We present the results of Z-scan studies carried out on fused silica at 1064nm and 532nm with two different nanosecond pulse durations. Such measurements in silica and in the nanosecond regime are possible thanks to a high sensitivity setting up of the Z-scan method and in-situ characterizations of the spatio-temporal parameters of the beam. Besides, with the use of a newly adapted numerical simulation only the calibration errors of the measurement devices are significant. In these conditions, we found a higher value of the nonlinear refractive index than in the femtosecond regime and we show that these values depend on pulse duration, which indicates the contribution of nanosecond mechanisms like electrostriction.