Research Article | Open Access
Jennifer J. Roberts, Andrew F. Bell, Rachel A. Wood, R. Stuart Haszeldine, "Geospatial Statistics Elucidate Competing Geological Controls on Natural CO2 Seeps in Italy", Geofluids, vol. 2019, Article ID 8147345, 9 pages, 2019. https://doi.org/10.1155/2019/8147345
Geospatial Statistics Elucidate Competing Geological Controls on Natural CO2 Seeps in Italy
Site selection for the geological storage of CO2 for long timespans requires an understanding of the controls on containment, migration, and surface seepage of subsurface CO2 fluids. Evidence of natural CO2 migration from depth to the surface is documented at 270 sites from Italy, a prolific CO2 province. Previous studies indicate that CO2 delivery to and from buried structures that host CO2 accumulations is fault controlled but competing controls on the CO2 flow pathways affect the location and style of CO2 release. Here, we conduct a meta-analysis using a novel geospatial approach to statistically determine the relationship between the geological setting and structures and the CO2 seep spatial distribution and characteristics (morphological type, flux, and temperature) in Central Italy. We find that seep distribution differs on two spatial scales corresponding to the geological setting. On large scales (>5 km), seeps are isotropically distributed and align with regional structures such as anticlines, decollements, and extensional faults. On local scales (<5 km), seeps cluster and align with subsidiary geologic structures, including faults and lithological boundaries. The detailed location and flux of seeps within clusters are influenced by the regional structural domain: in the Tyrrhenian, seeps tend to be located along fault traces, whereas seeps are located as springs in the tip and ramp regions of fault scarps in the Apennines. Thus, our geospatial approach evidences, at a regional scale, how macrocrustal fluid flow is governed by deep extensional and compressional features but once CO2 reaches shallower structures, it evidences how smaller scale features and hydrogeological factors distribute the CO2 fluids in the near surface, dependent on the geological setting. This work not only demonstrates useful application of a novel geospatial approach to characterize competing crustal controls on CO2 flow at different scales but also informs the design of appropriate site characterization and surface monitoring programs at engineered carbon stores.
Carbon capture and geological storage (CCS) can significantly reduce anthropogenic CO2 emissions from large industrial sources of CO2 [1, 2]. However, for CCS to contribute effectively to climate change mitigation, the CO2 must remain in the subsurface for tens of thousands of years [1, 3]. Examining naturally occurring CO2 seeps allows quantitative examination of the diverse crustal pathways taken by CO2 migrating from depth [4–7] and thus guides the selection of secure storage sites and the robust design of low-cost monitoring programs capable of detecting potential leakage to the surface. Further, understanding of CO2 flow pathways informs not only leak prevention but also leak remediation .
Natural CO2 seepage is widespread in Italy , where 308 CO2 seeps at 270 locations exhibit a variety of surface expressions (types), temperatures, and fluxes . These seeps have already proven being valuable for studying the environmental and social impact of CO2 escape [11, 12], storage site monitoring techniques , and CO2 leak pathways [14, 15].
The location of subaerial CO2 seeps in Italy is shown in Figure 1, along with major structural features. These structures are mostly derived from tectonic processes associated with the subduction of the Adria plate beneath the European margin [16, 17]. Initiating in the Miocene, NE-SW compression caused tectonic stacking of Mesozoic-Tertiary carbonate platform and foredeep sediments which concentrated in a NE-migrating thrust belt. Coeval back arc extension thinned the crust in the Tyrrhenian sector, leading to high heat flow and active volcanism since the Pliocene and developing distinct NW-SE-trending structural domains shown in Figure 1—the thinned Tyrrhenian back arc, the thrust belt, and the thickened Adriatic foredeep.
CO2 seep distribution and flux concentrate in the peri-Tyrrhenian  and decrease towards the Apennines, where modern-day seismicity concentrates. Few seeps occur towards the foredeep. Individual seep CO2 fluxes range from <1 to >2000 tonnes/day (t/d) , but 10-100 t/d is the most common [10, 12]. Overall nonvolcanic diffuse regional CO2 release from Central and Southern Italy is globally significant [20, 21]. Studies find that seeping CO2 may have a mixture of origins [9, 20] but the largest component derives from deep degassing from a mantle contaminated with subducted crustal carbonates [9, 22–24].
