Seismic Swarm S20160824.1: Analysis of Central Italy's 2016 Sequence

Central Italy's Apennine region experienced significant seismic activity with the registration of swarm S20160824.1. This event sequence began at 01:36 on 24 August 2016 and concluded at 12:56 on 8 September 2016, encompassing 415 earthquakes over 371 hours and 19 minutes. The swarm's primary location aligns with the tectonically active central Apennines, where extensional forces drive normal faulting along the mountain chain.

The geological setting of central Italy features the Apennine fold-and-thrust belt, influenced by ongoing extension linked to the rollback of the Adriatic slab. This produces shallow crustal earthquakes, typically at depths under 15 km, consistent with the swarm's recorded events. Historical seismicity in the area includes destructive events such as the 1703 Norcia earthquake and the 1997 Umbria-Marche sequence, underscoring the region's persistent hazard from active faults like the Mt. Vettore-Mt. Bove system.

Analysis of the first 100 events reveals a classic mainshock-aftershock pattern. The sequence initiated with a magnitude 6.2 earthquake at 4 km depth on 24 August 2016 at 01:36:32 UTC. Subsequent events showed rapid decay in magnitude, with peaks including a 5.6 event at 3 km depth roughly two hours later. Depths remained predominantly shallow, ranging from 1 to 15 km, with the majority clustered between 6 and 11 km. Magnitudes after the initial shock rarely exceeded 4.5, though several reached 4.2 and 4.6 within the first hours. Temporal distribution indicated intense activity in the initial 12 hours, gradually tapering with events spaced further apart by late 24 August.

This pattern aligns with aftershock productivity following a moderate-to-large mainshock in an extensional regime. The 100 events provided a detailed view of energy release, dominated by smaller tremors (magnitudes 2.4–3.7) that comprised over 80 percent of the sample. Depths showed slight variations but confirmed a crustal source without significant deepening over time.

Regionally, central Italy has recorded eight seismic swarms since 1 January 2000. Prior swarms occurred in 2007 (one), 2009 (three), 2010 (three), and 2011 (one). These episodes reflect episodic strain release along the Apennines, often without a single dominant mainshock. The 2016 sequence stands out for its higher event count and stronger initial magnitude compared to earlier swarms in the catalog.

The swarm's characteristics emphasize the importance of continuous monitoring in this high-hazard zone. Shallow depths amplified ground shaking, contributing to structural impacts in nearby communities. Updated assessments from national seismic agencies continue to refine fault models based on such sequences, aiding long-term hazard evaluation.

References

• Istituto Nazionale di Geofisica e Vulcanologia (INGV) seismic bulletins
• United States Geological Survey (USGS) earthquake catalog
• Journal of Geophysical Research: Solid Earth publications on Apennine tectonics