This is based on an old post by Carl, The World’s most ill begotten piece of real estate – Part III, which has been slightly reworked.
The Chinese have a saying, “May you live in interesting times”. And it is in no way a friendly thing to say; on the contrary it is a rather magnificent curse. In Naples people live all their lives in interesting times. If it was not enough with being the poorest city in Italy, they also have to contend with the Camorra (local mafia), drug-wars, corrupt politicians, strikes and general civil unrest. To top it off even further they have built their city on top, or around, no less than 3 active super volcanoes. Could the times get more interesting than that? Well you could add large earthquakes and tsunamis to the list.
Ischia, or more correctly Monte Epomeo, started it’s activity about 350 000 years ago. Technically it is of the complex volcano type. During the first 300 000 years it grew and developed a large edifice paired with an over-sized volcanic sub-structure.
56 000 years ago the volcano had reached the critical level where the edifice was too large and heavy to be sustained on top of the very large magma chamber. The eruption probably started as a very large VEI-6 eruption that emptied out the magma chamber sufficiently for the roof to collapse. And since Ischia is an island it then got messy as the ocean roared down into the open magma chamber. The ensuing VEI-7 explosion created the Green Tuff Ignimbrite. This Green Tuff Ignimbrite should not be confused with the even larger Pantelleria Green Tuff (Italy is rather interesting…) that covers most of the Mediterranean area.
After the eruption the island was completely gone. As far as is known a 23 000 year long period of dormancy followed, but there might have been minor subsurface eruptions that helped to start healing the roof of the volcanic chamber system.
33 000 (Ar/K-dating) years ago a new phase started where the volcano had frequent effusive eruptions that helped to weld the tuff together healing the roof of the magma chamber along the entire 10 kilometer wide caldera.
28 000 years ago things started to get really interesting. By then the roof above the chamber was sufficiently structurally sound to hold for the increasing pressure inside the chamber. That caused the entire roof to be pushed upwards.
Most of the readers in here are familiar with the concept of resurgent lava domes. We have all seen them being pushed out of craters like odd plugs. For those interested in seeing the phenomenon I recommend Soufriere Hills at Montserrat. Thing is though that it is normally smaller craters that suffer from this rather dangerous condition.
Problem here is that Monte Epomeo is a super-size resurgent dome. Just imagine the pressure needed to push up a ten kilometer wide plug 900 meters in 28 000 years.
I know, we are only talking about 3.2 centimeters per year on average, but it still requires rather stunning amounts of power. The uplift is though larger than that, the reason for that being failures in the resurgent dome with rock-slides and sector failures of the dome as it started to stick up above the caldera rim. 5 600 years ago the dome passed the rim. During the push up phase the dome had also dragged the caldera rim with it above surface, and around the island an elevated area has been created by the pressure. So, a lot of pressure has gone also into moving parts that technically are not a part of the resurgent dome.
Now a resurgent dome is in itself not eruptive and Mount Epomeo is not a volcano. It is a solidified mass, and the rising force is not just the magma below, but also the removal of the weight above, in the eruption. The resurgence is not a danger sign. But there is still magma around, as shown by the behaviour.
Eruptive and other behaviors
The most common type of eruption at Ischia is smaller eruptions taking place between the resurgent dome and the caldera rim. There are quite literally hundreds of fissures, cones, and other volcanic vent types encircling the dome. These eruptions normally follow episodes of rapid surging (uplift) of the dome.
There are two more dangers on top of the island caused by the resurgent dome. The first one is quite simply sector collapses, landslides and rock-falls as the brittle welded tuff suffers structural failure. Some of these slides and rock-falls have reached as far as the coast line.
The more dangerous version of failure is the lateral flank eruption. That happens as magma pushes upwards and builds up tremendous pressure and swelling of the side of the dome and the side of the caldera rim. Think Mount Saint Helens here and you get the picture. This causes a large pyroclastic flow going laterally over the island until it reaches the coast, then it will continue over the water. If it happens in the wrong direction it will hit inhabited land.
During the last 12 000 years there has also been 3 sub-surface collapses of the island causing massive debris flows running out into the Tyrrhenian Sea. And there are several spots along the coast line where parts of the Island have calved off into the ocean. When this happens large tsunamis will race into the Bay of Naples destroying any part not high up. The latest known widespread tsunami in the area is known to have happened 800BC according to written records.
In the end, any island volcano is a danger. Magma and water are not a good mix. The location at the Bay of Naples does not help either, as this benefits tsunamis (remember Krakatoa!). And flank collapses, from whatever cause, can cause such a tsunami even while the volcano remains deeply dormant.
