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)

摘要

We use local earthquake tomography and background seismicity to investigatestatic and transient features of the crustal velocity structure in the Val d’Agri (southernApennines, Italy), one of the regions in central Mediterranean with the highest seismogenicpotential. The upper crust is dominated by two broad high‐velocity anticlines of the buriedApulia Carbonate Platform ramping on two parallel high‐angle thrusts interpreted aspreexisting 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 Apuliancrystalline basement in the Apennine belt. These results provide valuable constraints onthe Apennine belt tectonic evolution, supporting a thick‐skinned interpretation for thePliocene terminal phase of the compressional tectonics. The geometry of the Val d’AgriQuaternary basin is controlled by these inherited compressive features, whereas thepresently active extensional tectonics barely reworked the structure. We find inconsistencybetween the structure of the Apulia Carbonate Platform and the location and geometryof the Quaternary normal faults mapped at the surface. This suggests either the immaturityof the normal faults or their secondary role in accommodating the extension. We observespatiotemporal (4‐D) changes of VP and VP/VS models defining transient variations ofpore fluid pressure in the upper crust. A strong change in the VP/VS ratio heralds a raise inthe seismicity rate that can be related to large water level changes in a nearby artificiallake. This evidence is consistent with a mechanism of reservoir‐induced seismicity byfluid pressure increase and pore pressure diffusion. The 4‐D velocity variations areconfined in the shallow portion of the upper crust (3–6 km depth) where fluids are storedin a highly fractured medium. Pore pressure fluctuations can affect the strength of faultsegments, favoring seismicity rate changes along the active faults and possibly promotinglarge future earthquakes.