The mystery of the Capri earthquake: why it was felt in the skyscrapers of Milan, in Sicily and Calabria but not in Campania

An anomalous seismic event has captured the attention of scholars and citizens, outlining a scenario that at first glance may seem contradictory. During the night, a magnitude 5.9 earthquake was recorded off the coast of Capri. Yet, contrary to expectations, in Campania — the region closest to the epicenter — the tremor was felt little or not at all. Instead, numerous and alarmed reports came from the North, particularly from Lombardy and Veneto, and from some southern areas like Sicily and Calabria.

The key to this apparent paradox has been explained by the INGV and lies entirely in geological dynamics. Unlike most Italian earthquakes, which are generally shallow, this tremor originated at an exceptional depth: 414 kilometers below sea level, in the upper portion of the Earth's mantle. At such depths, high temperatures and enormous pressures make the rocks ductile, hindering the sudden fractures typical of classic quakes.

In the Tyrrhenian Sea, however, a peculiar dynamic is at play: an ancient portion of oceanic crust belonging to the African plate is sinking due to subduction beneath the Calabrian arc, which is part of the Eurasian plate. This crust, colder and more rigid than the surrounding material, retains the ability to break in a brittle manner, generating very deep tremors, even between 400 and 500 kilometers.

This explains the first aspect: why did almost no one in Campania feel the quake? The perceptibility at the surface depends on the paths of seismic waves in the mantle and crust. To emerge in areas close to the epicenter, the waves must rise almost vertically through the Earth's crust, losing much of their energy and drastically attenuating. After about 400 kilometers of travel, they reach the surface with amplitudes and frequencies so reduced that they are imperceptible to most. The area above the hypocenter has remained, in fact, protected in a true "shadow zone".

Different is the behavior towards the more distant regions. To cover large distances, the waves traverse wide portions of mantle, a medium more homogeneous and efficient than the crust, following oblique trajectories. They then re-emerge at the surface retaining lower frequencies and longer oscillation periods. It is in these long-period waves that the reason for the strong perception in the North resides. The human body does not perceive them as a jolt, but rather as a slow oscillation, sometimes accompanied by nausea.

Moreover, their frequencies coincide with those of the tallest buildings and flexible structures. It is therefore not surprising that there are abundant reports from the upper floors of skyscrapers. In Milano, a city dotted with towers, the effect has been amplified, transforming the Lombard capital into a sort of perceptive antenna of the phenomenon. This has been compounded by the role of the subsoil of the Pianura Padana, which has acted as a gigantic resonating chamber, further enhancing the waves.

Even in Sicilia and Calabria, the earthquake was felt due to the same oblique propagation through the mantle. However, in the absence of the marked amplification effect typical of the Po Valley, the extent of reports in the Mezzogiorno was decidedly more limited.