When a tremor shook the Island of Ischia on August 21, causing two deaths and forcing 1500 people to leave their damaged homes, few seemed to remember that Ischia is a place prone to deadly quakes. The latest disaster on this beautiful Island happened during the same fatal summer of 1883, when Krakatoa wrought havoc in the southern hemispher, chancing Earth’s climate.
On July 28, 1883, Ischia was rattled by a 15-second, M 5.8 earthquake: 80% of the buildings in Casamicciola Terme, a quaint, thermal town in the northern part of the island, were reduced to bricks.
On August 21 this year, “only” two people lost their lives; on July 28, 1883, the death toll was staggering: 2300, among local residents and tourists, were fatally trapped underneath the rubble. The 1883 quake was almost 1000 times more powerful than the one that shook Ischia on August 21. Its Intensity was assessed as X (on a I to XII range) on the Mercalli Scale.
Alessandro Tibaldi, Professor of Structural Geology at Milan-Bicocca Universiy, stated that “This was a very shallow earthquake, typical of volcanic regions, so that many houses could not whitstand the ground acceleration…And most of the consequences we are seeing at Ischia are due to the poor quality of buildings”.
Back in 1883 the disaster was hundred times larger: Two of the victims were the parents of Benedetto Croce, one of Italy’s most renowned philosophists in the XX century, who was also severely wounded during the quake.
Why is Ischia prone to seismic hazard? The 42-square-km large volcanic island is part of a much broader, submerged volcanic complex in the southern Tyrrhenian Sea, off the Phlegrean coastline. It emerged from the waters approximately 150.000 years ago, and experienced five major phases of activity. Two of those were marked by gigantic eruptions that triggered the of a major caldera.
As underlined by Alessandro Tibaldi (University of Milan-Bicocca) and Luigina Vezzoli (Insubria University) in a key 2004 publication, about 30,000 years ago the western and central part of the caldera floor started being uplifted, due to the magma pressure underneath the island.
This led to the resurgence of Mt. Epomeo, a huge structural block that dominates the island: This steep block has been rising about 2-3 cm per year from under water up to its present-day altitude of 786 m asl. The steady uplifting of Mt. Epomeo is made possible by the actions of a number of faults bordering it; the epicentre of the 1883 and 2017 quakes are located along one of those faults.
This is one more confirmation that Ischia, on which the latest eruption occurred in 1302 AD, is far from being an extinct volcanic area: Magma is still pushing up underneath the island, and this is capable of triggering shallow, destructive earthquakes.
The only possible response we should give is to keep carefully monitoring volcanic and seismic hazard in the South of Italy, a region subject to long-lasting, powerful tectonic processes that make this touristic heaven one of the most geologically-active areas on Earth.
Federico Pasquaré Mariotto (Associate Professor, Insubria University, Varese, Italy)