Upper crustal structure, seismicity and pore pressure variations in an extensional seismic belt through 3‐D and 4‐D VP and VP/VS models: The example of the Val d’Agri area (southern Italy)

We use local earthquake tomography and background seismicity to investigate static and transient features of the crustal velocity structure in the Val d’Agri (southern Apennines, Italy), one of the regions in central Mediterranean with the highest seismogenic potential. The upper crust is dominated by two broad high‐velocity anticlines of the buried Apulia Carbonate Platform ramping on two parallel high‐angle thrusts interpreted as preexisting inverted normal faults. The deep core of the anticlines consists of very high VP (up to 6.9 km/s) and low VP/VS rocks, suggesting the involvement of the Apulian crystalline basement in the Apennine belt. These results provide valuable constraints on the Apennine belt tectonic evolution, supporting a thick‐skinned interpretation for the Pliocene terminal phase of the compressional tectonics. The geometry of the Val d’Agri Quaternary basin is controlled by these inherited compressive features, whereas the presently active extensional tectonics barely reworked the structure. We find inconsistency between the structure of the Apulia Carbonate Platform and the location and geometry of the Quaternary normal faults mapped at the surface. This suggests either the immaturity of the normal faults or their secondary role in accommodating the extension. We observe spatiotemporal (4‐D) changes of VP and VP/VS models defining transient variations of pore fluid pressure in the upper crust. A strong change in the VP/VS ratio heralds a raise in the seismicity rate that can be related to large water level changes in a nearby artificial lake. This evidence is consistent with a mechanism of reservoir‐induced seismicity by fluid pressure increase and pore pressure diffusion. The 4‐D velocity variations are confined in the shallow portion of the upper crust (3–6 km depth) where fluids are stored in a highly fractured medium. Pore pressure fluctuations can affect the strength of fault segments, favoring seismicity rate changes along the active faults and possibly promoting large future earthquakes