Solfatara is a shallow volcanic crater at Pozzuoli, near Naples, part of the Phlegraean Fields or Campi Flegrei volcanic area. It is a dormant volcano, which still emits jets of steam with sulfurous fumes.

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A magnitude 4.0 earthquake struck south of Naples, Italy, on Tuesday, just days after the volcanic region saw its strongest earthquake in 40 years, according to Italy’s National Institute of Geophysics and Volcanology (INGV).

Italy’s fire brigade said there was slight damage to buildings in the latest earthquake, but no people were injured. The 4.2 magnitude quake on September 27 rattled the region and was felt as far away as Rome.

Campi Flegrei, also known as the Phlegraean fields, is a volcanic area that encompasses multiple ancient volcanoes that date back 39,000 years.

Seismic activity in the area, which stretches 200 kilometers (125 miles) under the Bay of Naples and the islands of Ischia and Capri, has intensified this year, with the region experiencing many small quakes before the most recent two. Campi Flegrei experiences a seismic phenomenon known as bradyseism, defined by cycles of uplift and gradual lowering of the ground.

The last major eruption of Campi Flegrei was in 1538, which created a new mountain in the bay. So far in 2023, Campi Flegrei has logged 2,868 earthquakes — 1,118 in August alone.

Carlo Doglioni, head of the INVG, gave testimony on the potential outcomes in front of the Italian government’s Environmental Commission’s lower chamber on September 28.

“There are two possible scenarios relating to the evolution of the situation in the Campi Flegrei: the best is that the ongoing bradyseism crisis ends as happened in 1983-84, the worst is an eruption similar to that of 1538,” he said.

“It is an evolution that we do not know and that we are monitoring.”

During the episode of bradyseism from 1983 to 1984, the ground rose 3.5 meters (11.5 feet).

The INGV has requested that the municipality of Naples carry out evacuations of some residents closest to the volcanic area to check the places most vulnerable to underlying structural damage from the rising soil. Most of the structures in question have been built in the past 20 years.

Italy’s civil protection agency estimates that at least 800,000 people live in a designated “yellow zone” and 500,000 in the “red zone,” the highest-risk area in the vicinity of the seismic region.

The last time an evacuation plan was tested was in 2019. However, local residents have demanded an updated plan that outlines what should happen in case of an eruption.