Numerous studies of seep systems in Italy have highlighted the role of buried geological structures in Mesozoic carbonates on CO2 accumulation and leakage to the surface. These include shallow (~1 km) or deep (~5 km) anticlines [14, 19, 31] and horsts , but CO2 accumulations also occur in shallow pockets within Pleistocene sands [33, 34]. CO2 delivery to and from these structures tends to be fault associated [35–37]. Indeed, buried faults have been identified from CO2 or Radon gas anomalies in Pleistocene cover [34, 38, 39]. At depth, CO2 is known to affect fault properties [40–42] and seismogenesis [18, 43–47] in Italy. Seismic events are observed to affect CO2 seep flux and style [48, 49]. While faults affect crustal fluid flow by different mechanisms [50, 51] and offer important barriers or conduits for CO2 flow in the subsurface, along-strike permeability of faults is highly variable [51, 52], and towards the surface, many other factors influence local gas flow pathways, including topographic and hydrological factors  and vadose zone properties [36, 54]. As such, CO2 fluid pathways are affected by competing crustal controls, from regional geological structures, kms deep to top soil composition.
While several regional and subregional studies of CO2 seep occurrences are reported [55, 56], the dominant controls on CO2 seepage have not yet been systematically studied across a range of scales and geological settings. Here, we address this gap. We adopt a novel macroscopic approach to illuminate the competing crustal controls on natural CO2 fluid pathways by applying a novel geospatial statistical approach, the two-point spatial correlation function, to a database of CO2 seep characteristics integrated with geological data from Central Italy. The two-point spatial correlation function is a technique developed for cosmology  and previously used in earth science only to investigate earthquake aftershock distributions . The method quantifies the departure from homogeneity of point data, allowing the point distributions and orientations to be examined at a range of scales. As such, we do not examine each seep, cluster of seeps, or region of degassing on a case by case basis, given that these have been the subject of numerous previous studies. Rather, we focus on using the rich geospatial dataset of CO2 seepage in Italy to explore whether geospatial statistics can elucidate the geological controls on seep location, distribution, and characteristics over the entire region of Central Italy.
The database of CO2 seeps  quantifies seep location, morphological type, flux, and temperature (where data are available). We do not consider wells (boreholes known to leak CO2) or fumaroles in our analyses since man-made and volcanic seeps are not representative of leakage from geological CO2 stores. The remaining seep data are analyzed together with geological structures and geological boundaries in mainland Italy, including 1 : 1 M and 1 : 100 k scale geological maps  (in this dataset, only the location of the fault trace is known; there is no information on fault characteristics, such as type, throw, and age), normal fault scarps in the Apennines , seismic events, and subsurface carbonate structures . For more detail on these data, see SI Methods. A synthetic Poisson (random) point distribution is used as a “control” to compare against the seep data. The synthetic points are distributed within the areal extent of mainland Italy. A second Poisson distribution is created with the areal extent of the Tyrrhenian, since most seeps are located in this region (see SI Methods).
We used two approaches to test the scale dependence of point spatial relationships: (1)Proximity analyses determined the distance and azimuth of seeps to the nearest surface trace of a fault or lithological contact. We used the built-in ArcGIS proximity analysis tool to find the point on a fault line that is the shortest distance from a seep and then take the distance and azimuth between the seep and that point of the fault. This tells us how far the nearest fault is from each seep and where the seep is in relation to that fault(2)Point clustering was first examined using standard GIS tools (see SI Methods) and then analyzed more sophisticatedly using the two-point spatial correlation function. The two-point correlation method quantifies the departure from homogeneity of a distribution of points. The correlation function is expressed as the probability of finding a pair of points within an area and is usually explored over an area of incrementally increasing radius. The correlation function plots as a power law, , where is probability, is radius, and the constant describes the spatial distribution: for randomly distributed points, , for clustered points, , and for points randomly distributed on a line, . The distribution of azimuths between pairs of points can also be measured by this technique. Any change in point azimuths over the increasing area of the study indicates anisotropy in the point distribution (i.e., whether and how the location of points in relation to each other change as the area of study increases)
3.1. Seep Spatial Distributions
Two-point correlation function for seep and synthetic data (Figure 2(a)) shows that separation distances control distribution: (1)Between ~5 and ~100 km, the correlation function is the same for seep and synthetic data and , indicating that these points are isotropically distributed. The roll-off at distances greater than 100 km is a finite size (censoring) effect  from the spatial extent of Italy and is less notable in the synthetic data because points are distributed across the width of Italy whereas seepage focusses west of the Apennines. Indeed, roll-off is similar for synthetic and seep when the synthetic points are distributed only in the Tyrrhenian (see SI Figure 3)(2)At separation distances of <~5 km, decreases to ~0.5-1, indicating nonrandom spatial clustering ( indicates that seeps are aligned, and synthetic data remains ~2)
The distribution of azimuths between all pairs of seeps and synthetic points is separated above and below 10 km, the distance where the function begins to change (Figures 2(b) and 2(c)). At separation distances, <10 km seeps show several orientations approximately 30-40° apart. Synthetic data show peaks that relate to few point pairs rather than a preferred orientation. Above 10 km, seep pairs show a preferred NW-SE (140-160°) orientation in which synthetic data does not exhibit. Spatial relationships are unaffected by outcrop shape/extent or seep density (see SI Results, SI Figure 3).
