A new 3D velocity model of Central Apennines: insights from 2D/3D geological modelling and earthquake relocation.

The Central Apennines fold-and-thrust belt (Central Italy) is characterized by the presence of several active faults, potentially capable of generating damaging earthquakes. To support seismic hazard studies over the area, a new 3D velocity model was built, integrating a wide range of surface and subsurface data.

The tectonic framework of the area (from Sulmona plain to Maiella Mt), is still debated in literature, also due to the lack of both an adequate geophysical data set and a reliable velocity model at the crustal scale.

In addition, the low number of seismic stations available for the acquisition of Vp/Vs arrival times, and the very low seismicity detected in the study area (the Sulmona and Caramanico Apennine valleys are considered as “seismic gaps”), lead to a difficult interpretation of the subsurface tectonic structures.

3D velocity modelling could well represent an important tool to support these deep crustal reconstructions as well earthquake relocation studies and could enhance the definition of seismogenic faults deep geometries, hence supporting a better risk assessment over the area of these potential locked faults.

Using the knowledge developed within the oil&gas industry as well in gas/CO₂ storage projects for the construction of 3D velocity models, extensively used to obtain subsurface imaging and define the geometry of the reservoirs and traps in the depth domain, a similar methodological approach was implemented over the study area.

The subsurface dataset was partially inherited by the past hydrocarbon exploration activities (e.g. seismic lines, exploration wells and sonic logs) and by the literature (e.g. time/depth regional models). Tomographic sections and relocated earthquake hypocentres were also integrated form geophysical studies. Geological maps (1:50.000 & 1:100.000 scale) represent the surface dataset that we used to create the surface interpretation of the regional geology.

As a first step, 18 2D balanced regional geological cross-sections, dip-oriented (W-E) across the Central Apennine, were built define the structural picture at regional scale. The cross-sections were built using MOVE (Petroleum Experts) and Petrel (Schlumberger) software. The following modelling step was the 3D model construction, in which the surface/subsurface data as well as all the geological sections were integrated in the final 3D structural and geological model.

The main geological layers reconstructed in the 3D model were than populated using the appropriated interval velocity values, building the final 3D velocity model in which the lateral velocity variation due to the presence of different facies/geological domains were considered.

As one of the results, we defined several 1D-velocity models coherent with the regional 3D velocity model, in which the key seismic stations and the earthquakes hypocentres dataset for the most potential seismogenic faults were included. 1D models were characterized by different degree of simplification, in order to test diverse approaches for the earthquake relocation. For this exercise, we used public dataset extracted by the analysis of microseismicity of the Sulmona basin.

We believe that the proposed approach can represents an effective method for combining geological and geophysical data to improve the subsurface and seismogenic faults interpretation, contributing to the seismic hazard assessment.