Assessment of earthquake locations in 3-D deterministic velocity models. A case study from the Altotiberina Near Fault Observatory (Italy)

The accuracy of earthquake locations and their correspondence with subsurface geologydepends strongly on the accuracy of the available seismic velocity model. Most modern methods toconstruct a velocity model for earthquake location are based on the inversion of passive source seismologicaldata. Another approach is the integration of high-resolution geological and geophysical data to constructdeterministic velocity models in which earthquake locations can be directly correlated to the geologicalstructures. Such models have to be kinematically consistent with independent seismological data in order toprovide precise hypocenter solutions. We present the Altotiberina (AT) seismic model, a three-dimensionalvelocity model for the Upper Tiber Valley region (Northern Apennines, Italy), constructed by combining300 km of seismic reflection profiles, six deep boreholes (down to 5 km depth), detailed data from geologicalsurveys and direct measurements ofPandSwave velocities performed in situ and in laboratory. We assessthe robustness of the AT seismic model by locating 11,713 earthquakes with a nonlinear, global-searchinversion method and comparing the probabilistic hypocenter solutions to those calculated in threepreviously published velocity models, constructed by inverting passive seismological data only. Our resultsdemonstrate that the AT seismic model is able to provide higher-quality hypocenter locations than theprevious velocity models. Earthquake locations are consistent with the subsurface geological structures andshow a high degree of spatial correlation with specific lithostratigraphic units, suggesting a lithologicalcontrol on the seismic activity evolution