CO2 seeps in mainland Italy are significantly clustered (99.9% confidence) compared to a spatially random process and form small clusters (<5 km width) that occur ~20 km apart (see SI Figure 2). When analyzed by seep type, only springs are not significantly clustered (see SI Table 1).
3.2. Role of Geological Structures
Seeps spatially occur close to faults (Figure 2(d)), and all seep types are exponentially more common closer to fault traces except springs which show a much weaker, near-linear increase. Although the resolution of the fault populations limits the confidence of spatial interrogation at , 90% of vent, diffuse, and bubbling water seeps are located <1 km, increasing to 2 km for springs. These relationships are consistent for both geological datasets (1 M, 100 k). Seep-fault azimuths are principally SW (-NE).
Seeps are also preferentially located towards lithological boundaries; 76% of all seeps and all CO2 springs are located <1 km from lithological boundaries for both geological datasets (Figures 2(d) and 3(a)). Seeps show no favored orientation from lithological contacts, unlike faults. Seep flux and temperature datasets are incomplete but neither correlate with proximity to faults or lithological contacts.
Known seeps occur above structural highs of Mesozoic carbonate subsurface topography. For example, two CO2 seeps occur above an anticline crest known to host CO2 [19, 62] and others appear near to the crest, or local highs on the flanks, of carbonate structures and décollements (Figure 3(b)).
In the seismically active Apennines, seeps are rarely located along fault scarps. The few (37) CO2 seeps which are located <10 km of a fault scarp are mostly (70%) springs with high fluxes (all but one seep with quantified flux emit >10 t/d). Unlike seeps towards the Tyrrhenian, Apennine seeps are located SSE of the faults and typically positioned towards the fault tip or in ramp structures in fault stepover zones (Figure 3(c)). (a)Major and minor fault traces. Near Suio in Castelforte (Lazio), where 4 bubbling water and 2 vent seeps are located along, or close to, fault traces and lithological boundaries in 1 : 1 M and 1 : 100 k geological datasets  which do not specify the fault types(b)Leakage from subsurface structures. Close to Rocca San Felice in Avellino (Campania), where 2 CO2 vents are located above the Monte Forcuso anticline that is known to host the CO2 reservoir(c)Fluid flow at fault tip points. East side of Rieti Basin (Lazio) where 3 springs (2 high, 1 very high flux) emerge towards the fault tip points of a normal fault scarp, rather than along the fault trace. The scarp was mapped in detail by  (shown in the image)
4.1. Subsurface Plumbing of CO2 Fluids
Seeps are preferentially located near to the faults and show several preferred point pair azimuths within clusters (Figure 2). Regional NW-SE structures may be a primary control on CO2 seepage, but towards the surface, it seems that any fault (i.e., any range of orientation) is the secondary control that governs where seeps emerge within a cluster. Fault orientations are more varied in the 1 : 100 k dataset than the 1 : 1 M (see SI results). So, as well as subsidiary faults and fractures, which can exhibit a wide range of orientations to the primary deformation structure, there are also structures which predate the Miocene compression and extension . CO2 migrating from buried anticline or horst structures may do so via whichever of these features provide transmissive pathways.
As observed by previous authors, our analyses find that geological structures determine the presence and location of CO2 seeps in Italy. We also observe that distance from a fault influences the seep type (Figure 2(d)). Seep type may therefore indicate the degree of near-surface spread from geological structures and therefore the relative control of other geological and hydrological factors other than the fault trace . For example, compared with other seep types, the location of CO2 springs shows the weakest relationship with faults and the strongest relationship with lithological boundaries. It is not surprising that crustal migration pathways of aqueous CO2 fluids differ from gaseous or free-phase CO2. The location of CO2 springs is controlled by the hydrogeological characteristics of the aquifers. Assuming that the aquifer is well mixed, external CO2 could have entered the aquifer at any point(s) within the aquifer subsurface extent, in which case CO2-rich springs do not indicate the location of CO2 fluid flow pathways from depth, i.e., the spring may be located far from the fault trace(s) supplying the CO2.
