It's been 485 years since the last eruption at Italy's Campi Flegrei. As you might guess, a lot has changed. When Monte Nuovo formed in 1538, the population in and around Naples was a few hundred thousand people. Today, that number is closer to three million people. The 1538 eruption produced a small cinder cone in Pouzzuoli at the mouth of the Bay of Naples, burying parts of medieval villages. With increased restlessness under the Campi Flegrei, Italy has to contend with the fact that even a small eruption like 1538 might require evacuating half a million people from a densely settled area.
Italy's Restless Caldera
Right now, the Campi Flegrei caldera is definitely restless. Over the last few weeks, there have been hundreds of earthquakes under the east side of the caldera near La Solfatara thermal area, including a few as large as M4. The INGV, Italy's volcano monitoring agency, reports that most of the earthquakes are happening at ~1 mile (2.2 kilometer) depths under the surface. More troubling is the uplift of the land surface in the same area that has averaged 0.5 inch (15 mm) per month since the start of the year with a slight increase as of late.
The take home message right now from the INGV is that, yes, the Campi Flegrei is acting up more than background. However, there is also no clear sign that an eruption is imminent in the near term. They have the volcano on alert status Yellow, meaning people should pay attention to where this unrest goes and be prepared for potential evacuation.
What Might Happen?
When it comes to what might happen at the Campi Flegrei, the best odds are on nothing. Unrest at calderas like the Campi Flegrei are common, sometimes related to magma movement, sometimes to faults slipping and sometimes the hydrothermal waters under the caldera. Even if an eruption were to happen, the likelihood is that it would be an eruption on the scale of Monte Nuovo rather than potentially an unimaginable explosive eruption like the Neapolitan Yellow Tuff from ~15,000 years ago. An eruption of that scale would dwarf blasts like 1991's Pinatubo.
The Campi Flegrei is a challenge from a monitoring and hazards standpoint. Imagine plopping the Yellowstone Caldera down in the outskirts of Houston or Phoenix. Suddenly, even minute changes in earthquakes, gas emissions or deformation need to be watched closely because the margins are much narrower with all the people to potentially evacuate if an eruption is in the cards.
This kind of seismic unrest has happened multiple times over the past 50 years at the Campi Flegrei. The caldera began to show increased activity in the 1960s after relative quiet for centuries. Large scale evacuations of Pozzuoli happened in 1983 when earthquakes and 3 meters (10 feet) of uplift suggested that an eruption could be coming ... but nothing happened. There as another vigorous earthquake swarm during 2012, but that didn't directly lead to any new eruption either.
The Challenge of Making the Call
That is the challenge for volcano monitoring and emergency management personnel: when do pull the trigger. Italy is suffered from confusion and misinformation about potential geologic disasters in the recent past, so public trust in such agencies may be thin. Combine that with the fact that the Campi Flegrei has been "crying wolf" during the last few seismic swarms, the worry is how seriously will people take a real crisis.
There is little doubt that people in the area are worried. Local authorities in the Naples area have been running drills to prepare for a potential large evacuation. An eruption could lead to hundreds of thousands people leaving their homes for potentially an indeterminant amount of time. During the 1983 crisis, the conditions that people found themselves when evacuating was horrific and a repeat of such a humanitarian disaster would be devastating.
The INGV has dozens of instruments watching every movement of the Campi Flegrei. The Italian government has reiterated that the decision to evacuate won't come lightly, but new plans are brewing in case the call is made. The margin for error is very small, both in calling or not calling for evacuation.
