Seismic Swarm S20180410.1: Central Italy Earthquake Sequence Analysis

Central Italy lies within the Apennine mountain belt, where ongoing extensional tectonics driven by the rollback of the Adriatic slab produces frequent seismic activity along normal faults. The region experiences shallow crustal earthquakes, typically at depths of 5–15 km, reflecting brittle failure in the upper crust. Historical records document major events such as the 2009 L’Aquila and 2016–2017 Amatrice–Norcia sequences, underscoring the area’s persistent seismic hazard.

Swarm S20180410.1 was recorded in this tectonic setting. The sequence began at 11:40 on 9 April 2018 and concluded at 01:22 on 14 April 2018, spanning 109 hours and 41 minutes. During this interval, 68 earthquakes were detected. Event depths ranged from 5 km to 11 km, consistent with the regional crustal seismicity pattern.

The swarm featured a mainshock of magnitude 4.7 at 03:11 on 10 April 2018 (depth 10 km). Subsequent activity included events of magnitude 3.8 (13 April, depth 10 km), 3.5 (two occurrences on 10 April, depths 9–10 km), and 3.4 (two occurrences on 11 April, depths 5–8 km). The majority of events registered between magnitude 2.0 and 2.9, with depths predominantly clustered around 8–10 km. A notable concentration of activity occurred on 10 April, accounting for over half of the recorded events.

This swarm aligns with the historical pattern of seismic swarms in Central Italy. Since 2000, six such sequences have been documented: one each in 2007 and 2010, three in 2016, and one in 2017. These episodes typically involve clusters of small-to-moderate events without a single dominant mainshock, reflecting fluid migration or aseismic slip along fault segments within the extensional regime.

The 2018 swarm’s temporal evolution shows an initial phase of low-magnitude events on 9 April, followed by the peak on 10 April, and a gradual decline through 14 April. Depths remained stable within the upper crust, indicating no significant migration to greater depths. Such characteristics are typical of swarm behavior in the Apennines, where distributed fracturing accommodates tectonic strain.

SeismoSight internal classification identifies this sequence as Swarm S20180410.1 based on its spatiotemporal clustering and lack of a clear foreshock–mainshock–aftershock progression.

References
Istituto Nazionale di Geofisica e Vulcanologia (INGV) seismic bulletins
SeismoSight internal swarm database (2000–2018)