The robustness of our results is of course limited by the resolution of the geological data and the completeness of the gas seep information. However, our meta-analysis identifies three different seep settings in Central Italy (Figure 3). These settings are distinct but are not mutually exclusive and align with the current understanding of crustal controls on fluid flow. (1)Major and minor fault traces. In the Tyrrhenian, the extended back arc region, 90% of vent, bubbling water, and diffuse seeps are located within 1 km of a fault (Figure 3(a)). The location of seeps suggests that in this geological setting, CO2 fluids are channeled by barrier/conduit properties of the fault wall and so seeps emerge along it, close to fault traces(2)Leakage from subsurface structures. In many cases, deep geological structures supply CO2 to surface seeps. As such, due to the structural trend of compression and extension structures in Central Italy, the resulting seep clusters supplied by buried CO2 accumulations will be located NW-SE of each other (Figure 3(b)). The orientation of faults related to, or pre- or postdating, these subsurface structures is likely to be responsible for the leakage of CO2 to the surface. For example, at Mefite d’Ansanto, the example in Figure 3(b), observed polarization of ambient seismic noise, may indicate the presence of faults governing gas escape from the Monte Forcuso CO2 reservoir (3)Fluid flow at fault tip points. There are fewer CO2 seeps located within the Apennines compared to the Tyrrhenian sector, and Apennine seeps tend to be springs with high fluxes and occur at lithological boundaries (Figure 3(c)). This indicates that there are limited pathways to surface for free-phase CO2 fluids in this region, which is also the most seismically active part of Central Italy. Instead, CO2 from depth enters the aquifers and its emergence as CO2 springs is then controlled by hydrogeology. We find that springs tend to emerge close to fault tips or in ramp structures in stepover zones. While these fault scarps are clearly an important control on crustal fluid flow in the Apennines, it is not necessarily the case that fault tips or ramp structures in stepover zones offer pathways for CO2 migrating all the way from depth to the surface
We propose that orientation of regional geological structures leads to the observed surface distribution of seep clusters in Central Italy (Figure 2). Extensional faults of the Apennines and major normal faults in the Tyrrhenian sector align NW-SE (see SI Results), and although compressional structures are more variable in their orientation, these are also predominantly NW-SE in Central Italy, where CO2 degassing concentrates. This means that our analyses cannot distinguish which fault types exert greatest control on CO2 seep distributions and characteristics. Ghisetti et al.  found that extension-related structures in Italy permit fluid flow during deformation, whereas contraction-related structures were initially closed but opened during subsequent exhumation and extension. Regional extensional and compressional features in Italy may therefore be important for governing crustal fluid flow, supplying deep-derived CO2 to buried structures within the tectonized Mesozoic carbonates and ultimately to seep clusters (Figure 3(b)). However, at a global scale, Tamburello et al. (2018) have shown that there is a spatial correlation between CO2 discharges and the presence of active fault systems and particularly with normal slip faulting.
4.2. Implications for Carbon Capture and Storage
Understanding the geological controls on CO2 fluid flow can aid the prevention of leakage from engineered CO2 stores by informing effective site selection criteria. Moreover, should unintended leakage to the surface occur, understanding the geological controls on CO2 fluid flow can inform the assessment of the potential CO2 seep locations and characteristics.
In Italy, we observe that CO2 seepage is clustered and that the location, distribution, and type of seepage within and between these clusters are controlled by a number of factors. These include, in the order of importance (as highlighted by our study), the orientation of regional structures, the geological setting, the density, and orientation of local geological structures and whether CO2 is migrating in spring water or as a separate phase. It is therefore important not only to characterize the storage formation and overburden but to consider the storage system in the context of the geological setting and near-surface geology.
Our work contributes towards predictive models of CO2 leak pathways. These models are important to de-risk sites selected for engineered storage, since site selection protocols can minimize the risk of leakage . Further, whether CO2 migrates and/or is emitted to the surface as gas or as a dissolved constituent of springs has implications for the environmental and social risk and impact of CO2 leakage [12, 65, 66] and so the design of robust and cost-effective monitoring programs to detect CO2 migrating to the surface should CO2 migrate from its primary storage formation .
The analyses presented in the paper used publicly available datasets, as specified in the article text and the SI Methods.
Conflicts of Interest
There are no conflicts of interest to declare.
J. J. Roberts designed the research, conducted the data analysis and wrote the paper. A. F. Bell contributed to the research design and data analysis, and R. A. Wood and R. S. Haszeldine contributed towards the research design and writing of this paper.
