Seismicity patterns before MD ≥ 4.1 earthquakes in the Friuli-Venezia Giulia (N.E. Italy) and western Slovenia areas

The spatial-temporal pattern of seismicity, preceding 5 moderate-sized earthquakes with MD ≥ 4.1, that occurred in the Friuli-Venezia Giulia region (north-eastern Italy) and in western Slovenia, is investigated with the Region-Time-Length (RTL) and the PI algorithms. These algorithms were originally formulated to reveal the variation of seismicity patterns before large earthquakes. The RTL algorithm detects seismic quiescences before the 1996 Claut earthquake (MD 4.3), the 1998 Bovec earthquake (MD 5.6), the 2002 Sernio earthquake (MD 4.9) and the 2004 Bovec earthquake (MD 5.1). No seismic quiescence is found before the 1988 Mena earthquake (MD 4.1). A stage of foreshock activation is clearly observed only in the Claut case. The PI method reveals space-temporal changes in the earthquake rate only before the Mena, Claut and Sernio earthquakes. The cases analyzed and those available in literature suggest that the seismicity anomalies like quiescence and foreshock activation preceding the moderate-sized events seem less evident than those preceding the severe earthquakes.