Current status of Ischia
Even though Ischia is currently not showing any sign of erupting other than the steady uplift she is deemed by Guido Bertolaso (head of the dipartimento della Protezione Civile) to be the most likely volcano to erupt due to the rapid buildup of magma that they have recorded. Bertolaso even went so far as stating “if I had to say which is the volcano with the most loaded gun barrel, I’d say it’s not Vesuvius but the island of Ischia”. He though went on to state that no eruption is imminent. This becomes evident if one looks at the lack of heightened volcanic tremor, and minimal amount of magmatic earthquakes.
Risks of Ischia
The risks are roughly discussed below in the order of likelihood. Ischia is the volcano most likely to have a large eruption in the Naples area. One should though remember that it is most likely to have a normal VEI-1 to VEI-4 eruption when it erupts next. This would mainly affect the 60 000 residents on the island, and the same amount of tourists.
Rock falls, dome failures and landslides from Monte Epomeo is also fairly likely to happen in the foreseeable future due to the resurgent dome uplifting. This will also only affect the local residents and tourists.
Large landslides either at the coast, or out on the elevated shelf that surround the island is fairly likely to happen within the next few thousand years as the pressure building up raises the land up and weakens the structure of the flanks. When this happen large tsunami waves will hit the Bay of Naples causing widespread destruction. This is also the risk that is hardest to predict and mitigate.
And now Ischia is back in the news. A minor earthquake hit August 21, which caused a surprising amount of damage, as well as two fatalities. The response of the Italian emergency services has been exemplary, and their rescuing efforts certainly have avoided further fatalities. We salute their work. But why did such a small quake (M4.3) do so much damage? And what does it mean for the future?
The earthquake was very shallow: initial estimates put it at a (default) 10 kilometer, but now it is placed at only 2 kilometer. The shallowness magnified the effects on the surface. The buildings may also not have been sufficient resistant to shaking. Some people have expressed concern about the use of the local tuff as building material.
There is no indication that this earthquake is volcanic in nature, even though the entire island is of volcanic origin. It also appears completely unrelated to the increase of activity around Solfatara, where scientists have considered the possibility of a small eruption in a few years time. The most likely cause may be local subsidence. A very sudden drop would also have increased the surface acceleration, and would fit with the enhanced damage.
Ischia has shown strong subsidence since Roman times. On the northeast, a roman foundry is now 7 meters below sea level: much of the subsidence may have happened around the 1st century AD. On the southern side, a beach has been uplifted by 15-30 meters but the region is currently taking part in the subsidence of the island of around 3 mm per year.
Has this happened before? The last major earthquake here was on July 28, 1883, at the town of Casamicciola. It was 9:25 pm and the richer people were at the theatres. (The location of the less wealthy people was not reported.) 25 seconds of shaking left the town completely destroyed, as were nearby Laco and Forio. There were an estimated 2300 fatalities. There was little damage at Ischia (the town on the east coast of the island of the same name), and the quake was only just felt at Naples. The fact that the damage was so localized suggest it also was not that strong (M5.5, perhaps?) but shallow. There have been no significant earthquakes since. The damage corresponded to XI on the Mercalli scale. (An interesting aside is that Mercalli himself studied the aftermath of the 1883 earthquake on Ischia.) Compared to Naples itself, which was hit by an M7 37 years ago, Ischia is not a major fault zone.
Casamicciola was build on the northern slopes of Epomeo. Forio is on the west coast: here the shaking was vertical, suggesting it was near the epicentre. The locations suggest that the 1883 earthquake occurred at the same fault and place as the 2017 one.
The most worrying aspect is that there had been an earthquake at the same location, two years earlier, on March 4, 1881, which had already severely damaged Casamicciola. This was also not a strong quake, VIII on the Mercalli scale, with a depth of 2 kilometer – identical to the 2017 one. There was also a weak earthquake on 25 July 1880.
And the region had been hit before. On 18 March 1793, some houses were damaged and 7 people died in Casamicciola. On 11 April 1827 there was a quake felt throughout Ischia, followed by a much stronger quake on 2 Feb 1828 which hit -hold it – Casamicciola. The two reported further earthquakes in 1867 and 1868.
Landslides are a major hazard. The uplift of Epomeo has left gradients as steep as 1:4, and rock falls are common. A rock slide in 1228 killed as many as 700 people.
The message here is that there is a tendency for quakes to occur in pairs, 1-3 years apart, and the last quake may be the strongest. This would be a good time to check on the earthquake resilience of the surviving buildings. This area has history.
Carl 2012; updated Lurking & Albert, 2017