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Note:This page contains AI-generated content for informational and entertainment purposes only. It may contain inaccuracies. Raw event data is from USGS and EMSC. All statistics, lists, and derived information are generated by this site. Full disclaimerFound an error?
Location:
CENTRAL ITALY
Period:
8 Apr 2009 13:05:47 - 11 Apr 2009 15:32:57 (3 days 2 hours 27 minutes)
Volcanoes in 100km radius:
Colli Albani(83km)
Earthquakes:
130
10 swarms found nearby.
2009
S20090330.1(15.4km)
30 Mar
98 days 16 hours
6163 earthquakes
6 Apr
14 days 13 hours
720 earthquakes
11 Apr
8 days 23 hours
375 earthquakes
20 Apr
20 days 8 hours
436 earthquakes
28 Apr
1 day 20 hours
28 earthquakes
30 Apr
1 day 9 hours
26 earthquakes
28 Jun
9 days 0 hours
138 earthquakes
3 Jul
14 days 1 hours
237 earthquakes
S20090801.1(25.8km)
31 Jul
19 days 15 hours
224 earthquakes
2011
S20110215.1(20.8km)
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 Analysis: Central Italy, April 2009

Central Italy occupies a tectonically active segment of the Apennine chain, where ongoing extension along northwest-southeast trending normal faults accommodates the rollback of the Adriatic slab. This structural setting produces frequent moderate-to-large earthquakes, with historical events documented in the L’Aquila basin since Roman times and instrumentally recorded sequences throughout the twentieth century. The 2009 L’Aquila mainshock of 6 April (Mw 6.3) ruptured the Paganica fault and was followed by an extended aftershock sequence that included multiple seismic swarms.

SeismoSight internal records classify Swarm S20090409.1 as the first swarm recorded in the region since 1 January 2000. The swarm initiated at 13:05 UTC on 8 April 2009 and terminated at 15:32 UTC on 11 April 2009, spanning 74 hours and 27 minutes. During this interval, 130 earthquakes were detected.

Analysis of the first 100 events reveals a compact spatiotemporal cluster. Magnitudes ranged from 1.1 to 3.7, with the large majority falling between 1.8 and 2.5. Four events exceeded magnitude 3.0, the largest reaching 3.7 at 11:53 UTC on 10 April. Focal depths concentrated between 7 km and 11 km, consistent with the brittle–ductile transition in the Apennine crust; a minority of events occurred at shallower depths of 2 km, possibly reflecting fluid migration or fault segmentation effects. The temporal distribution shows two main pulses of activity on 9 and 10 April, separated by a brief lull, followed by a gradual decay on 11 April. No single event dominated the sequence, confirming the swarm character rather than a classical mainshock–aftershock pattern.

Such swarms are common in the central Apennines and are often linked to pore-pressure transients along mature fault zones. The 2009 swarm occurred within the broader aftershock volume of the L’Aquila sequence, illustrating how residual stress and fluid flow can sustain elevated seismicity for days to weeks after a major rupture.

Since 2000, only two swarms have been identified in the same sector according to the same classification criteria, underscoring the relative rarity of this mode of seismic release compared with isolated mainshocks.

References

  • Italian National Institute of Geophysics and Volcanology (INGV), Bollettino Sismico Italiano, 2009.
  • Chiaraluce, L. et al., The 2009 L’Aquila (Italy) seismic sequence, Tectonophysics, 2011.
  • SeismoSight internal swarm catalogue, 2000–present.