The earthquakes of September/October 1997
in the frame of tectonics and long-term seismicity
of the Umbria-Marche (Central Italy) Apennines


M. Stucchi (Istituto di Ricerca sul Rischio Sismico - IRRS, CNR, Milano)
F. Galadini (Istituto per la Tettonica Recente - ITR, CNR, Roma)
G. Monachesi (Osservatorio Geofisico Sperimentale - OGSM, Macerata)

with the collaboration of E. Cova (DISTART, BO), I . Leschiutta, A. Zerga (IRRS, MI),
A. Rebez (OGS, TS)

Some notes on the main earthquakes quoted can be found in
Main historical earthquakes in the Umbria-Marche area
by V. Castelli (GNDT at OGSM, MC)

The Umbria-Marche Apennines are a seismic area, the activity of which is not uniform. According to the seismogenic model of GNDT (release ZS 4.0, 1996, from Scandone et al., 1990), this area belongs to several seismogenic zones. According to the Italian parametric earthquake catalogue NT4.1 (Camassi and Stucchi, 1997), the most active of them are zones 45, 46 and 47 (Fig. 1).

These zones are part of the axial belt of the Apennine chain, which is characterized by mainly NW-SE to N-S trending normal and oblique faults, whose activity is generally highlighted by the displacement of Late Pleistocene-Holocene deposits and landforms.

The epicentres of the earthquakes of Septembre and October 1997 belong to the zone 47 and are located in the area between Serravalle del Chienti, Foligno and Nocera; this area shows a seismic activity lower than the one of Alta Val Nerina.
From the tectonic point of view the epicentral area of these earthquakes is characterized by the presence of two primary faults (Colfiorito and Cesi faults, NW-SE to N-S trending, see Pizzi, 1992 and Cello et al., 1997), which border to the east the Colfiorito and the Cesi basin, and are 7.5 km e 6 km long respectively.

In this area, the following earthquakes with Ms >= 5.0 are located

The magnitude of the last two events is too low to infer the faults which might be responsible for them on the basis of geological data.

Slightly at SE a few more earthquakes are located:

Fig. 2 presents, with varied colour for each earthquake, the intensity datapoints >= 7/8 MCS of the most damaging earthquakes located along the axial belt (intensity data from DOM4.1, Monachesi and Stucchi, 1997).

It shows that the source of the earthquakes of September/October 1997 is located in an area which did not release strong earthquakes since 1279 at least. In this case the fault responsible for the 1997 earthquakes may be the same of that responsible for the 1279 event and the recurrence time for the re-activation of this fault may be at least 700 years, or even larger if the earthquake of 1279 was generated by other faults.

Time distribution of the strong earthquakes
The time distribution of the seimic activity in the Umbria-Marche Apennines is far from being stationary. Fig. 3 shows the time sequence of the events which belong to seismogenic zones 45, 46, 47.

The destructive earthquakes of 1279, (Monachesi, 1987), 1328, (Monachesi, 1987), 1352 e 1389, epicentral area Alta Val Tiberina (Castelli et al., 1996) were followed by a period of moderate activity, with a small number of destructive earthquakes over 250 or even 350 years: 1458, Alta Val Tiberina (Castelli et al., 1996); 1599, Cascia (GNDT, 1994); the earthquakes of 1639 near Amatrice (Monachesi and Castelli, 1992) might also belong to zone 47 (Fig. 4). Studies on these earthquakes are also found in Boschi et al. (1995 & 1997).

