The Friulian
earthquake of 1976 was the first major Italian earthquake to be studied using a
substantial amount of scientific data and robust analyses. It prompted
coordinated research at a national level, resulting in the establishment of the
Geodynamics Targeted Project. This interdisciplinary initiative, involving
geologists, seismologists, and engineers, underscored the necessity of a
national civil protection system, which was formalised in 1982. On many levels,
this marked the beginning of modern seismological research in Italy. This
foundation led to the development of new instruments for monitoring,
territorial classification and prevention, providing a better understanding of
how the Earth moves and reducing the impact of future earthquakes.
At the
beginning of 1976, scientific organisations in Friuli Venezia Giulia were not
particularly interested in local seismic activity. This was one of the reasons
why the population and local authorities were unprepared for the magnitude of
the earthquake that struck on May 6 that year. At that time, there were only 33
operational seismometer stations in Italy, all of which were located at
universities or geophysical institutions. In Northeast Italy, these were
located in Trieste, Udine, Padua, Salò and Bolzano. Within a 250-kilometre
radius of Friuli, there were operational stations in Slovenia (Ljubljana and
Cerknica), Croatia (Labin, Puntijarka and Zagreb), Austria (Kremsmünster,
Mariazell, Molln and Innsbruck) and Germany (Bad Reichenhall, Fürstenfeldbruck
and Garmisch). In June 1976, the University of Vienna installed new stations in
Carinthia, in Klagenfurt and Bad Bleiberg.
The task of
monitoring seismicity in Italy was assigned to the National Institute of
Geophysics of Rome, now known as the National Institute of Geophysics and
Volcanology (Istituto Nazionale di Geofisica e Vulcanologia – INGV). During
that period, the institute issued two annual bulletins. A seismographic station
had existed in Trieste since 1906, located at the Maritime Observatory, and it
ensured the collection of seismological data on the Eastern Alps until it was
absorbed by the Experimental Geophysical Observatory (Osservatorio Geofisico
Sperimentale – OGS) in 1958. The OGS is now the National Institute of
Oceanography and Experimental Geophysics (Istituto Nazionale di Oceanografia e
di Geofisica Sperimentale). In 1963, the station was relocated from the site of
Campo Marzio, in the centre of Trieste, to the Grotta Gigante cave, which has
minimal background environmental noise. It was connected to the World-Wide
Standardized Seismograph Network (WWSSN), installed by the United States
Geological Survey and part of a global network dedicated to monitoring both US
and, above all, Soviet nuclear explosions.
The shock of
May 6 1976, and the seismic sequence that followed in the subsequent days were
studied by OGS initially only using the data recorded at the Grotta Gigante
cave installation, because it was very difficult to connect with nearby
stations due to interruptions and the congestion of telephone lines. It was
therefore difficult to identify the epicentre of the earthquake, and for the
estimation of depth, it was only possible to assess the shallow nature of the
event, whose shocks, including the aftershocks, were located within the first
seven to ten kilometres. Initially, the epicentre was believed to be located on
Mount San Simeone, near the confluence of the Fella River with the Tagliamento
River. However, subsequent consideration was given to the foothill belt of the
Julian Prealps to the east of Gemona and the Val Resia to the north-east of
Venzone. Ultimately, it was determined that the epicentre was located in the
area of Monteprato, between the settlements of Taipana and Lusevera, in the
Julian Prealps.
The extensive
data collected by the station in Trieste enabled 695 earthquakes to be located
between May and October 1976. It also made it possible to identify a
north-westward migration of seismic activity.
The earthquake
of May 6 1976 affected an area of 5,700 square kilometres in central-eastern
Friuli. At the time, its strength was estimated at a magnitude of 6.5 degrees
(ML 6.5, now Mw 6.42), among the highest ever recorded in northern Italy. It
was felt over a very wide area, as far as the Baltic Sea, and was preceded one
minute earlier by a shock of magnitude 4.5 (ML 4.5, now Mw 4.5), considered a
warning shock, or a foreshock, and not a separate event from the
sequence. The maximum macroseismic intensity reached degree X on the
Medvedev-Sponheuer-Karnik scale (MSK) at Gemona, Venzone, Trasaghis, Bordano,
Forgaria, Maiano and Osoppo. The earthquake did not strike any densely
populated urban areas. The nearest city, Udine, was located only 30 kilometres from
the epicentre and suffered only minor damage, as the ground motion rapidly
subsided towards the south.
At 9 p.m. on
May 6 1976, one of the strongest and most devastating seismic sequences of the
second half of the 20th century in Italy began in Friuli. It
continued with gradually weaker and less frequent shocks, eventually ceasing in
the summer. The most intense aftershock, measuring Mw 5.08, occurred on May 9.
