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We monitor the relative variations of crustal velocity during a stop of water injection at the Val d'Agri oilfield (Italy) in January-February 2015 from the analysis of the ambient seismic noise cross-correlations. This technique allows the continuous estimations of the relative velocity variations occurred in the superficial layers of the Earth crust independently from the earthquake occurrence. Our results show a relative decrease in seismic velocity of about 0.08%, detected seven days after the injection restart of fluids injection and can be compatible with an increase of fluids in the medium. We estimate the medium diffusivity from this delay time obtaining a value of about 2.0 m2/s. Independently, we compute diffusivity from the observed delay time of small-magnitude (ML≤ 1.8) seismicity induced by the first injection tests in June 2006, finding a similar value. The high diffusivity values found from the two independent …