We investigate the temporal evolution of background seismicity rate in the Umbria-Marche sector of the northern Apennines that was struck by the 1997–1998 Colfiorito seismic sequence. Specifically, we apply the Epidemic-Type Aftershock Sequences (ETAS) model to separate the background seismicity rate from the coseismic triggered rate of earthquake production. Analyzed data are extracted from the Catalogo della Sismicità Italiana (CSI1.1, catalog of Italian seismicity) 1981–2002, which contains for the study area 12.163 events with ML>1.5. The capability of the ETAS model to match the observed seismicity rate is tested by analyzing the model residuals and by applying two nonparametric statistical tests (the runs and the Kolmogorov-Smirnov tests) to verify the fit of residuals to Poisson hypothesis. We first apply the ETAS model to the seismicity that occurred in the study area during the whole period covered by the CSI1.1 catalog. Our results show that the ETAS model does not explain the temporal evolution of seismicity in a time interval defined by change points identified from time-evolution of residuals and encompassing the Colfiorito seismic sequence. We therefore restrict our analysis to this period and analyze only those events belonging to the 1997–1998 seismic sequence. We again obtain the inadequacy of a stationary ETAS model with constant background rate to reproduce the temporal pattern of observed seismicity. We verify that the failure of the ETAS model to fit the observed data is caused by the increase of the background seismicity rate associated with the repeated Colfiorito mainshocks. We interpret the inferred increase of background rate as a consequence of the perturbation to the coseismic stress field caused by fluid flow and/or pore-pressure relaxation. In particular, we show that the transient perturbation caused by poroelastic relaxation can explain the temporal increase of background rate that therefore represents a fluid signal in the seismicity pattern.

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