Non-random spectral components in the seismicity of NE Italy

The time evolution of seismicity in one of the most active seismic areas of the Alpine Arc has been investigated with the aim of detecting statistically reliable non-random features. The statistical evaluation regards the spectral analysis of the fluctuations of earthquake occurrence with respect to the mean seismicity. The seismic catalogue of the seismometric network of Friuli–Venezia Giulia (NE Italy) (Istituto Nazionale Oceanografia e Geofisica Sperimentale, OGS) is used, limiting the magnitudes to M≥1.8 and covering the time period from 1977 to 1997. The analysis is carried out on the full and on the aftershock-depleted catalogue. The result is that a significant spectral peak is found, centered on the period of 1.4 years. At the present stage, some doubt remains whether the fluctuations really reflect variations in natural seismicity, or whether they are connected to varying levels of completeness in the data acquisition. A spectral peak of about 1.4 years is also found in the variations of the water table, calculated over the same time interval.