Quantitative analyses of background seismicity and earthquake clustering in extensional seismogenic settings - case studies from the Central-Southern Apennines of Italy

The Central-Southern Apennines of Italy are a region with high seismic risk zone and experienced destructive earthquakes both in historical and in instrumental time. Geological data and historical seismicity indicate that the fault structures in this area are able to produce earthquakes with magnitude greater than 6.5. In fact the sector, stretching between the Irpinia 1980 (Mw 6.9) and the Accumuli-Visso-Norcia 2016 (Mw 6.5) seismic sequences, was struck by eleven events (MW ≥ 6.5) among the largest historical and early-instrumental earthquakes  since 1349. On the contrary, if we exclude the Barrea seismic sequence occurred in 1984 (Mw 5.9), the instrumental catalogue shows that this area is predominantly characterized by a low background level of seismicity and by earthquake clustering characterized by low release of strain energy.

We analyzed the seismicity occurring in this area from 1985 to 2018 (0.0 ≤ ML ≤ 5.0)  and  by a declustering algorithm  we indentified a set of 45 spatio-temporal clusters where the earthquake number stem out significantly from the background seismicity rate. The background seismicity (6196 events, 0.0≤ML≤4.1) is characterized by a b value of 0.96 ± 0.4, a magnitude of completeness of 1.4 and it is strictly controlled by known fault patterns. The earthquake clusters accounts for a non-negligible (45%) part of the total seismicity. A close inspection to the individual clusters allowed us to identify 4 seismic sequences characterized by isolated mainshock-aftershocks behaviour and 41 tectonic earthquake swarms (TESs). TESs have a duration ranging 2-12 days, 2.5-3.0 characteristic magnitude and 1.2 km/d migration rate. They are constituted by mono and/or polyphase episodes and they do not show a spatial complementary along the system of activated fault rather they are often spatially overlaid occupying the same fault segment. The latter behavior seems to indicate TES occurrences be driven by an underlying transients loading of the fault faster than the few mm/yr long-term extension active along the Apennines chain. The best candidates to explain these transients are likely presence of pressurized fluid abundant and/or  possible small scale creeping. The focal mechanisms and the depth of foci well correlate with the mapped normal fault systems and TESs illuminate regions of these faults adjoining ruptures of past large earthquakes. The spatio-temporal distribution of TESs suggests that the system of faults in the southern and central Apennines is characterized by heterogeneous rheology where small fault patches systematically release strain through TESs and other parts are to date locked. These findings are of great importance to better improve models for the assessment of seismic risk in the area.