This research was funded by the Knowledge Transfer Partnership, Scottish Carbon Capture and Storage (SCCS), and the University of Strathclyde. SCCS is funded by awards from the Scottish Funding Council, Scottish Government, EPSRC EP/P026214/1, and NERC and a consortium of commercial UK energy companies.
SI Figure 1: polar plot of normalized azimuths from the analyzed fault datasets. SI Table 1: results of point cluster analysis. SI Figure 2: Ripley’s function for seeps and the synthetic random dataset. SI Figure 3: two-point correlation results for seep data compared with results of synthetic points located in the Tyrrhenian sector (where seeps are most numerous) and seeps hosted in turbiditic rock units (the most common outcropping rock type in Central Italy). (Supplementary Materials)
- IPCC, “IPCC special report on carbon dioxide capture and storage,” Prepared by Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2005.
- V. Scott, S. Gilfillan, N. Markusson, H. Chalmers, and R. S. Haszeldine, “Last chance for carbon capture and storage,” Nature Climate Change, vol. 3, no. 2, pp. 105–111, 2013.
- G. Shaffer, “Long-term effectiveness and consequences of carbon dioxide sequestration,” Nature Geoscience, vol. 3, no. 7, pp. 464–467, 2010.
- N. M. Burnside, Z. K. Shipton, B. Dockrill, and R. M. Ellam, “Man-made versus natural CO2 leakage: a 400 k.y. history of an analogue for engineered geological storage of CO2,” Geology, vol. 41, no. 4, pp. 471–474, 2013.
- F. Gal, M. Brach, G. Braibant, C. Bény, and K. Michel, “What can be learned from natural analogue studies in view of CO2 leakage issues in carbon capture and storage applications? Geochemical case study of Sainte-Marguerite area (French Massif Central),” International Journal of Greenhouse Gas Control, vol. 10, pp. 470–485, 2012.
- J. M. Miocic, S. M. V. Gilfillan, J. J. Roberts, K. Edlmann, C. I. McDermott, and R. S. Haszeldine, “Controls on CO2 storage security in natural reservoirs and implications for CO2 storage site selection,” International Journal of Greenhouse Gas Control, vol. 51, pp. 118–125, 2016.
- J. M. Pearce, S. Holloway, H. Wacker, M. K. Nelis, C. Rochelle, and K. Bateman, “Natural occurrences as analogues for the geological disposal of carbon dioxide,” Energy Conversion and Management, vol. 37, no. 6-8, pp. 1123–1128, 1996.
- IEA Greenhouse Gas R&D Programme (IEA GHG), “Remediation of leakage form CO2 storage reservoirs, 2007/11,” 2007.
- A. Minissale, “Origin, transport and discharge of CO2 in Central Italy,” Earth-Science Reviews, vol. 66, no. 1-2, pp. 89–141, 2004.
- G. Chiodini, M. Valenza, C. Cardellini, and A. Frigeri, “A new web-based catalog of earth degassing sites in Italy,” Eos, vol. 89, no. 37, p. 341, 2008.
- S. E. Beaubien, G. Ciotoli, P. Coombs et al., “The impact of a naturally occurring CO2 gas vent on the shallow ecosystem and soil chemistry of a Mediterranean pasture (Latera, Italy),” International Journal of Greenhouse Gas Control, vol. 2, no. 3, pp. 373–387, 2008.
- J. J. Roberts, R. A. Wood, and R. S. Haszeldine, “Assessing the health risks of natural CO2 seeps in Italy,” Proceedings of the National Academy of Sciences, vol. 108, no. 40, pp. 16545–16548, 2011.
- L. BATESON, M. VELLICO, S. BEAUBIEN et al., “The application of remote-sensing techniques to monitor CO2-storage sites for surface leakage: method development and testing at Latera (Italy) where naturally produced CO2 is leaking to the atmosphere,” International Journal of Greenhouse Gas Control, vol. 2, no. 3, pp. 388–400, 2008.
- J. J. Roberts, M. Wilkinson, M. Naylor, Z. K. Shipton, R. A. Wood, and R. S. Haszeldine, “Natural CO2 sites in Italy show the importance of overburden geopressure, fractures and faults for CO2 storage performance and risk management,” Geological Society, London, Special Publications, vol. 458, no. 1, pp. 181–211, 2017.
- F. Trippetta, C. Collettini, M. R. Barchi, A. Lupattelli, and F. Mirabella, “A multidisciplinary study of a natural example of a CO2 geological reservoir in Central Italy,” International Journal of Greenhouse Gas Control, vol. 12, no. 0, pp. 72–83, 2013, v.
