Skip to main content

Seismogenic COmpression in southern Italy? – High-resolution topography (Lidar) and mOrphotectonic analySis to test the active nature of the Southern ApenninE Outer Thrust Front

Periodic Reporting for period 1 - COLOSSEO (Seismogenic COmpression in southern Italy? – High-resolution topography (Lidar) and mOrphotectonic analySis to test the active nature of the Southern ApenninE Outer Thrust Front)

Reporting period: 2018-10-01 to 2020-09-30

Earthquakes are a major natural hazard in Italy [1]. The bulk of the seismic energy is released along the Apennine belt’s topographic high, where most of the historical- [2] and recent instrumental seismicity [3] nucleates. However, in the last 50 years, moderately energetic sequences (4.0≤Mw≤6.0) with compressional/transpressive sense of motion [4] have unexpectedly occurred also along the external sectors of the Apenninic chain (Fig. 1), in areas characterized by low (<2 mm/y) deformation rates.
Geological, seismic, and morphotectonic evidence of ongoing shortening is reported along the northern- and central Apenninic Outer Front (NAOF and CAOF, respectively, Fig. 1), along the Sicilian outer front of the Maghrebian chain and along the Calabrian Arc (both offshore and on land). On the other hand, orogenic activity along the NW-SE trending Southern Apennines Outer Front (SAOF, Fig.1) is well documented only until the Lower- and part of the Middle Pleistocene (~0.7 Ma) [5, 6]. Nearly 23% of the Italian population is concentrated in southern Italy (http://demo.istat.it/bil2019/index.html). The cost of human losses consequent to earthquakes in Italy (in the last fifteen years) dramatically pointed out the significant vulnerability of the territory [7, 8] even if exposed to moderate magnitude events. This implies a pressing need to fill a gap of understanding on the seismic potential of SAOF, the latter representing about 1/3 of the Apennine outer compressional front.
The COLOSSEO project aims to detect evidence of Late Quaternary (post-125 ka) shortening along the SAOF and document the geometry of possible seismogenic faults. The research strategy envisages a multidisciplinary approach adequate to the peculiar geological-tectonic setting of the SAOF, which is typically buried under the Plio-Quaternary foredeep successions [9] and not strongly indicative of its recent activity. Tectonic structures belonging the front are difficult to investigate even because of the low deformation rates.
The exploited methodology includes a) analysis of High Resolution Topography (HRT) data (e.g. lidar data) to capture fault zone structure and/or offsets at fine geomorphic scale; b) assess relative rock uplift rate across the landscape using geomorphic indices from topographic- and fluvial network analysis; c) regional scale field geology to determine deformation rates from geomorphic markers; d) seismotectonic analysis exploiting available instrumental seismicity, focal mechanisms, borehole breakouts and geodetic data to constraint the SAOF’s kinematics.

[1] Gruppo di Lavoro MPS, 2004. http://zonesismiche.mi.ingv.it/; [2] Rovida et al., 2019. https://doi.org/10.13127/CPTI/CPTI15.2; [3] Chiarabba et al., 2016. https://doi.org/10.1111/ter.12233; [4] Montone & Mariucci., 2016. DOI: 10.4401/ag-7235; [5] Patacca, E. and Scandone P. (2004). In: Special Volume of the Italian Geological Society for the IGC 32, 93-129, Florence (2004);[6] Vezzani et al. (2010). DOI: 10.1130/2010.2469; [7] M. Dolce (2004). Seismic safety of schools in Italy. In: Keeping schools safe in earthquakes, Organisation for Economic Co-operation and Development, Paris (OECD, 2004); [8] Ceci et al., 2010. https://doi.org/10.1016/j.engstruct.2009.12.023. [9] CNR, 1992, Structural Model of Italy, Quaderni de La Ricerca Scientifica, 114, sheet 4, scale 1:500,000, S.E.L.CA. Firenze.
The research started by focusing on the SAOF northern section (peri-Adriatic sector) where it structurally connects with the active and seismogenic CAOF (black inset, Fig. 1). The methodological approach involved an integrated topographic- and fluvial network analysis along with morphotectonic investigation of fluvial terraces to identify evidence of differential rock uplift and transient signals related to thrust/reverse fault activity. Using 10m- px resolution DEM [11] and combining GIS tools (ArcMap by ESRI) with MATLAB-based software [11, 12], local relief- and residual maps, swath profiles, ksn map, and χ-z profiles have been computed in the study area. Results were compared with the main geo-lithological units and structural elements. In comparison with the morphometric analysis, the morphotectonic setting of the Quaternary fluvial terraces preserved along the main river flowing in the study area (Pescara river) has been investigated using 1-to-2m px resolution lidar data (http://www.pcn.minambiente.it/mattm/en/tag/lidar-data/). The interpretation of seismic reflection sections was also exploited to assess evidence of deformation affecting the late Quaternary successions and support a possible correlation between the shortening structures (in the sub-surface) and the pattern of relief- and stream network anomalies.
While the correlation of ksn anomalies and high local- and residual relief resulted in ambiguous evidence of localized uplift, the along river χ-z analysis turned out more successful results (Fig. 2). The spatial distribution of the knickpoints pointed out clustering of those likely driven by active tectonics. Most of them located along the Adriatic piedmont and Majella foothill (Fig. 2). In addition, the observed tilting of the fluvial terraces highlighted post-Middle Pleistocene deformation unlikely relatable to regional and large-scale uplift. All evidence converge in suggesting a possible correlation between the collected evidence of localized uplift with the late Quaternary activity of buried compressional segments along a ~80 km-long sector belonging the SAOF (Fig. 2).

[10] Tarquini et al., 2012. https://doi.org/10.1016/j.cageo.2011.04.018; [11] Schwanghart and Scherler, 2014. https://doi.org/10.5194/esurf-2-1-2014; [12] Forte and Whipple, 2019. https://doi.org/10.5194/esurf-7-87-2019.
Despite the challenge to test evidence of deformation in an area characterized by low deformation rates, the first results of the COLOSSEO project represent an improvement in the knowledge of the seismotectonic setting of the investigated area. The evidence of localized uplift (unlikely correlated with regional uplift) asks to reconsider, in terms of seismic hazard assessment, the existence of active (and possibly seismogenic) structures in a densely populated sector of the Apenninic outer front. The next steps of the research envisage the application of the same methodology along the SOAF southern section, to fill the gap of knowledge in southernmost Italy (Fig. 1), where the front connects with sections recognized as active in the late Quaternary. In addition, new field surveys and detailed dating (e.g. through OSL-Optically Stimulated Luminescence) of the late Quaternary continental deposits are expected along the main rivers of the peri-Adriatic region, to ease the application of morphotectonic analysis. This would provide, as in the Pescara river case, new hints supporting or further invalidating the hypothesis of the late Quaternary SAOF activity for that area