Location:

Period:

18 Oct 2011 21:03:30 - 20 Oct 2011 01:39:54 (1 day 4 hours 36 minutes)

Volcanoes in 100km radius:

Colli Albani(94km)

Earthquakes:

14 swarms found nearby.

2009

S20090510.1(18.0km)

9 May

13 days 16 hours

178 earthquakes

S20090626.1(10.4km)

25 Jun

3 days 7 hours

118 earthquakes

S20090713.1(9.3km)

12 Jul

6 days 5 hours

128 earthquakes

S20090801.1(26.1km)

31 Jul

19 days 15 hours

224 earthquakes

S20090823.1(8.1km)

22 Aug

6 days 9 hours

83 earthquakes

2010

S20100831.1(7.2km)

30 Aug

25 days 22 hours

427 earthquakes

S20101001.1(9.4km)

30 Sep

11 days 3 hours

111 earthquakes

S20101020.1(1.8km)

19 Oct

6 days 18 hours

375 earthquakes

S20101104.1(6.3km)

3 Nov

6 days 0 hours

176 earthquakes

S20101206.2(27.6km)

5 Dec

1 day 1 hours

33 earthquakes

2011

S20110215.1(26.7km)

15 Feb

1 day 8 hours

27 earthquakes

S20110306.1(10.0km)

5 Mar

8 days 1 hours

87 earthquakes

2016

S20161030.1(21.6km)

30 Oct

3 days 20 hours

72 earthquakes

2017

S20170118.1(14.5km)

18 Jan

5 days 9 hours

231 earthquakes

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Seismic Swarm S20111019.2 in Central Italy

Central Italy lies within a tectonically active zone shaped by the convergence of the African and Eurasian plates. This interaction produces extensional stresses along the Apennine mountain chain, resulting in normal faulting and frequent seismic activity. The region experiences both isolated earthquakes and clustered events known as swarms, where numerous tremors occur over short periods without a single dominant mainshock.

Swarm S20111019.2 was recorded in this setting. It began at 21:03 on 18 October 2011 and concluded at 01:39 on 20 October 2011, spanning 28 hours and 36 minutes. During this interval, 25 earthquakes were detected. Magnitudes ranged from 1.5 to 3.1, with most events occurring at depths between 6 and 15 km. The sequence showed a typical swarm pattern of gradual onset, fluctuating activity, and eventual decay.

The events unfolded as follows. The initial shock measured 2.6 at 10 km depth. Subsequent tremors on 18 October included magnitudes of 1.9, 1.6, and 1.9, all near 10 km. Activity continued overnight into 19 October with events of 2.0 and 1.9 at similar depths. Mid-morning brought the strongest shocks: a 2.8 at 10 km, followed by 2.7 at 8 km and a peak of 3.1 at 12 km. Later events remained modest, with the final recorded tremor of 1.9 at 6 km depth closing the sequence.

Such swarms reflect fluid migration or slow slip along faults rather than abrupt rupture. Depths clustered around 10 km indicate activity within the upper crust, consistent with the extensional regime of the central Apennines. No damage or injuries were associated with this low-to-moderate energy release.

Seismic swarms have occurred repeatedly in central Italy. Since 1 January 2000, twelve such episodes have been identified. Earlier swarms took place in 2009 (five events) and 2010 (five events), with two recorded in 2011. These clusters underscore the episodic nature of strain release in the area, where background seismicity remains elevated due to ongoing plate-boundary deformation.

The 2011 swarm fits within this established pattern. Its modest magnitudes and shallow focal depths align with historical observations of swarm behavior in the region. Monitoring by national seismic networks continues to track these sequences to improve understanding of precursory signals and long-term hazard assessment.

References

Istituto Nazionale di Geofisica e Vulcanologia (INGV) seismic bulletins
SeismoSight internal swarm classification records
Geological Survey of Italy tectonic summaries