Seismic Swarm S20161128.1: Analysis of Central Italy Seismicity in Late 2016

Central Italy occupies a tectonically active segment of the Apennine chain, where ongoing extension along northwest-southeast trending normal faults accommodates the divergence between the Adriatic and Tyrrhenian domains. This extensional regime, driven by the rollback of the subducting Adriatic slab, produces frequent moderate earthquakes at depths typically between 5 and 15 km. The region’s seismic history includes destructive events such as the 1997 Umbria-Marche sequence and the 2009 L’Aquila earthquake, underscoring a long-term pattern of clustered activity rather than isolated mainshocks.

Swarm S20161128.1 was recorded between 04:12 on 27 November 2016 and 15:45 on 2 December 2016, spanning 131 hours and 33 minutes. During this interval, 68 earthquakes were detected, with magnitudes ranging from 2.5 to 4.0 and focal depths predominantly between 5 and 12 km. The largest events reached magnitude 4.0 on 27 November at 21:41 and again on 1 December at 11:30, both at approximately 10 km depth. Activity was most intense on 27 November, when multiple events above magnitude 3.0 occurred within a few hours, followed by sustained lower-level seismicity that gradually declined by early December.

The temporal distribution shows classic swarm characteristics: no single dominant mainshock, but rather a diffuse release of energy across numerous events of comparable size. Notable clusters occurred in the evening of 27 November, with events of 3.5, 3.6, and 4.0 within roughly two hours, and again on 28 and 29 November when several magnitude 3.5–3.6 shocks took place. Depths remained stable around 8–10 km for the majority of events, consistent with the brittle-ductile transition zone in the upper crust of the Central Apennines.

Since 2000, six seismic swarms have been documented in the same region, with one swarm in 2007, two in 2010, and three in 2016. This recurrence highlights the propensity of Central Italian faults to produce swarm-like sequences, often linked to fluid migration or aseismic slip transients along segmented normal faults. The 2016 swarms, including S20161128.1, occurred within a broader period of elevated seismicity that affected the area throughout the year.

The data indicate a typical swarm duration of several days with rapid onset and gradual decay, maximum magnitudes below 5.0, and hypocenters confined to the upper 12 km. Such patterns align with the structural complexity of the Apenninic extensional belt, where fault interaction and heterogeneous stress fields favor distributed rather than localized rupture.

References
SeismoSight internal classification records for swarm S20161128.1.
INGV (Istituto Nazionale di Geofisica e Vulcanologia) seismic bulletins on Central Apennines tectonics and historical seismicity.
Scientific literature on extensional faulting and swarm mechanisms in the Italian Apennines (post-2016 peer-reviewed studies).