- D. Cosentino, P. Cipollari, P. Marsili, and D. Scrocca, “Geology of the Central Apennines: a regional review,” Journal of the Virtual Explorer, vol. 36, 2010.
- F. Ghisetti and L. Vezzani, “Normal faulting, transcrustal permeability and seismogenesis in the Apennines (Italy),” Tectonophysics, vol. 348, no. 1-3, pp. 155–168, 2002.
- G. Chiodini, C. Cardellini, A. Amato et al., “Carbon dioxide Earth degassing and seismogenesis in Central and Southern Italy,” Geophysical Research Letters, vol. 31, no. 7, 2004.
- G. Chiodini, D. Granieri, R. Avino et al., “Non-volcanic CO2Earth degassing: case of Mefite d’Ansanto (Southern Apennines), Italy,” Geophysical Research Letters, vol. 37, no. 11, 2010.
- A. Ascione, G. Ciotoli, S. Bigi et al., “Assessing mantle versus crustal sources for non-volcanic degassing along fault zones in the actively extending Southern Apennines mountain belt (Italy),” GSA Bulletin, vol. 130, no. 9-10, pp. 1697–1722, 2018.
- G. Chiodini, F. Frondini, C. Cardellini, S. Caliro, G. Beddini, and A. Rosiello, “Measuring and interpreting CO2 fluxes at regional scale: the case of Apennines, Italy,” Journal of the Geological Society, 2017.
- G. Chiodini, S. Caliro, C. Cardellini, F. Frondini, S. Inguaggiato, and F. Matteucci, “Geochemical evidence for and characterization of CO2 rich gas sources in the epicentral area of the Abruzzo 2009 earthquakes,” Earth and Planetary Science Letters, vol. 304, no. 3-4, pp. 389–398, 2011.
- B. Gambardella, C. Cardellini, G. Chiodini et al., “Fluxes of deep CO2 in the volcanic areas of Central-Southern Italy,” Journal of Volcanology and Geothermal Research, vol. 136, no. 1-2, pp. 31–52, 2004.
- F. Italiano, P. Bonfanti, M. Ditta, R. Petrini, and F. Slejko, “Helium and carbon isotopes in the dissolved gases of Friuli region (NE Italy): geochemical evidence of CO2 production and degassing over a seismically active area,” Chemical Geology, vol. 266, no. 1-2, pp. 76–85, 2009.
- F. Brozzetti, “The Campania-Lucania extensional fault system, Southern Italy: a suggestion for a uniform model of active extension in the Italian Apennines,” Tectonics, vol. 30, no. 5, 2011.
- E. Patacca, P. Scandone, E. Di Luzio, G. P. Cavinato, and M. Parotto, “Structural architecture of the Central Apennines: interpretation of the CROP 11 seismic profile from the Adriatic coast to the orographic divide,” Tectonics, vol. 27, no. 3, 2008.
- R. Di Stefano, I. Bianchi, M. G. Ciaccio, G. Carrara, and E. Kissling, “Three-dimensional Moho topography in Italy: new constraints from receiver functions and controlled source seismology,” Geochemistry, Geophysics, Geosystems, vol. 12, no. 9, 2011.
- G. P. Roberts, “Visualisation of active normal fault scarps in the Apennines, Italy: a key to assessment of tectonic strain release and earthquake rupture,” Journal of the Virtual Explorer, vol. 29, 2008.
- R. A. Calabro, S. Corrado, D. Di Bucci, P. Robustini, and M. Tornaghi, “Thin-skinned vs. thick-skinned tectonics in the Matese Massif, Central-Southern Apennines (Italy),” Tectonophysics, vol. 377, no. 3-4, pp. 269–297, 2003.
- L. Improta, A. Zollo, P. P. Bruno, A. Herrero, and F. Villani, “High-resolution seismic tomography across the 1980 (Ms 6.9) Southern Italy earthquake fault scarp,” Geophysical Research Letters, vol. 30, no. 10, 2003.
- G. Bicocchi, F. Tassi, M. Bonini et al., “The high pCO2 Caprese Reservoir (Northern Apennines, Italy): relationships between present- and paleo-fluid geochemistry and structural setting,” Chemical Geology, vol. 351, pp. 40–56, 2013.
- M. L. Carapezza and L. Tarchini, “Accidental gas emission from shallow pressurized aquifers at Alban Hills volcano (Rome, Italy): geochemical evidence of magmatic degassing?” Journal of Volcanology and Geothermal Research, vol. 165, no. 1-2, pp. 5–16, 2007.
