INGV reported that a seismic swarm at Campi Flegrei consisted of about 150 earthquakes recorded from 1951 on 20 May to 0031 on 21 May. The largest event in the swarm, a M 4.4 located at a depth of 2.6 km beneath the Solfatara, was the largest recorded since the current cycle of seismicity associated with uplift began in 2005. A total of 450 earthquake events were recorded the previous month. The rate of inflation was 2 cm per month and remained unchanged. The report noted that during a 1982-84 seismic crisis there were more than 1,300 monthly events recorded, with associated uplift as high as 9 cm per month.

Geological Summary. Campi Flegrei is a 13-km-wide caldera that encompasses part of Naples and extends to the south beneath the Gulf of Pozzuoli. Episodes of significant uplift and subsidence within the dominantly trachytic caldera have occurred since Roman times. The earliest known eruptive products are dated 47,000 years BP. The caldera formed following two large explosive eruptions, the massive Campanian ignimbrite about 36,000 BP, and the over 40 km3 Neapolitan Yellow Tuff (NYT) about 15,000 BP. Following eruption of the NYT a large number of eruptions originated from widely scattered subaerial and submarine vents. Most activity occurred during three intervals: 15,000-9,500, 8,600-8,200, and 4,800-3,800 BP. The latest eruption were in 1158 CE at Solfatara and activity in 1538 CE that formed the Monte Nuovo cinder cone.

Source: Sezione di Catania - Osservatorio Etneo (INGV)