By Giovanni Diaferia, Ph.D., University of Roma TRE and Angela Stallone, Ph.D., INGV
On June 23, a Mw=3.6 earthquake struck Colonna, Italy, southeast of Rome. Despite initial fears that it was yet another large quake near the central Apennines, it struck instead nearby the Alban Hills volcanic complex and the source of the famed Tivoli marble.
Citation: Diaferia, G., and Stallone, A., (2019), Earthquake rumbles the gates of Rome, Temblor, http://doi.org/10.32858/temblor.033
A travertine quarry near the epicenter of the earthquake. These blocks, formed through natural heat, pressure, are on their way to becoming kitchen countertops.
On the evening of June 23, strong shaking was felt at the gates of Rome and people started fearing the worst. Since Rome doesn’t experience a lot of seismic activity, the population tends to associate any strong ground shaking with distant and destructive earthquakes in the nearby central Apennines. On June 23, people’s minds inevitably went to the memory of the hundreds of victims following the central Italy earthquakes of 2016 and 2017.
Fortunately, this time things were different. The INGV (National Institute of Geophysics and Volcanology) reported a Mw=3.6 earthquake near the city of Colonna, 34 kilometers (21 miles) southeast of Rome. No damage to people or things was recorded, though thousands of people reported to the INGV site “Did you feel the earthquake?” the oscillations of liquids and chandeliers within a radius of 20 kilometers.
The quake struck in an area of modest seismic hazard where a Mw=6 shock is likely to be experienced in one’s lifetime, which is about 30,000 times larger than a Mw=3.6.
What was its origin?
The epicenter of the earthquake is located near the Alban Hills (Colli Albani) volcanic complex, which produced major explosive eruptions between 560,000 and 350,000 years ago and was active until a few tens of thousands of years ago. Several citizens noted this detail on social media, wondering if the June 23 quake could signal a resumption of volcanic activity. This hypothesis was promptly denied by the National Seismic Network, which highlighted the tectonic origin of the event. Seismic data showed it was related to an extensional fault oriented northwest-southeast, and parallel to the numerous and well known tectonic structures of the central Apennines.
What do we know?
A direct study of this fault is not feasible, as the thick volcanic deposits preclude any surface evidence. However, it is possible to make inferences on the basis of indirect observations: some previous studies carried out by the University of Roma TRE (e.g., De Rita et al., 1995; Faccenna et al., 1994) suggested the existence of an active faults system buried under the Pleistocene volcanic deposits. These studies focused on the thick travertine deposits in the area east of Rome (especially around the city of Tivoli, just north of the epicentre of the June 23 quake), whose different chemical-physical characteristics and thicknesses were explained in the light of past seismic events. Also thanks to the geophysical and well surveys that were useful for clarifying the spatial relationships among the travertine deposits in the area, the existence of an extensional tectonic structure with a strike-slip component was hypothesized. Given the extensional character of the recent event in Colonna, it is therefore likely that it has occurred on this structure. In other words, the June 23 quake supports the hypothesis made in those studies.
Geological map of the city of Rome and surroundings. The June 23 quake in Colonna is nearby the tectonic structure hypothesized in several studies. Modified from Faccenna et al. (2008).
This shows that the study of the geological and structural characteristics of the subsurface is an essential tool for the identification of active faults that are not directly detectable on the surface. In this way, it is possible to improve the estimation of seismic hazard of a certain area, with obvious repercussions on the management and planning of the territory.
We thank Claudio Faccenna and Andrea Sembroni for providing references and Luigi de Filippis for the travertine quarry picture.
De Rita D., Faccenna C. Funiciello R. & Rosa C. (1995). Structural and geological evolution of the Colli albani volcanic district. In: The Volcano of the Alban Hills, eds. R. Trigila, 33-71, Rome.
Faccenna C. (1994). Structural and hydrogeological features of Pleistocene shear zones in the area of Rome (Central Italy). Annali di Geofisica, vol. .37, 1, 121-133
Faccenna C., Funiciello R., Montone P., Parotto M., Voltaggio M. (1994). An example of late Pleistocene strike-slip tectonics: the Acque Albule basin (Tivoli, Latium). Memorie descrittive della carta geologica d’Italia, vol.XLIX, 63-76.
Faccenna, C., Soligo, M., Billi, A., De Filippis, L., Funiciello, R., Rossetti, C., & Tuccimei, P. (2008). Late Pleistocene depositional cycles of the Lapis Tiburtinus travertine (Tivoli, Central Italy): possible influence of climate and fault activity. Global and Planetary Change, 63(4), 299-308.