- F. Barberi, M. L. Carapezza, M. Ranaldi, and L. Tarchini, “Gas blowout from shallow boreholes at Fiumicino (Rome): induced hazard and evidence of deep CO2 degassing on the Tyrrhenian margin of Central Italy,” Journal of Volcanology and Geothermal Research, vol. 165, no. 1-2, pp. 17–31, 2007.
- S. Bigi, S. E. Beaubien, G. Ciotoli et al., “Mantle-derived CO2 migration along active faults within an extensional basin margin (Fiumicino, Rome, Italy),” Tectonophysics, vol. 637, pp. 137–149, 2014.
- F. Agosta, A. Mulch, P. Chamberlain, and A. Aydin, “Geochemical traces of CO2-rich fluid flow along normal faults in Central Italy,” Geophysical Journal International, vol. 174, no. 2, pp. 758–770, 2008.
- A. ANNUNZIATELLIS, S. BEAUBIEN, S. BIGI, G. CIOTOLI, M. COLTELLA, and S. LOMBARDI, “Gas migration along fault systems and through the vadose zone in the Latera caldera (Central Italy): implications for CO2 geological storage,” International Journal of Greenhouse Gas Control, vol. 2, no. 3, pp. 353–372, 2008.
- F. Ghisetti, D. L. Kirschner, L. Vezzani, and F. Agosta, “Stable isotope evidence for contrasting paleofluid circulation in thrust faults and normal faults of the Central Apennines, Italy,” Journal of Geophysical Research: Solid Earth, vol. 106, no. B5, pp. 8811–8825, 2001.
- G. Ciotoli, G. Etiope, F. Marra, F. Florindo, C. Giraudi, and L. Ruggiero, “Tiber delta CO2-CH4degassing: a possible hybrid, tectonically active sediment-hosted geothermal system near Rome,” Journal of Geophysical Research: Solid Earth, vol. 121, no. 1, pp. 48–69, 2016.
- G. Etiope, M. Guerra, and A. Raschi, “Carbon dioxide and radon geohazards over a gas-bearing fault in the Siena Graben (Central Italy),” Terrestrial Atmospheric and Oceanic Sciences, vol. 16, no. 4, p. 885, 2005.
- C. Collettini, C. Cardellini, G. Chiodini, N. De Paola, R. E. Holdsworth, and S. A. F. Smith, “Fault weakening due to CO2 degassing in the Northern Apennines: short- and long-term processes,” in The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties, C. A. J. WIBBERLEY, W. KURZ, J. IMBER, R. E. HOLDSWORTH, and C. COLLETTINI, Eds., vol. 299, pp. 175–194, the Geological Society of London, 2008.
- C. Collettini and R. E. Holdsworth, “Fault zone weakening and character of slip along low-angle normal faults: insights from the Zuccale fault, Elba, Italy,” Journal of the Geological Society, vol. 161, no. 6, pp. 1039–1051, 2004.
- S. A. F. Smith, C. Collettini, and R. E. Holdsworth, “Recognizing the seismic cycle along ancient faults: CO2-induced fluidization of breccias in the footwall of a sealing low-angle normal fault,” Journal of Structural Geology, vol. 30, no. 8, pp. 1034–1046, 2008.
- M. Bonini, “Mud volcano eruptions and earthquakes in the Northern Apennines and Sicily, Italy,” Tectonophysics, vol. 474, no. 3-4, pp. 723–735, 2009.
- C. Collettini and M. R. Barchi, “A low-angle normal fault in the Umbria region (Central Italy): a mechanical model for the related microseismicity,” Tectonophysics, vol. 359, no. 1-2, pp. 97–115, 2002.
- F. Di Luccio, G. Chiodini, S. Caliro et al., “Seismic signature of active intrusions in mountain chains,” Science Advances, vol. 4, no. 1, p. e1701825, 2018.
- L. Malagnini, F. P. Lucente, P. De Gori, A. Akinci, and I. Munafo, “Control of pore fluid pressure diffusion on fault failure mode: insights from the 2009 L’Aquila seismic sequence,” Journal of Geophysical Research: Solid Earth, vol. 117, no. B5, 2012.
- S. A. Miller, C. Collettini, L. Chiaraluce, M. Cocco, M. Barchi, and B. J. P. Kaus, “Aftershocks driven by a high-pressure CO2 source at depth,” Nature, vol. 427, no. 6976, pp. 724–727, 2004.