This period of moderate (assuming that catalogues are complete) activity was suddenly interrupted at the beginning of 18th century by the strong earthquakes of Norcia and L'Aquila of 1703, followed by a century of destructive earthquakes. Actually, in this century ten heavy damaging earthquakes (7/8 <= Imax <= 8 MCS) and eight destructive events (Imax >= 8/9 MCS) took place in this area (Fig. 5). Their parameters, as supplied by the catalogues NT4.1 (Camassi e Stucchi, 1997) and CFTI (Boschi et al., 1997), are:

	year      epic.area                Imax(NT)     Imax(CFTI)      Ms(NT)      Me(CFTI)
	1703      Norcia/L'Aquila          10           11              6.7         6.7
	1730      Norcia                   9            9               5.9         6.3
	1741      Fabrianese               9            9               6.2         6.2
	1747      Fiuminata/Gualdo T.      9            8/9             6.2         5.7
	1751      Gualdo Tadino            10           10              6.7         6.2
	1781      Cagliese                 10           10              6.7         6.1
	1789      Alta Val Tiberina        8/9          10              5.9         5.7
	1799      Camerino                 9            9/10            6.2         5.9

Studies on these earthquakes are found in: Stucchi (1985), Pergalani et al. (1985) Monachesi (1987), Conversini et al. (1990), Postpischl, ed. (1990), Stucchi et al. (1991), Castelli et al. (1996), Boschi et al. (1995 & 1997).

In the next two centuries (19th and 20th) the seismic acivity was high, though it did not reach the level of 18th century. A few destructive earthquakes (Fig. 6): 1832, Foligno (Monachesi, 1987); 1859, Norcia (GNDT, 1994); 1917, Alta Val Tiberina (Castelli et al., 1996); 1979, Norcia/Cascia (Spadea et al., 1981), and about ten heavy damaging earthquakes are reported. Studies on these earthquakes are also found in Boschi et al. (1995 & 1997).

Some elements of active tectonics
The key events of this sequence are the earthquakes which affected the Norcia and L'Aquila basins in 1703 (Monachesi et al., 1987, Boschi et al., 1995). Proceeding from South to North there are few doubts that the earthquake which struck the city of L'Aquila on February 2nd, 1703 is related to the activity of the two fault-branches which affect the eastern border of the Aterno river valley-L'Aquila basin, that is the Mt. Marine and Mt. Pettino fault-branches (8.5 km and 9.5 km long respectively). These faults are responsible for the displacement of Late Pleistocene slope deposits and for the formation of impressive carbonate bedrock scarps (Blumetti, 1995, Galadini et al., 1997).
The Norcia area is another typical fault-bound intermontane basin of the Central Apennines (Blumetti et al., 1992). The main Quaternary faults, located along the eastern and western borders of the basin, are responsible for the displacement of Middle Pleistocene alluvial deposits. By contrast, the Cascia basin, affected by the 1599 earthquake, is characterised by a NW-SE normal fault which border the basin to the NE (Cello et al., 1997); along this fault Blumetti (1995) reports geological effects related to the 1703 earthquake.
More work has to be done before having a clear tectonic framework of the earthquakes which have affected the Alta val Nerina, including the earthquake of 1328.
In the case of the Gualdo Tadino basin (1751 earthquake) some data are available from an unpublished map of Bosi et al. (1983). In this map some geomorphological features are reported (such as fault scarps) which highlight that the Gualdo Tadino basin is another typical intermontane depression of the Central Apennines, bound by a Quaternary fault to the east. Finally, more work has to be carried out in order to explain the seismicity of Alta Val Tiberina (Sansepolcro, Città di Castello) and Cagli, with reference to the earthquakes of 1389, 1789 and 1781 respectively.

Fig. 7a presents some primary active faults of the area, as compiled from Blumetti (1995), Pizzi (1992), Cello et al. (1997), Galadini et al. (1997) and unpublished data by A. Basili, F. Galadini, P. Galli, P. Messina. Fig. 7b compare these faults with the earthquake data of Fig. 2.

Time-space seismicity evolution
Fig. 8 presents the time-space distribution of the strongest earthquakes (Ms > 5.0) which belong to the axial belt (bold blue in the small figure). Distances are calculated between the epicentres and the indicated observation point.
Some trends can be observed, mostly concerning the 18th century (red), which might suggest influence of the strain release between each event and the following one. This influence, if any, needs further explanation according to quantitive analysis; similar considerations have been performed for the Southern Apennines by Nostro et al., 1996.


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