Location:

Period:

20 Apr 2009 06:33:37 - 10 May 2009 15:30:44 (20 days 8 hours 57 minutes)

Volcanoes in 100km radius:

Colli Albani(81km)

Earthquakes:

436

10 swarms found nearby.

2009

S20090330.1(17.0km)

30 Mar

98 days 16 hours

6163 earthquakes

S20090407.1(4.3km)

6 Apr

14 days 13 hours

720 earthquakes

S20090409.1(2.2km)

8 Apr

3 days 2 hours

130 earthquakes

S20090412.1(1.2km)

11 Apr

8 days 23 hours

375 earthquakes

S20090429.1(4.6km)

28 Apr

1 day 20 hours

28 earthquakes

S20090501.1(6.3km)

30 Apr

1 day 9 hours

26 earthquakes

S20090629.1(5.2km)

28 Jun

9 days 0 hours

138 earthquakes

S20090704.2(3.4km)

3 Jul

14 days 1 hours

237 earthquakes

S20090801.1(27.7km)

31 Jul

19 days 15 hours

224 earthquakes

2011

S20110215.1(22.3km)

15 Feb

1 day 8 hours

27 earthquakes

AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20090421.1: Analysis of Early Events in Central Italy

Central Italy occupies a tectonically active segment of the Apennine chain, where ongoing extension driven by rollback of the Adriatic slab produces normal faulting at shallow crustal depths. The region has a well-documented history of moderate-to-large earthquakes, including the 1703 Norcia event, the 1915 Avezzano earthquake, and the 2009 L’Aquila mainshock, all linked to the same Quaternary fault system.

SeismoSight internal records classify Swarm S20090421.1 as the first swarm recorded in central Italy since systematic monitoring began in 2000. The sequence initiated at 06:33 UTC on 20 April 2009 and concluded at 15:30 UTC on 10 May 2009, spanning 488 hours and 57 minutes and comprising 436 events.

Insight drawn from the first 100 recorded events shows a compact cluster of low-magnitude earthquakes. Magnitudes ranged from 1.3 to 3.6, with the largest shock (M 3.6) occurring on 21 April at 16:20 UTC. Depths remained consistently shallow, averaging 9–10 km and never exceeding 14 km. Temporal distribution was irregular yet continuous, with no prolonged quiescence exceeding a few hours. The majority of events clustered between 8 and 11 km depth, consistent with the brittle upper crust of the Apennines.

Such characteristics align with swarm behaviour driven by fluid migration or slow slip along pre-existing normal faults rather than classical foreshock–mainshock–aftershock sequences. The 2009 timing places the swarm within the post-seismic phase of the L’Aquila earthquake, suggesting possible triggering by static stress changes or pore-pressure diffusion.

Since 2000, four swarms have been identified in the same sector under SeismoSight classification, underscoring the recurrent nature of diffuse microseismicity in central Italy. Continued high-resolution monitoring remains essential for distinguishing background swarm activity from potential precursors to larger events.

References

SeismoSight internal swarm catalogue (S20090421.1 parameters)
INGV official seismic bulletins (2009 L’Aquila sequence)
USGS Earthquake Catalog (regional tectonics and historical events)