- M. Bonini, “Structural controls on a carbon dioxide-driven mud volcano field in the Northern Apennines (Pieve Santo Stefano, Italy): relations with pre-existing steep discontinuities and seismicity,” Journal of Structural Geology, vol. 31, no. 1, pp. 44–54, 2009.
- J. Heinicke, T. Braun, P. Burgassi, F. Italiano, and G. Martinelli, “Gas flow anomalies in seismogenic zones in the Upper Tiber Valley, Central Italy,” Geophysical Journal International, vol. 167, no. 2, pp. 794–806, 2006.
- J.-H. Choi, P. Edwards, K. Ko, and Y.-S. Kim, “Definition and classification of fault damage zones: a review and a new methodological approach,” Earth-Science Reviews, vol. 152, pp. 70–87, 2016.
- D. R. Faulkner, C. A. L. Jackson, R. J. Lunn et al., “A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones,” Journal of Structural Geology, vol. 32, no. 11, pp. 1557–1575, 2010.
- D. Curewitz and J. A. Karson, “Structural settings of hydrothermal outflow: fracture permeability maintained by fault propagation and interaction,” Journal of Volcanology and Geothermal Research, vol. 79, no. 3-4, pp. 149–168, 1997.
- J. J. Roberts, R. A. Wood, M. Wilkinson, and S. Haszeldine, “Surface controls on the characteristics of natural CO2seeps: implications for engineered CO2stores,” Geofluids, vol. 15, no. 3, 463 pages, 2015.
- J. J. Roberts and L. Stalker, “What have we learned about CO2 leakage from field injection tests?” Energy Procedia, vol. 114, pp. 5711–5731, 2017.
- G. Chiodini, F. Frondini, and F. Ponziani, “Deep structures and carbon-dioxide degassing in Central Italy,” Geothermics, vol. 24, no. 1, pp. 81–94, 1995.
- F. Frondini, S. Caliro, C. Cardellini, G. Chiodini, N. Morgantini, and F. Parello, “Carbon dioxide degassing from Tuscany and northern Latium (Italy),” Global and Planetary Change, vol. 61, no. 1-2, pp. 89–102, 2008.
- P. J. E. Peebles, “The large-scale structure of the Universe, Princeton,” in Princeton Series in Physics, Princeton University Press, 1980.
- K. Richards-Dinger, R. S. Stein, and S. Toda, “Decay of aftershock density with distance does not indicate triggering by dynamic stress,” Nature, vol. 467, no. 7315, pp. 583–586, 2010.
- ISPRA, ISPRA GeoMapViewer, vol. 2010, ISPRA, 2010.
- C. Nicolai and R. Gambini, “Structural architecture of the Adria-platform-and-basin system,” CROP-04, pp. 21–37, 2007.
- E. Bonnet, O. Bour, N. E. Odling et al., “Scaling of fracture systems in geological media,” Reviews of Geophysics, vol. 39, no. 3, pp. 347–383, 2001.
- G. Chiodini, A. Baldini, F. Barberi et al., “Carbon dioxide degassing at Latera caldera (Italy): evidence of geothermal reservoir and evaluation of its potential energy,” Journal of Geophysical Research, vol. 112, no. B12, 2007.
- S. Bigi and P. Costa Pisani, “From a deformed Peri-Tethyan carbonate platform to a fold-and-thrust-belt: an example from the Central Apennines (Italy),” Journal of Structural Geology, vol. 27, no. 3, pp. 523–539, 2005.
- M. Pischiutta, M. Anselmi, P. Cianfarra, A. Rovelli, and F. Salvini, “Directional site effects in a non-volcanic gas emission area (Mefite d’Ansanto, Southern Italy): evidence of a local transfer fault transversal to large NW–SE extensional faults?” Physics and Chemistry of the Earth, Parts A/B/C, vol. 63, pp. 116–123, 2013.
- D. G. Jones, S. E. Beaubien, J. C. Blackford et al., “Developments since 2005 in understanding potential environmental impacts of CO2 leakage from geological storage,” International Journal of Greenhouse Gas Control, vol. 40, pp. 350–377, 2015.
- J. M. Lemieux, “Review: the potential impact of underground geological storage of carbon dioxide in deep saline aquifers on shallow groundwater resources,” Hydrogeology Journal, vol. 19, no. 4, pp. 757–778, 2011.
- C. Jenkins, A. Chadwick, and S. D. Hovorka, “The state of the art in monitoring and verification—ten years on,” International Journal of Greenhouse Gas Control, vol. 40, pp. 312–349, 2015.
Copyright © 2019 Jennifer J. Roberts et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.