While reconstruction work was
underway, two earthquakes occurred on September 11 (Mw 5.25 and Mw 5.60,
maximum Mw 5.58) and two more on September 15 (Mw 5.93 and Mw 5.95, maximum Mw
6.02), which caused further collapses and additional victims. A further strong
earthquake (Mw 5.14) occurred one year later, on September 16 1977, followed by
a series of aftershocks that lasted for more than one month.
The entire sequence represents the
evolution of a complex phenomenon that involved many compressive Alpine
tectonic structures, often blind, and some Dinaric faults, as in the case of
the strong earthquakes of May 7 and 9 1976. The shock of May 6, attributable to
a low-angle Alpine reverse fault initially associated with the Susans-Tricesimo
fault, was later referred to the Buja-Tricesimo line. Other studies instead
preferred to link the event both to the Tricesimo-Cividale thrust and to the
Periadriatic thrust, as in the case of the September 1977 event (Mw 5.14),
located to the west of the Tagliamento River.
Examining these interpretations
confirms the existence of a catalogue of active faults in Italy. This catalogue
identifies four sources of the Friulian sequence: the Gemona South fault, for
the main event of May 6 1976; the Tarcento line, for the earthquake of
September 11; the Montenars line, for the shock of magnitude (Mw 5.93) of
September 15; and, finally, the Gemona East fault, for the event of magnitude
(Mw 5.95) of September 15.
The tectonic complexity of the
Friulian territory, characterised by numerous, mainly reverse, fault systems
and intermittent elastic energy release, prompted the Autonomous Region of
Friuli Venezia Giulia to establish a Regional Seismometric Network. Initially
comprising three stations active in the epicentral area, the Network was
inaugurated on May 6 1977. The number of active installations increased in
subsequent years, reaching the current total of 20. This comprises 15
short-period and 5 broadband stations. In addition, there are stations in
Veneto and the Italian Autonomous Provinces of Trento and Bolzano. Combined,
they constitute the Seismometric Network of North-Eastern Italy. This system is
complemented by the Regional Accelerometric Network (Rete Accelerometrica
Regionale – RAF), comprising 14 stations that measure ground shaking during the
strongest earthquakes. This provides invaluable data for anti-seismic design
and assessing local effects. Together, the two networks provide comprehensive monitoring
and measurement of seismic activity.
Using data from public and private
stations, both permanent and temporary, located less than 250 kilometres from
central Friuli, a revision was recently carried out of the 2,012 events with
focal depths between approximately 5 and 14 kilometres that occurred between
the sequence of May 6 1976 and December 31 1977. It was thus possible to
produce a new interpretation of the seismic fault structures in the Friulian
foothill belt, proposing a new interpretative key that is useful for better
understanding the development of the Friulian sequence. A new structural model
for the seismicity of Friuli was thus proposed, suggesting a close relationship
between the right-lateral strike-slip Idria-Ampezzo system, extending from the
Isonzo River valley in western Slovenia to the upper Tagliamento River valley
in Carnia, and the thrusts present in the Julian Prealps and in the Friulian
foothill area (Gemona-Caporetto fault, Susans-Tricesimo fault, the Buja,
Pozzuolo and Udine seismic lineaments).
Three precursor phenomena of the
event were identified:
1. On January 26 1973, in the Grotta
Gigante cave in Trieste, a noticeable shift was recorded on both components of
the Marussi horizontal pendulums, with abnormal disturbances in the following
hours. The phenomenon repeated itself frequently, then stopped abruptly with
the earthquake on May 6, only to reappear weakly before the tremors in
September. Since that time, this type of disturbance has never been recorded
again.
2. During the winter of 1975-1976, four
low-intensity earthquakes occurred in the area of Latisana in the lower Friuli
plain facing the Adriatic Sea. This zone had previously been considered
earthquake-free. The tremors, which had magnitudes between 2.5 and 3.5, were
clearly felt by the local population. After the events of May 6, it was
speculated that these episodes were a precursor phenomenon because they were
caused by micro-fractures that had developed during the phase of compressive
stress accumulation preceding the earthquake.
3. Some clinometric variations were
recorded at the Ambiesta dam on the Tolmezzo River, south-west of Tolmezzo, as
had happened during the earthquake that struck the area of Gemona on October 11
1954.
Most of the research carried out has
therefore also enabled major advances in the field of prevention. «Protection
against earthquakes», wrote Marcello Riuscetti, a lecturer in Applied
Seismology at the University of Udine and one of the designers of the National
Seismic Network, «is a complex task that can be achieved with the following
prerequisites: solid knowledge of seismicity; modern seismic legislation that
is appropriate for the culture, economy and characteristics of the nation’s
buildings and socio-administrative organisation; rigorous application of this
legislation to old and new buildings; and an efficient post-earthquake response
system (emergency management)».