Report on Campi Flegrei (Italy) — August 2012
Bulletin of the Global Volcanism Network, vol. 37, no. 8 (August 2012)
Managing Editor: Richard Wunderman.
Campi Flegrei (Italy) Analysis of seismic swarms (Mw =1.9; ~219 events) during September 2012
Please cite this report as:
Global Volcanism Program, 2012. Report on Campi Flegrei (Italy). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 37:8. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.BGVN201208-211010.
40.827°N, 14.139°E; summit elev. 458 m
All times are local (unless otherwise noted)
219 low-magnitude earthquakes occurred at Campi Flegrei during September 2012, a comparatively large number with respect to the previous year (figure 22). The earthquakes chiefly were contained within two swarms (with events up to Mw 1.9; Mw indicates moment magnitude) occurring on 7 and 15 September. Peak ground accelerations (PGA) were non-trivial (up to ~0.5 g), and some earthquakes were widely felt by area residents. Analysis revealed that the strain release rate of the 7 September swarm fell within values seen for other swarms during the last 20 years. The observations reported by the Vesuvius Observatory (who provided the material for this report) were limited to those associated with the earthquakes and related seismic analysis. Other reporting on topics such as deformation appears on the Observatory’s website (see Information Contacts, below). The observatory is part of Italy’s National Institute of Geophysics and Volcanology (INGV).
Almost all of the earthqaukes that occurred during September took place in two swarms (figures 22 and 23). The first swarm occurred in the area of Pozzuoli during 0715-0935 UTC on 7 September. The two largest events of that swarm were Mw 1.9 (a duration magnitude, Md, value of 1.7; figure 24); these events were the largest recorded events of the prior year (figure 24A). The 7 September swarm was dominant over the 15 September swarm both in terms of the number and magnitude of events.
The second swarm of September 2012 took place between 0901 and 1012 UTC on 15 September (figure 22), with the strongest events (Md -0.3) occurring at 0947 and 0954 UTC. This swarm was recorded by only one station (STH, Agnano, figure 23B) and thus was plausibly located in close proximity to that station at shallow depth. This swarm is absent on the depth plot in figure 25 (depth not available).
The hypocenters of 49 events were determined during September 2012; their depths were generally less than 4 km (figures 23 and 25). The seismological parameters did not show significant anomalies (figures 24 and 25). However, September 2012 was the most seismically energetic time period of the prior year (figure 26); seismicity during September produced >3 times the cumulative energy released during the preceding year.
|Figure 26. Cumulative seismic energy released at Campi Flegrei during (A) October 2011-September 2012 and (B) September 2012. Courtesy of Vesuvius Observatory-INGV (Naples).|
Analysis of the 7 September seismic swarm. For the two main events (0734 and 0825 UTC) on 7 September, source parameters were determined from S-wave displacement spectra (results shown in figure 27).
The duration and strain release of the 7 September swarm were similar to other seismic swarms at Campi Flegrei since at least 1994 (figure 28).
|Figure 28. A plot showing duration and strain release rate for Campi Flegrei seismic swarms since 1994. Courtesy of Vesuvius Observatory-INGV (Naples).|
Some of the events in the swarm were widely felt in the urban area of Pozzuoli. Peak ground acceleration values (PGA, units of %g, the acceleration due to gravity) recorded by the accelerometer in Pozzuoli (CPOZ, figure 23B) show two prominent peaks corresponding to the two largest events that occurred at 0734 and 0825 UTC (figure 29).
Reference. Mooney, W.D., 1989. Seismic methods for determining earthquake source parameters and lithospheric structure, in Pakiser, L.C. and Mooney, W.D. (eds), Geophysical framework of the continental United States, Geological Society of America Memoir 172.
Geologic Background. Campi Flegrei is a large 13-km-wide caldera on the outskirts of Naples that contains numerous phreatic tuff rings and pyroclastic cones. The caldera margins are poorly defined, and on the south lie beneath the Gulf of Pozzuoli. Episodes of dramatic uplift and subsidence within the dominantly trachytic caldera have occurred since Roman times. The earliest known eruptive products are dated 47,000 yrs BP. The caldera formed following two large explosive eruptions, the massive Campanian ignimbrite about 36,000 BP, and the >40 cu km Neapolitan Yellow Tuff (NYT) about 15,000 BP. Following eruption of the NYT a large number of eruptions have taken place from widely scattered subaerial and submarine vents. Most activity occurred during three intervals: 15,000-9500, 8600-8200, and 4800-3800 BP. Two eruptions have occurred in historical time, one in 1158 at Solfatara and the other in 1538 that formed the Monte Nuovo cinder cone.
Information Contacts: Vesuvius Observatory, National Institute of Geophysics and Volcanology (INGV), Via Diocleziano 328, 80124 Napoli, Italy (URL: http://www.ov.ingv.it/ov/).