2 Domes
Another common theme we can see in Roman buildings from antiquity is the domed roof. The Romans were famous for their invention of the arch, whose load-distribution capabilities enabled far larger and heavier structures to be erected. The concept of the dome merely took the arch and stretched it out 360 degrees. The key feature here is that the weight of the structure is evenly distributed over the entirety of the surface, so there are no weak points that shaking might exploit.
One of the largest and most famous examples of a dome in Rome is the Pantheon. It also happens to be a great example for this paper, because not only has the domed roof prevented any seismic damage to this day; it’s also constructed with large portions of volcanic rock. Boasting the world’s largest unreinforced concrete dome, the Pantheon offers a masterclass in Roman understanding of volcanic materials such as pumice, basalt, and tufa combined with conventional materials such as bricks and travertine in such a way the dome has stood undamaged for over two centuries.
In 2005, the University of British Columbia’s department of Civil Engineering conducted shake tests on a 24-foot diameter wooden dome in order to simulate several earthquake events of differing magnitudes. During phases of testing, the top of the dome was even loaded with up to 24 tons of sandbags and steel plates, but the dome never failed. Remarking on this study, Dr. Arnold Wilson, an engineer in the field of thin shell concrete construction, posed “earthquake forces do not even approach the design strength the monolithic Done was built to withstand”(South Clark, 2014).
3.3 Metamaterials
Recent investigations into the foundations of Roman buildings have unearthed ‘archaeological metamaterials’ that have been working to deflect ground waves for centuries. According to Edwin Cartlidge’s article, a metamaterial is “a medium engineered to acquire one (or more than one) property not found in naturally occurring materials.” (Cartlidge, 2019) In this instance, metamaterial structures divert seismic waves around buildings to protect them from earthquakes.
There are a couple of modern metamaterials that are being implemented as a means of protecting buildings from seismic waves, including auxetic metamaterials, above surface resonators, and buried mass-resonators, which work to either dissipate ground waves or gather information on them (Brûlé et al, 2020). These are very recently developed technologies, however, only implemented in the past fifty years.
One more basic form of metamaterial is soil-metamaterials, or “structured soils made of cylindrical voids and or rigid inclusions” (Brûlé et al, 2020). The star pupil of this study is the Colosseum. The cylindrical shape of the building and its footers work incredibly well to deflect incoming seismic waves, as seen in Figure 14. An additional beneficial component of the foundation was the hypogeum. The complex maze of chambers and tunnels likely acted as voids and rigid inclusions in soils as mentioned above, further stabilizing the Colosseum.
The Colosseum has been one of the most studied Roman buildings in terms of metamaterials. Its circular shape helps deflect shear waves in the earth, whether or not the Romans were aware at the time. With the floor gone, the hypogeum’s labyrinth of corridors is visible, as seen in Figure 15.
The question then arises if the Romans were aware of the seismic stability of their construction at the time. I believe that answer is no. There is no evidence that Romans had ulterior motives for the design of the Colosseum beyond fantastic show and entertainment. Similarly, there is no evidence that knowledge of the seismic resilience of the dome was a motivating factor for the creation of such buildings like the Pantheon.
4. Earthquake Preparedness in Antiquity
In somewhat of an ironic turn, according to Kent Harries’ paper Earthquake Resistance Construction in Classical Rome, the ancient Romans were absolutely not intentionally building structures to withstand earthquakes. “The Roman understanding of earthquakes was based almost entirely on the belief that they were due to divine intervention” (Harries, 1996). They attributed earthquakes first to Neptune, god of the sea, then as an act of the singular god ‘God’. While today we analyze earthquake damage to find flaws in construction, the ancient Roman reaction was to conclude they had offended a divine overlord, and act accordingly. For all of the credit we give Romans in creating marvels that have stood the test of time, there was an acute lack of awareness at the continuous cause of damage their buildings kept sustaining.
5. Conclusion
Researching for this paper, touring Rome with the group, and wandering and pondering for myself has given me a massive amount of respect for the engineering skills of ancient Romans. The civilization was dealt a mixed deck of cards; bountiful useful volcanic material for construction, but also a very active seismic region to build upon. Through centuries of social upheaval, wars, and literal earth shattering events though, the empire created massive lasting structures as a testament to its power and prestige.
That being said, I don’t think I would feel comfortable living in a city with such little understanding of faults underlying ancient structures pancaked on top of each other. I do currently live in a city that’s within the potential span of a massive earthquake itself, though, so maybe ignorance is bliss.
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