Recent seismic activity in central Italy and Malta

Distribution of main seismic sequences in Italy between 1997 and November 2, 2016. Source: EMSC

Distribution of main seismic sequences in Italy between 1997 and November 2, 2016. Source: EMSC

On August 24, a magnitude 6.2 earthquake struck the Amatrice region in central Italy, killing around 300 people and causing severe damage and collapse to buildings. Hundreds of aftershocks followed the earthquake.

On October 30, a magnitude 6.5 earthquake again hit central Italy, this time near Norcia. Even though buildings were again destroyed, no casualties were reported after this earthquake, mainly because people had been evacuated after the August earthquake. Another 4.8 magnitude earthquake hit the area four days later.

Aldo Zollo, Professor of Seismology at the University of Naples Federico II, explained that the occurrence of multiple shocks in earthquake sequences in Italy is not a rare event. In the past 36 years, there have been a number of cases where such phenomena were experienced, including Irpinia in 1980, Abruzzo in 1984 and Colfiorito in 1997.

The difference between these cases is the time between the strong shocks. In the Irpinia case, three earthquakes each with epicentres in a different place, struck within 80 seconds. In the Colfiorito case, the two largest shocks were of magnitude 5.7 and 6 respectively, and hit the area with a time delay of around nine hours. In the present case of earth­quake sequences, the time delay bet­ween the strongest shocks was more than two months.

The area currently being struck by these earthquakes hosts the central Appenines, a densely faulted and complex mountainous region. Due to the huge number of faults, the area is considered as having high seismic activity. This also increases the probability that, given certain conditions, a rupture of around magnitude 6 can trigger a new rupture on adjacent, similar size segments by shifting stress in the rocks, and create what can be viewed as a ‘domino effect’.

Antonella Peresan of Italy’s Istituto Nazio­nale di Oceanografia e di Geofisica Sperimentale in Trieste explained that these events “can hardly be considered as independent ones”. The area is “often characterised by double events and sequences of rather strong earthquakes”, and so “the likelihood of further quakes was relatively high according to the different statistical and geophysical models”.

These earthquakes have no connections or implications to Malta

Although the magnitude 6.2 earthquake caused around 300 casualties, such a magnitude is considered to be moderate when compared to magnitudes 8 and above. Thus, when analysing the cause of damage and casualties, one has to look at other factors, such as the distance, the geology on which the buildings were built, the topographic effects and also the construction of the building itself. In fact, most of the buildings destroyed were old, not designed to sustain damage in a moderate earthquake and also close to the fault where the earthquake started.

These earthquakes have no connections or implications to Malta. From the archived documentation, which is generally very scant for the period before the arrival of the Knights of the Order of St John in 1530, at least six earthquakes that caused various degrees of damage to the Maltese islands have been documented. The sources of these earthquakes were Sicily, Greece and the central Mediterranean Sea area.

The highest intensity of earthquake experienced in Malta since 1500 was the result of the quake that struck southeast Sicily on January 11, 1693. This caused over 60,000 deaths in eastern Sicily, together with total destruction of several towns and villages in this region.

Its effect in Malta was extensively documented in the archives. The earthquake was strongly felt throughout the whole of the islands. Damage was reported in Valletta, Mdina (where the Cathedral partially collapsed), Rabat and the Citadel in Gozo. The damage to the Mdina Cathedral and Citadel was reported to be most probably due to “long years of neglect”, as was the damage to coastal towers.

Two large earthquakes in Greece in the late 19th century also caused damage in the Maltese islands, and in 1911, an earthquake left considerable damage in Gozo, while in Malta, damage was apparently limited to some cracks. No epicentre location for this last earthquake is available, due to the scarcity of seismic instrumentation at that time. However, there is reason to believe that the earthquake was located close to Gozo.

The Seismic Monitoring and Research Group at the University of Malta’s Department of Geosciences carries out research on all the above aspects of earthquake-related phenomena, including historical research, site effects and calculations of hazard and risk to the country.

Daniela Farrugia is a PhD student at the Seismic Monitoring and Research Group of the University of Malta’s Department of Geosciences.

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