Seismic Swarm S20111217.2: Characteristics and Context in Central Italy

Seismic swarm S20111217.2 occurred in central Italy between 23:27 on 16 December 2011 and 01:23 on 18 December 2011. Over 25 hours and 56 minutes, the sequence produced 29 earthquakes. All events were recorded at focal depths between 16 km and 24 km, with local magnitudes ranging from 1.5 to 3.8. The largest shock reached magnitude 3.8 at 03:04 on 17 December; two additional events exceeded magnitude 3.0.

The temporal distribution shows the highest rate of activity during the first eight hours, followed by a steady decline. Magnitudes remained modest throughout, and no single mainshock dominated the sequence. Depths clustered tightly around 18–20 km, consistent with the brittle–ductile transition zone in the region. These features are typical of swarm-type seismicity driven by fluid migration or slow aseismic slip rather than rapid tectonic stress release.

Central Italy lies within the Apennine extensional belt, where ongoing northeast–southwest extension occurs at rates of 2–4 mm per year. The belt is segmented by normal faults that accommodate crustal thinning above the subducting Adriatic slab. Historical and instrumental records document repeated moderate-to-large earthquakes along these structures, including the 1997 Umbria–Marche sequence and the 2009 L’Aquila event. The 2011 swarm is located within one of these active fault segments, where background seismicity is persistently elevated.

Instrumental monitoring since the 1980s shows that swarms of similar duration and magnitude range recur every few years in the same area. Such episodes rarely culminate in damaging earthquakes, yet they provide valuable data on fault-zone permeability and stress transfer. Depths of 16–24 km place the events near the base of the seismogenic layer, where fluids released from the subducting slab may facilitate slip on pre-existing fractures.

No damage or injuries were reported from the 2011 swarm. The sequence nevertheless contributed to refined three-dimensional velocity models and improved understanding of how small-magnitude clusters relate to larger seismic cycles in the Apennines.

References

Istituto Nazionale di Geofisica e Vulcanologia (INGV) seismic catalogue
European-Mediterranean Seismological Centre (EMSC) event bulletins
DISS Working Group, Database of Individual Seismogenic Sources (2020 version)
Serpelloni et al., 2022, Tectonic extension rates in the Apennines (J. Geophys. Res.)