| 18 Sep 2025
First Tomographic Imaging of Mid-Crustal Doubling at the Abruzzi Outer Thrust Front, Central-Southern Italy
Abstract. The geometry, deep structural style, and seismotectonic setting of the outer Abruzzi thrust system are less understood than those of other segments of Italy's Late Pliocene–Quaternary contractional belt. This knowledge gap arises from the region's complex surface geology, low seismicity rates, and the limited resolution of existing geophysical data.
Here, we present a local earthquake tomography of a large and previously unexplored area that encompasses the Abruzzi thrust system and spans from the Apennine extensional province in the west to the foreland strike-slip province in the east. The model is based on the inversion of 42,176 P-wave and 29,045 S-wave arrival times from earthquakes with ML ranging from 0.2 to 5.5.
Our results show low seismic velocities at upper crustal levels in the western sectors, correlating with continental basins of the extensional domain. In contrast, marked Vp inversions at mid- to lower-crustal depths in the eastern sector delineate a crustal doubling.
We interpret the tomographic results in the context of geological, geophysical, and seismological data to construct a 3D conceptual model of the region. This includes the first geometric reconstruction of the Abruzzi Arc basal thrust, an eastward convex arcuate structure extending ~170 km and reaching depths of ~24 km. The model also incorporates strike-slip faults in the footwall and east-dipping normal faults to the west.
The structural affinity between the Abruzzi Arc basal thrust and other seismogenic structures of the Padan–Adriatic belt located in the same structural position, suggests potential seismogenic behavior, although slow deformation rates and long recurrence intervals obscure its seismic expression. This conceptual model provides new insights into regional geodynamics and has significant implications for seismic hazard assessment in the central–southern Apennine transition zone.
GENERAL COMMENTS
The manuscript “First Tomographic Imaging of Mid-Crustal Doubling at the Abruzzi Outer Thrust Front, Central-Southern Italy” by De Nardis et al. presents a local earthquake tomography study of the Outer Abruzzi Thrust Front in Central Italy. By integrating Vp and Vs models with geological, geophysical, and seismological information, the Authors propose a three-dimensional conceptual model of the crustal structure, including a geometric reconstruction of the basal thrust of the Abruzzi Arc. The study addresses a geologically significant region and has the potential to provide valuable insights into the geodynamics and seismic hazard of the central Apennines. While the topic is appropriate for Solid Earth and the manuscript is generally well written, several substantive issues remain, mainly concerning the tomographic inversion and the resolution of the resulting models. On this basis, I recommend publication after major revision.
SPECIFIC COMMENTS
For the tomographic inversion the Authors used the Fast-Marching Tomography algorithm (FMTOMO). Although the Authors properly cite the related literature, the manuscript lacks a concise explanation of how the algorithm works. A brief methodological description would substantially strengthen the paper by allowing readers to better understand the inversion setting and assess the reliability of the results. For example, the manuscript mentions the use of two different grids (the propagation grid and the velocity grid) but it remains unclear why they differ and how each is employed during ray tracing and velocity inversion.
The P- and S-wave velocity models are displayed using effective velocity values, but the chosen color scales do not adequately highlight the anomalies discussed in the text. Adopting color scales with stronger contrasts, ideally diverging around appropriate background reference values, would improve the readability and interpretation of the results.
Figure 5a presents multiple cross-sections with different orientations, yet only four are shown clearly. All sections should be displayed, at least in the Supplementary Material, to provide a more complete visualization of the Vp model. The treatment of seismicity also requires improvement. Relocated earthquakes are shown only in Figure 5a, and the varying orientations of the sections hinder a coherent assessment of the relationship between seismicity and velocity structure. A map displaying the whole relocated dataset, along with seismicity plotted on each tomographic section, would greatly clarify this aspect.
Additional ambiguity arises from the statements in lines 164–166 and 203–204, where the Authors indicate that seismicity was used to interpret the 3D velocity models and to construct the final 3D fault model. It is not stated, however, which seismicity dataset was actually used. Without this information, it is impossible to properly evaluate the modelling strategy. The authors should clearly specify whether the interpretation relies solely on relocated events, only on catalogue data, or on both, and they should provide a justification for their approach.
The assessment of model resolution also requires a more comprehensive analysis and evaluation. The Authors performed three checkerboard (CB) tests with different cell sizes, but they show in the Supplementary material results only at 10 km depth. A complete evaluation of the tomographic models cannot be made without access to the full CB test results. Maps at multiple depths for all three tests should be provided, at least in the Supplementary Material. Furthermore, in the manuscript is not reported any comparison between the sizes of CB cells with those of the anomalies interpreted in the real velocity models. Without such a comparison, it is difficult to distinguish between robust anomalies and those that may be artefacts. In this framework, some more layers are actually displayed in Figure 4, but for these maps the cell dimension is not specified. At a pure visual inspection, it seems that the CB model shown is the one with 15 km horizontal and 12 km vertical cell size. Is it correct? The recovered model exhibits unexpectedly reduced resolution at 8 km depth compared to the layers above and below. The authors should explain how this behaviour was evaluated and why it occurs.
Finally, the authors state in lines 242–243 that the velocity models are well resolved down to 20 km and locally to approximately 24 km. However, the CB test of Fig. 4 indicates poor recovery at 20 km depth. Since one of the main features discussed in the manuscript lies within roughly 14–24 km, the resolution of this depth interval needs to be carefully revised and reassessed to avoid potentially misleading interpretations.
TECHNICAL CORRECTIONS
In Figure 1a the FM solutions are reported with red or light-red background. Explain in the caption the difference.
The temporal range of the historical earthquakes shown in Figure 2a is not specified. Add this information in the caption.
Caption of Figure 3: the sentence “Red circles and green-blue contour lines as in legend panel d.” should be moved in the description of panel b.
Line 215: Fig. S5 or Fig. 5?
Line 496: “(see yellow dots in Fig.7)”. It should be “squares”.