The “precursor” signals of the 1908 earthquake and crustal deformations in the Strait of Messina – Tempo Stretto

In the last 2 years preceding the earthquake, the soil near Messina rose by +2.3 cm per year

Following the catastrophic earthquake of 28 December 1908, the geomorphological structure of the Strait of Messina it has been studied by many scientists from all over the world to understand the complex geological structural conformation of the area. Even today we have more than one theory about the birth and evolution of the Strait during the various geological eras. Among these, the one that can be well applied to the reality of the strait attributes to this area a structure a “Graben”. Translated into the vocabulary of geology the term “Graben” indicate an ocean rift, i.e. a portion of the earth’s crust that has collapsed due to the combined action of a system of normal (or direct) faults in a relaxing tectonic regime. In a nutshell, this means that the Strait of Messina is nothing more than a depression of a tectonic nature formed in 125 thousand years, thanks to the repetition of large earthquakes that have gradually removed the north-eastern tip of Sicily from the coasts of southern Calabria and from the rest. of the continent.

The image of Messina destroyed by the earthquake of 28 December 1908

Of great importance are the rocky and lithological characteristics of the reliefs that surround the Strait. It is no coincidence that the Peloritani (Sicilian shore) and the Aspromonte massif (Calabrian shore) have the same type of conformation. This shows us that in past times Sicily was united with the rest of the continent, thus confirming the theory that sees the “Graben” as the dominant structure of the area. As we know, the Strait of Messina is located right in the center of the Mediterranean, in a very unstable area from a tectonic point of view. In fact, three continental plates converge right here, each of them has its own movement that leads them to collide thus forming a system of normal faults, a total of 370 km long, which in various studies (see Monaco, Tortorici, Gasparini, Anderson, Jackson, ..) was named as “Siculo-Calabrian Rift zone”. This complex system of faults extends continuously from the Calabrian Tyrrhenian coasts, extending through the Strait of Messina, along the Ionian coast of Sicily to reach the eastern Iblei (south-eastern Sicily) and the area around the island of Malta. The orientation of the “Rift” is well documented by the focal mechanisms of numerous and violent crustal earthquakes that in the past centuries have struck eastern Sicily, in particular the Hyblean area, the coast around Catania (1169-1693) and the Strait of Messina (394-1908), such as southern Calabria, especially the Aspromontana belt (above all the seismic crisis of 1783). Just under the Strait of Messina pass several faults of a relaxing nature, directly connected to the so-called “Siculo-Calabrian Rift zone”. These tectonic structures are characterized by particular horizontal movements due to the continuous displacements of the continental plates.

Unfortunately many of these faults are still unknown by geologists as they are located at approx 7-8 km under the seabed of the Strait, for this reason they cannot be the object of study or constant monitoring, unlike what happens in California with the famous San Andreas fault that flows on the surface. Given the impossibility of studying it and the almost total absence of signs of breakage left on the surface during the terrible earthquake of 1908 the fault that continues to shape and deform the morphology of the Strait of Messina is also referred to as a “blind” fault, which acts in depth without being able to emerge on the surface. A real puzzle for geologists and seismologists who for decades have continued to study the geological evolution of the Strait. The earthquake of 1908 has made available numerous useful information for the purposes of a more reliable definition of the geodynamics of the area. In reality, just a few months before the enormous seismic catastrophe, a campaign to level the coasts of the Strait, between Reggio Calabria and Messina, had ended. The measurements were repeated shortly after the event, thus providing a reliable estimate of the vertical cosismic shifts (Loperfido, 1909; De Stefani, 1909). Such information showed a clear uplift of Calabria compared to Sicily. But anomalous movements of the soil, before strong earthquake of 28 December 1908, have been highlighted by the tidal data (Omori, 1913).

In the port of Messina registrations of this type, by the Mareographic Service National, took place continuously since 1887. From the data relating to the station operating in Messina, a phase of raising of the Messina coast up to 1900, and a subsequent lowering of the same, in the period 1900-1906, was highlighted. But the very strange thing, which made many internationally renowned seismologists think for a long time, concerns the fact that in the last two years before the powerful earthquake of 1908 the ground, near Messina, began to rise suddenly upwards, even reaching a significant anomaly, of well +2.3 cm / year, straddling between 1906 and 1907 (Mulargia and Boschi, 1983; Bottari et al, 1992). This sudden rise of the coasts bordering the Strait of Messina by some seismologists it has been interpreted as a “warning sign”, indicating a strong stress of the fault of the Strait, now at the breaking point, and therefore charged at the right point to trigger a high magnitude earthquake. In fact, the earthquake was accompanied by a considerable lowering of the coast on the Messina side, of several tens of centimeters, with a maximum of about 70 cm south of Messina. The tsunami that preceded the earthquake damaged the Mareographic station of Messina which, restored in 1912, allowed the recording of a strong “post-seismic” uplift of the soil (physiological after the subsidence of more than 70 cm which caused the coast around Messina to sink during the earthquake of 28 December), di –2.4 cm / year (average value over a five-year period following 1912).

The magnitude of the observed displacements suggests that the deformation state that will probably precede an earthquake similar, in terms of force and release of energy, to that of 1908, can be detected by permanent monitoring systems with the aid of GPS. In the last thirty years, numerous geodetic campaigns have been carried out on the shores of the Strait of Messina, to monitor any deformations in the area. In 1970, as part of the project launched by the Italian Government for the construction of the Bridge of the Strait, a geodetic network was established repeatedly measured with terrestrial techniques until 1982. The comparison between the leveling measurements of 1981-1982 and 1970 showed a moderate “subsidence” of about 0.1 cm / year (Achilli and Broccio, 1982; Baldi et al., 1983; Anzidei et al., 1998). The GPS monitoring network, starting from 1994, has been extended towards the southern part of the Strait of Messina, connecting the local network to a regional network (TYRGEONET), aimed at controlling crustal deformations in the central Mediterranean. This network, which now covers the entire area of ​​the Strait of Messina and the adjacent territories, in the near future will be able to detect any type of crustal deformation ready to affect the area, giving us useful information in real time on the trend of seismicity. on the shores of the Strait of Messina, and above all by indicating in time those “anomalies” that can in some way predict significant seismic events for the area between Reggio and Messina.